Department of Physics Master Thesis in Physics

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Department of Physics

Master Thesis in Physics

Investigation of PEN as a Scintillator for Low Background Experiments:

Characterisation of Light Yield Properties

Author:

Thesis Supervisor:

Thesis Reviewer:

Thesis Co-Advisor:

Thesis Submission Date:

Thomas Kraetzschmar PD Dr. Béla Majorovits Prof. Dr. Stefan Schönert Dr. Elena Sala

April 30, 2018

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Acknowledgements

I would like to express my gratitude to my thesis advisor, PD Dr. Béla Majorovits of the Max Planck Insitute for Physics (Werner-Heisenberg In- situte), for offering me this interesting thesis topic and for his guidance and support throughout the thesis. I also thank Prof. Dr. Schönert for accepting to be the co-advisor and second referee for my thesis, and his valuable advice given in the meetings we had.

My special gratitude also goes to my thesis co-advisor Dr. Elena Sala, who acquainted me with particle detectors and gave valuable advice throughout the thesis.

Another great experience was to work with the research groups at MPP, the GERDA group and the Future Detectors. I always appreciated the friendship and cooperation of the group members. Many thanks to everyone who has provided feedback on ideas and/or drafts of this thesis.

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Abstract

Polyethylene Naphthalene (PEN) presently attracts the interest of various groups as a relatively cheap, radio-pure scintillating material, which can be moulded in arbitrary geometries. Various potential application fields, spanning from low-background experiments to high-energy physics calorimeters to potential use in education could profit from the use of PEN.

The light output of PEN as custom-made scintillator is not yet well understood. The goal of this thesis was the investigation of optical properties of PEN.

Three dedicated setups were used, two of which had to be built and characterised as part of this master thesis. A photospectrometer setup for measuring the spectral shape and light output of different samples was installed, optimised, and characterised. For light output measurements, a setup with a photomultiplier tube (PMT) was built and first measurements were performed. The test of temperature dependence of PEN light output was conducted in an available experimental setup.

The measurements with the spectrometer showed, that some moulding parameters influence the light output. Using a PMT setup, it could be demonstrated that PEN tiles can be used with similar efficiency as standard BC408 scintillator tiles for veto purposes. Also, clear evidence for increasing light output at lower temperatures could be found.

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Chapter 1 Introduction

Motivation: The matter-antimatter problem is one of the biggest yet to be solved mysteries in physics. As we believe based on our current understanding of the formation of the universe, that there were equal parts of matter and antimatter right after the big bang, it is unclear how baryogenesis, the survival of matter over antimatter took place.

One of the possible solutions is the CP violating decay of Majorana neutrinos, which are their own antiparticles. Neutrinoless double-beta decay can be used to test the Majorana nature of neutrinos, because it is only allowed if neutrinos are Majorana particles.

Since the lifetime of the postulated neutrinoless double-beta decay is very long, the event rate is extremely low. This makes quasi background- free experiments with high detector mass, such as the GERmanium Detec- tor Array (GERDA) experiment, necessary to search for this process with sufficient sensitivity. For the next generation experiment, the Large En- riched Germanium Experiment for Neutrinoless double-beta Decay (LEG- END), it is proposed to suppress the background even further. New tech- niques of background reduction and the optimisation of existing ones are necessary to achieve this goal. A common technique is the use of multi purpose materials, which fulfil the need of radio-purity, but can also be used for actively vetoing background radiation. In GERDA, most of the components used fulfil these requirements, except of the detector array’s structural material, made of ultra-pure copper and silicon. For LEGEND, the use of an ultra-pure, transparent, and active structural material could improve the background identification efficiency. Unfortunately, common scintillators are usually unsuitable as they do not fulfil the set requirements. A possible candidate as structural material is Polyethylene naphthalate (PEN), a polyester polymer which is commonly used for everyday tools such as bottles or cups. Aside from its sturdy properties it also scintillates when exposed to radiation. It is widely used in industry, thus, its price is much lower compared to other scintillation materials. PEN could reduce the va- riety of materials for the detector array structure, using it as detector and structural material as well as copper wire isolations. It was shown that PEN is mechanically hard in cryogenic environments [Eck18] and PEN can

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be laminated and used as an isolator for cables [BCG⁺13]. Furthermore, PEN is relatively radio pure. These properties make it interesting for the use in low background experiments, such as the LEGEND project.

Problem Statement: Since the goal is to use custom-made PEN in LEGEND, it is crucial to understand the scintillation properties of the material and how the customisation process influences the desired properties of light output and transparency, as well as radio-purity and mechanical hardness. Furthermore, it also should be possible to produce cups for the Ge detectors. Our project at the Max-Planck-Insitut for Physics in Munich is investigating the possibility of using Polyethylene naphthalate (PEN) for that purpose.

The main focus of this thesis was on the investigation of the light output properties. The first question, when considering a new scintillator material, is what the light spectrum of the emitted light looks like. Especially the peak wavelength is of interest, as it determines the possible light detectors which can be used. Second, the light output and pulse shapes when using a common light detection system are of interest. At last, additional properties like the temperature or pressure dependence of the light output are also of interest. Since it is intended to use the material in cryogenic environments, the behaviour of the light output for different temperatures was investigated.

Approach: In order to answer the stated questions, three dedicated were used. Two of which were designed, implemented and characterised specifi- cally for this work. These setups were used for studying the light spectrum and the light output and behaviour of a common light detection system.

The last experiment, for studying the behaviour of the light output at different temperatures, could be conducted in an existing setup, available at the institute.

Work Performed: The two setups for investigating the scintillation properties of PEN were successfully implemented and characterised in the course of this thesis work. Light spectra of custom-made PEN were recorded and the peak wavelength was determined with a common value of 450 nm. For the customisation process of moulding, a moulding parameter investigation focusing on the influence of the moulding parameters on the light output was performed and two parameters influencing the light output were iden- tified. Furthermore, first pulses with a PMT were recorded revealing a long decay tail of the event pulse and the possibility of pulse shape discrimina- tion for PEN, and energy spectra of different scintillators were recorded.

At last, a clear indication of a temperature dependence of the light output was found, with an increase of light output for a decrease in temperature.

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Chapter 2 Physics Introduction

2.1 The Matter-Antimatter Problem

In physics, the simplest questions to ask are often the hardest to answer.

One of these questions is: Why is there matter? When considering the standard model (SM), there is no reason why the observed amount of matter should have formed in the early universe instead of being annihilated right after its production. We believe that baryons were formed in a process called baryogenesis, from a state of equal numbers of baryons and antibaryons. Otherwise, there would have been a highly fine-tuned initial condition, with 6.000.001 quarks for every 6.000.000 antiquarks. Empiri- cally, such a fine-tuning is very implausible in physics. Furthermore, the model of an inflation is well motivated by the observed features of the cosmic background [DNN08].

2.2 Beyond the Standard Model: Majorana Particles

For a dynamical generation of baryons, we have three criteria given by Sakharov [DNN08]:

1. Baryon number violation

2. Charge conjugation (C) and Charged conjugation Parity (CP) trans- formation violation

3. Out-of-equilibrium dynamics

These criteria are all found in the SM. However, none of the known mecha- nisms generates a large enough baryon asymmetry. This is pointing to new physics beyond the SM, which must extend the model with new sources of CP violation and either provide a departure from thermal equilibrium in addition to the electroweak phase transition (EWPT) or modify the EWPT itself. One of the favoured candidates introducing a new CP violating source are Majorana neutrinos. Departure from thermal equilibrium

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can occur for example during electroweak symmetry breaking. Majorana particles violate the lepton number as they have a Majorana mass and are their own antiparticle [DNN08].

In 1937 Ettore Majorana proposed "an essentially new theory for particles without electric charge (neutrons and hypothetical neutrinos)" [Maj37].

In this theory the complex field Ψ(x) for Weyl-fermions is given by Ψ(x) = χ₁+χ₂

√2 (2.1)

with

χ1,2(x) = Ψ(x)±Ψ^C(x)

√±2 (2.2)

fulfilling the Dirac equations, with the Dirac matrices γ^α:

(iγ^α∂_α−m)χ_1,2(x) = 0 (2.3) Additionally, χ_1,2(x) also fulfils the Majorana condition with the charged conjugate operator C:

χ_1,2(x) = χ^C_1,2(x) =Cχ^T_1,2(x) (2.4) Here, χ_1,2(x) indicates the field of a particle while χ_1,2(x) indicates the field an antiparticle. It is possible to show that, for Majorana particles, the creation operator of particles and antiparticles are the same, which means that they are their own anti-particles [Bil10].

2.3 Investigating the Nature of Neutrinos

The neutrino is only weakly and gravitationally interacting, which is why it has earned the nickname "ghost particle". Its detection is technically chal- lenging, making it hard to measure its properties. First proposed by Pauli in 1911 in order to explain the continuous electron energy spectrum of the βdecay, it took decades until it could finally be measured in 1956 by Reines and Cowan with their famous "Poltergeist" experiment, using the inverse β decay [CRH⁺56]. In 1962, a second flavour state, the muon neutrino ν_µ, was found by Ledermann, Schwartz, and Steinberger [DGG⁺62] in the AGS Neutrino experiment at Brookhaven National Laboratory. Finally, in 2000 the DONUT experiment proved the existence of the tau neutrino ν_τ [KUA⁺01].

Since the "solar-neutrino problem", it is known that neutrinos are mas- sive particles, and not mass-less as assumed in the SM. Here, the flux of neutrinos was measured and the one calculated from well understood processes in the sun did not coincide [BU88]. It turned out that the neutrinos are os- cillating, which means that they can change their flavour when propagating.

This was measured independently by experiments like Super-Kamiokande for atmospheric neutrinos and the Sudbury Neutrino Observatory (SNO) for solar neutrinos [F⁺98], [A⁺02].

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CHAPTER 2. PHYSICS INTRODUCTION Page 5

The flavour eigenstates hν_α|=hν_e|,hν_µ|,hν_τ|, and the mass eigenstates hν_i| =,hν₁|,hν₂|,hν₃| are not the same but superpositions of each other.

The mixing is determined by the unitary matrix U given by:

hν_α|=X

i

U_αihν_i| (2.5)

This rotation matrix is named PMNS matrix after Pontecorvo, Maki, Nak- agawa and Sakata [GGMS14]. It is an equivalent to the CKM matrix in the quark sector and can be parametrised with three mixing anglesθ_ij and δ_CP the CP violating Dirac Phase. The entries of the PMNS matrix are denoted as follows:

U =





U_e1 U_e2 U_e3 U_µ1 U_µ2 U_µ3 U_τ1 U_τ2 U_τ3



 (2.6)

With s_ij =sinθ_ij and c_ij = cosθ_ij the rotation in the state space is given by the rotation matrix R:

R=





c₁₂c₁₃ s₁₂c₁₃ s₁₃e^−iδ^CP

−s₁₂c₂₃−c₁₂s₁₃s₂₃eîδ^CP c₁₂c₂₃−s₁₂s₁₃s₂₃eîδ^CP c₁₃s₂₃ s₁₂s₂₃−c₁₂s₁₃c₂₃eîδ^CP −c₁₂s₂₃−s₁₂s₁₃c₂₃eîδ^CP c₁₃c₂₃



 (2.7) Furthermore, if neutrinos are Majorana particles, two additional phases the CP violating phasesα1, α2, given by the phase matrix A in Equation 2.8, have to be considered.

A =





e^iα¹ 0 0 e^iα² 0

0 0 1



 (2.8)

Finally, this gives the PMNS matrix stated in Equation 2.9 [GGMS14]:

U =R·A (2.9)

For a flavour state with energy E the probability that an oscillation has taken place with a change from flavour state α to a stateβ, at distance L from the source, is described by:

P(α→β, L, E) =X

kl

U_αkU_αl^∗U_βk^∗ U_βle

∆m2 kl

2E L (2.10)

This probability can be non-zero if the masses are not the same. With the mass squared difference∆m²_kl and the neutrino energy E. On the one hand this means the neutrino mass squared differences are measurable in neutrino oscillation experiments [GGMS14], [AS09]. On the other hand oscillation experiments can not give information on the absolute masses m_1,2,3.

M.S. Thesis Thomas Kraetzschmar April 30, 2018

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By investigation of theβ decay, the effective electron neutrino massm_ν_e can be measured in a direct way. The measured quantity is a superposition of the neutrino mass eigenstates given by:

hmβi= q

c²₁₂c²₁₃m²₁+s²₁₂c²₁₃m²₂+s²₁₃m²₃. (2.11) The KATRIN experiment aims to improve the limit given by one order of magnitude [EBB⁺18]. The energy released in a β decay, the Q-value, is known. This means that the upper endpoint of the electron energy spectrum of this decay is shifted bym_βc² with respect to the Q-value. The current upper limit m_β < 2eV was obtained by the Mainz and Troitsk experiments [K⁺05], [ABB⁺11].

It is also possible to constrain the absolute neutrino mass scale Σ =X

i

m_i (2.12)

from cosmological measurements. Taking into consideration CMB measurements, model-dependent limits on structure formation can be given. A limit of Σ < 0.13 eV with a 95% confidence level [CNV16] can be determined.

2.3.1 Neutrinoless Double-Beta Decay

All data of the experiments presented so far established a finite but very small mass of neutrinos, which cannot be explained in the SM. A possible explanation could be given with the Majorana theory explained in the previous section. If one has a neutrino field ν_L, satisfying the Majorana conditions, one can allow for a mass term in the form of

L^M =−1

2ν_LM_L^M(ν_L)^c+h.c. (2.13) with M^M being a complex3×3 matrix and h.c. being the hermitian con- jugated. This mass term is called Majorana mass term [Bil10].

For neutrinoless double-beta decay (0νββ) to occur a non-vanishing Ma- jorana neutrino mass is necessary, independent of the mechanism. Double- beta decay can in general be observed, if an isotope with(A, Z), the number of nucleiA and the number of protonsZ, has less binding energy than the isotope(A, Z±1)but more than the isotope (A, Z±2). In this case single β^± decay is energetically forbidden while double β^± decay is allowed. The Majorana nature would allow for the exchange of a neutrino as illustrated in Figure 2.1. This process would yield two electrons, but no neutrinos in the final state. Hence the total energy freed in this process, the Q-value, could be detected. An observation of double-beta decay would not only prove the Majorana nature of the neutrino, but it would also yield a deeper insight into the neutrino mass [O⁺14].

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CHAPTER 2. PHYSICS INTRODUCTION Page 7

Figure 2.1: Feynman diagram illustrating a double-beta decay, on the left, and a neutrinoless double-beta decay, on the right [AEE08].

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Chapter 3 Low Background Experiments

As examples for low background experiments, this chapter briefly surveys GERDA, an already operational0νββ-decay experiment, and LEGEND, a next generation 0νββ-decay experiment [The17] [A⁺17].

3.1 Background Contributing Components

Rare event physics experiments are searching for processes which have a very low event rate. Therefore, it is mandatory to measure in an almost background-free environment. In general, there are several sources of background: cosmic radiation, contamination of radioactive isotopes in con- struction materials, and environmental radioactivity [The96].

In underground laboratories muons are the main contribution of cosmic radiation. They are produced in the atmosphere by primary cosmic rays and are very penetrating. While for most particles produced in hadronic cosmic ray showers a relatively shallow underground laboratory with about 15 meters of water equivalent is enough to shield them, it is necessary to go deep underground in order to reduce the muon flux by several orders of magnitude. Furthermore, muons can produce secondary radiation, such as neutrons, which also contributes to background [The96].

Placing an experiment in deep underground reduces the muon flux, but contamination of the surrounding material has still to be taken into account.

In most of earth’s building materials including stones and sand, there are primordial radioactive remnants of the nucleosynthesis the solar system was born from. The main contribution to the background are isotopes of Potassium: ⁴⁰K, Thorium: ²³²Th, and Uranium: ²³⁵U. ²³²Th and ²³⁵U have long half lives, compared to the age of the universe, and decay chains and the decay of some of their daughters emit gammas in a wide energy spectrum. Amongst the isotopes produced in these decay chains Radon is a particular problem for a low level radiation experiment. Radon is a noble gas and can hence not be chemically bound when produced. Especially

222Rn is troublesome. Because of its comparably long half-life of 3.82 days, it can dissipate and spread in the experiment, leading to constantly renewed contamination [The96].

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Using the GERmanium Detector Array (GERDA) experiment as an example, methods to reduce the background either by rate reduction or by other means are presented below. Furthermore, as a prospect for how to improve the background further in next generation experiments, some R&D for the Large Enriched Germanium Experiment for Neutrinoless double- beta Decay (LEGEND) is presented.

3.2 GERDA and LEGEND

The following section closely follows [The17]. The idea of GERDA and LEGEND is to operate high purity Germanium detectors enriched with the double beta decaying isotope⁷⁶Ge, in a radio-pure cryogenic environment shielded from radiation outside the experiment and equipped with veto detectors for radiation events in the experiment. The elegance in GERDA is that most infrastructural components serve two goals. They act as shield of the detectors from radiation and veto radiation events in the experiment at the same time. Furthermore, the detectors themselves also serve as decay volume in which theββ-decay occurs.

In order to shield the GERDA experiment from cosmic muons, it is located at the Laboratori Nazionali del Gran Sasso (LNGS), an underground laboratory in Italy. It has an overburden of 3500 meters of water equivalent. Overburden is the distance, through which cosmic ray particles must pass in order to penetrate into the laboratory multiplied by the density of the material; it is stated in meters of water equivalent. The overburden at LNGS removes the hadronic components of cosmic ray showers and reduces the muon flux to 1.2 muons per (m² h).

The detectors are placed in a cryostat surrounded by radiopure liquid Argon (LAr), cooling the Ge detector down to its operation temperature of less then 90 K. Additionally, 2 m of LAr shields the Ge detectors from external radiation and acts as a scintillator, i.e. it produces light when exposed to radiation. The cryostat itself is placed in a tank filled with ultrapure water that again acts as a shield from external radiation and as active material, thanks to the readout of Cherenkov light from muons passing through the experiment with PMTs. The muon veto is completed by three layers of plastic scintillator panels on top of the experiment. This experimental structure is illustrated in Figure 3.1 a).

The closer components are to the Ge detectors, the more effort needs to be devoted to selecting them to guarantee low levels of radioactive material. While the 66 photomultpliers (PMT) monitoring the water tank for Cherenkov light are well shielded, inside the cryostat for the LAr veto, low background PMTs have to be used in order to ensure low enough background. These photomulipliers are part of the light collection system used in GERDA, as shown in Figure 3.1 b).

The light collection system of the LAr veto is located around the Ge detector array, depicted in Figure 3.1 c), and collects the scintillation light produced by energy deposits in LAr. It is instrumented with low back-

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CHAPTER 3. LOW BACKGROUND EXPERIMENTS Page 11

Figure 3.1: a) Overview: (1) water tank with muon veto system PMTs (590m3, diameter 10 m), (2) LAr cryostat (64m3, diameter 4 m), (3) floor and roof of clean room, (4) lock, (5) glove box (6) plastic muon veto system;

b) LAr veto system: (1/5) bottom/top plate (diameter 49 cm) with 7/9 3" PMTs (R11065-10/20 MOD), low radioactivity of U and Th (<2 mBq/PMT), (2) fiber curtain (h 90 cm) coated with wavelength shifter, (3) optical couplers and SiPMs, (4) thin-walled (0.1mm) Cu cylinders (h 60 cm) covered with a Tyvek reflector on the inside (6) calibration source entering slot in top plate (7) slot for second of three calibration sources;

c) detector array: (1) Ge detectors arranged in 7 strings, (2) flexible bias and readout cables, (3) amplifiers;

d) detector module, view from bottom: (1) BEGe diode (2/3) signal/high voltage cables attached by (4) bronze clamps to (5) silicon support plate, (6) bond wire connections from diode to signal and high voltage cable, (7) Cu support rods.

Figure and caption cited from [The17].

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ground PMTs and silicon photomultipliers (SiPM). The light collection system is made of a curtain of wavelength shifting fibers surrounding the Ge detector array from the side. There are read outs at both ends with SiPMs. At the top and bottom of the light collection system, 16 three-inch PMTs collect the scintillation light independently. The distance of the Ge detectors to each PMT is at least 0.7 m and hence enough to limit the detectable background of the PMTs intrinsic Th and U contamination.

The Ge detectors are connected to custom-made low-radioactivity charge- sensitive pre-amplifiers. In order to guarantee enough shielding, the amplifiers are placed 35 cm above the detectors, as illustrated in Figure 3.1 c).

The detectors are placed on an ultrapure silicon plate and are surrounded by scintillating nylon cylinders. These are needed to limit contamination of the detector surface by the LAr volume with ⁴²K, which emerges in the decay of ⁴²Ar. The detector array is held in place by strings made of ultrapure Copper.

In LEGEND, one of the ideas to reduce the background even further is to replace the inactive materials of the detector array such as Si and Cu with an active structural material.

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Chapter 4 Interaction of Ionising Radiation with Matter

When considering radiation detection, the following types of radiation have to be distinguished [Bir64]:

1. Heavy charged particles: protons, heavy ions, and mesons 2. Leptons: mainly electrons or positrons and muons

3. Electromagnetic radiation 4. Neutrons

These radiation types are categorised into two groups which have funda- mentally different ways of interaction with a scintillation medium. Charged particle radiation can continuously interact through Coulomb force with the electrons and the nuclei of the medium. Because of this, the probability of detecting them is high. Electrically neutral particles are not influenced by the Coulomb force, thus are not hindered by the charge of the nucleus of the electron clouds and the detection probability is low as explained below.

4.1 Charged Particle Radiation

When considering charged particle radiation, the maximum transferred energy in matter per event for a particle of mass m with kinetic energy E_kin is given by Equation 4.1, withmi being the mass of the incident particle of the medium.

∆Ekin = 4m_i

mEkin (4.1)

4.1.1 Heavy Charged Particle Radiation

For heavy charged particles (HCPs) the following approximation holds:

∆E_kin ≈ 1 500E_kin 13

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HCPs need many interactions to transfer a considerable amount of energy to the medium. The mean energy loss dE, along a path with the unit lengthdr is given by the Bethe-Bloch Equation:

dE

dr = 4πe⁴z²

m_ev² N Z[ln2mev²

I −ln(1−β²)−β²] (4.2) wherez·eis the charge of the interacting particle –edenotes the electronic charge, z is an integer and indicates the number of charges – and v is the velocity of the particle with v = βc; c being the velocity of light. N and Z are the number of atoms and the atomic number and I is an empirical constant for a given homogeneous medium [Bir64] [Kno00].

At non-relativistic energies, Equation 4.2 is dominated by the overall 1/β² factor, leading to a decrease of dE/dr for increasing velocity. The minimum is atv ≈0.95cand particles at this point are known asminimum ionizing particles (MIPs). For particles with energies beyondEv≈0.95c,1/β² becomes almost constant anddE/dr increases again [Leo87].

4.1.2 Lepton Radiation

For low-energetic leptons, especially electrons, the energy loss in the medium has a lower rate than that of HCPs, but the energy loss per event is much higher leading to a even more tortuous path through the medium then for HCPs. Additionally to the Coulomb interactions, energy losses through electro magnetic (EM) radiation – mostly bremsstrahlung – are much more prominent and have to be taken into account, leading to a modified version of the energy depositions function [Kno00] [Bir64].

On the other hand high-energetic leptons, mostly cosmic muons, are commonly used as MIPs, which follow the energy deposition function de- scried by Equation 4.2. MIPs have an essentially straight path through the medium and only depositing a fraction of the their energy in the material [Leo87].

4.1.3 Energy Loss Distributions in Matter

One has to consider the actual energy deposition of charged particles in thin scintillators. Up until now all energy deposition functions treated the mean energy deposited in the material. Now we are interested in the amount of energy transfer per event. In a thick absorber it holds that N_i → ∞ the condition for the number of interactions is approximated well enough, which means that the Central Limit Theorem for the probability distribution of energy deposition holds. The energy deposition is distributed Gaus- sian with the most probable energy deposited being at the mean. This is not true in thin absorbers. In thin scintillators the probability of a single event energy deposition is much greater then in thick absorbers, because N_i is much lower. The so-called Landau distribution f(x,∆) describes the complicated probability distribution function for energy deposited in the

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CHAPTER 4. INTERACTION OF RADIATION WITH MATTER Page 15

material. It is given by:

f(x,∆) = Φ(λ)

ζ (4.3)

with ζ being the first order approximation of the mean energy loss given by Avogadros numberNa, the density of the mediumρ, and r_e² = (e²/mc)² with m being the mass of the particle:

ζ = 2πN_ar²_em_ec²ρZ A

z β

2

r (4.4)

The universal function Φ(λ) is given by:

Φ(λ) = 1 π

Z ∞ 0

exp(−uln|u| −uλ) sin(πu)du (4.5) being only dependent on λ, which is given by:

λ= 1

ζ[∆−ζ(ln|ζ| −ln||+ 1 +C)] (4.6) with ∆ being the energy loss, Euler’s Constant C = 0.577. . . and ln||

given by:

ln||= ln

(1−β²)I² 2mc²β²

+β² (4.7)

This distribution is valid if the ratio of the mean energy loss ∆¯ and the maximum energy transfer allowable in a single collisionWmax, given by:

κ=

∆¯

W_max, (4.8)

fulfils the condition ofκ≤0.01[Leo87].

4.2 Uncharged Particle Radiation

4.2.1 Gamma Ray Radiation

EM radiation has three major interactions: photoelectric absorption, Comp- ton scattering, and pair production. For low-energetic photons, the photoelectric effect is the dominant interaction mechanism. With increasing energy the Compton effect becomes dominant until at approximately 5 MeV pair productions is most probable.

The fraction of incident photons interacting with the material, which yields the detection probability, is described by:

f = 1−exp(−µd) (4.9)

f is the detection efficiency, µ the linear attenuation coefficient, d is the thickness of the scintillator. With a thin scintillator, in the order of some

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millimetres and the density of lead, and a gamma above 100 keV, the probability is about 1%. This decreases several orders of magnitude for higher energies and lower densities of the scintillator material. From this, we can see that large structures of scintillator material are necessary to detect gamma rays efficiently [Bir64] [The96].

Photo Electric Absorption

The photoelectric absorption is an interaction in which the photon completely disappears in an interaction with an atom in the medium. A photoelectron is ejected from the atom which absorbed the photon; the kinetic energy Ee of this photo electron is given by:

E_e =hν−E_b (4.10)

whereE_bdenotes the binding energy which has to be overcome,his Planck’s constant andν is the frequency of the electromagnetic wave corresponding to the radiation. In this interaction, the photon does not interact with a single electron, but with the atom as a whole.

A secondary electron can be ejected and interact with the medium as described before. Furthermore the ionised atom refills its shell from which the electron was ejected. The refilling process emits a characteristic X-ray which again interacts with the medium, involving less tightly bound shells, close to the original incident site [Kno00].

Compton Scattering

When the gamma is Compton scattered, the photon transfers some portion of the energy to the recoil electron and is scattered away with an angle θ from the beam path. The energy transferred is dependent on θ:

hν⁰ = hν 1 + _m^hν

ec²(1−cosθ) (4.11) withν being the frequency of the incoming photon and ν⁰ the frequency of the scattered photon. The more energy the photon transfers, the more it gets deflected. Furthermore, the interaction probability is linearly dependent on the atomic numberZ, as it gives the available electrons of an atom with which interaction is possible [Kno00].

The angular distribution of scattered gamma rays is given by the Klein- Nishina formula [Kno00] and indicating the strong tendency of forward scattering.

Pair Production

If the photon energyEγ fulfils:

E_γ ≥2m_e (4.12)

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CHAPTER 4. INTERACTION OF RADIATION WITH MATTER Page 17

pair production is possible. In this process, the gamma particle changes into an electron and a positron.

The kinetic energy E_kin of the decay products is given by:

E_kin =E_γ−2m_e (4.13)

Since there is a low probability of pair production until Eγ reaches several MeV this effect is confined to high-energetic gammas. Furthermore, pair production has a characteristic signal due to the positron annihilation in the medium [Kno00].

4.2.2 Neutron Radiation

Neutrons can reach the nucleus unhindered, as they are not electrically charged. If an interaction occurs, the neutron can get absorbed by the nucleus or alter its energy and direction. A nucleus can be in a highly excited state, after interacting with the neutron, leading to considerable secondary particle production. The detection efficiency is analogous to gamma radiation, with the necessity of dense or thick materials [Kno00]

[Bir64].

At last one also has to account for the response function of the detector, which can be approximated with a Gaussian. The two response functions in effect lead to a function called Langaussian. [Leo87]

By radiation, energy can be transferred to the scintillator and an electron gets excited from the ground state S₀₀ to any vibrational state S_1j⁰ , with j denoting a vibrational state ofS₁⁰. The electron can relax within the vibrational structure of S₁⁰ due to internal conversion. This energy can be transferred to neighbouring molecules and lost for any luminescence process. After relaxing within the S₁⁰ quasi-band the electron can only get to a lower state via luminescence. The actual scintillation process in this case would be fluorescence. Another relaxation that can happen is via phosphorescence. In this case the electron is transferred via inter-system crossing into the T₁⁰ quasi-band. It relaxes to the vibrational ground state T10 and performs phosphorescence afterwards. The number of electrons transferred by inter-system crossing depends on the ionisation density, as this can be different for interactions of different types of radiation. The ratio of the fast fluorescence pulse to the slow phosphorescence pulse can be used for deter- mination of the type of incident radiation. Gamma excitation should have a much shorter waveform, as gammas do not enhance transitions to theT₁⁰ quasi-band, whereas α particles are expected to give a much longer waveform, by enhancing the transition from S₁⁰ to T₁⁰ via inter-system crossing [Win86] [Ber71] [Ber71].

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Chapter 5 Generation of Scintillation Light

One of the most common methods to measure ionizing radiation is detection of scintillation light. Instead of detecting particles directly, we can observe the characteristics of the emitted light with a suitable detection system. Nowadays, a manifold of materials optimised for light emission exists. In the following, the underlying physics of these materials, in particular organic scintillators, are described.

5.1 Organic Scintillators

Organic scintillators are aromatic hydrocarbon compounds consisting of linked or condensed benzene ring structures. When exposed to ionising radiation they can exhibit luminescence, i.e. emit light. Luminescence of organic molecules is not only used for scintillator applications but has various other applications, e.g. dye lasers. The process of converting various types of radiation into detectable light is well understood [Leo87] [Win86].

The underlying fundamental principle of luminescence is explained in the following subsections. First, the electronic structure is described, which is necessary to understand the luminescence process explained afterwards.

Finally, the scintillation mechanism is explained.

5.1.1 Electronic Structure of Organic Molecules

In general, the luminescence of organic molecules is associated with compounds containing either benzene or triple-bonded carbon atoms in molecules [Bir64]. Since PEN contains two benzene rings, only benzene is considered in the following description.

Carbon (C) atoms define the structure of organic molecules. In benzene, it is in a trigonal or sp² hybridization state, which describes the mixing of the electronic states of carbon. One of the states of carbon, the p orbital, does not mix with another state. This is shown in Figure 5.1 on the left side by the dumbbell shapes. Three hybrid orbitals are produced by mixing one s and the remaining two p states. The bonds are known as σ-bonds, and they are in the same plane with an 120^◦ angle to each other, leading

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to the characteristic hexagonal structure of benzene, shown in Figure 5.1.

The unchangedpstate of the carbon is mirror-symmetric towards the plane

Figure 5.1: Molecular structure of benzene with the bonds displayed. The black lines between the Carbon atoms indicateσ-bonds. On the left the unchanged p states are displayed in yellow and blue dumbbell shapes. On the right these p states have formed a delocalisedπ orbital structure, indicated by the yellow and blue torus. [aut].

of the benzene and the associated orbital is known as aπ-orbitals. The six π-orbitals of the benzene Carbon atoms interact and form six π-molecular- orbitals which are completely delocalised, as illustrated in Figure 5.1 on the right side [Bir64].

In principle, the π-orbital structures are in singlet energy states, from now on denoted as S with S₀ being the ground state, and triplet energy states, denoted as T with T₁ being the lowest triplet state. Each electromagnetic energy state has vibrational and rotational substates. The energy ratio of the level splitting is about 400:20:1 for the electrical, vibrational, and rotational states. A certain electric state is indicated by the first index, e.g. S₀. The vibrational state is denoted by the second index number, eg.

S₀₀ orS₀₁, the energy levels are depicted in Figure 5.2.

For understanding energy levels the following model is helpful: (i)States are viewed as planes of energy and the nuclear coordinate. For each existing energy level there is a plane. (ii)An electron at an energy level can wander around on this plane freely. (iii) Vibrational and rotational energy levels have a much smaller energy splitting, the energy planes overlap more easily than electronic states. (iv) An overlap of energy planes allows the electron to transfer into another state, converting its energy into heat. Lower energetic states are in general more favourable as the electron has less energy and is more tightly bound. This process is called internal conversion. In complex systems such as organic molecules the overlap of energy planes is already high. Furthermore, in non-gaseous media broadening effects lead to an even greater overlap resulting in quasi-bands instead of strictly sep- arated states. In this setting, molecules excited to higher electronic states S_nm, with n ≥ 1 and n, m ∈ N, dissipate their excess energy in the form of phonons, until the electron reaches the S₁₀ state in Figure 5.2. The

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CHAPTER 5. GENERATION OF SCINTILLATION LIGHT Page 21

S₀₀ S₀₁ S₀₂ S₀₃ S₁₀ S₁₁ S₁₂ S₁₃ S₂₀ S₂₁

T₁₀ T₂₀ T₂₀

Singlet Triplet

Absorption Fluorescence Phosphorescence

Internal Conversion

Inter-system Crossing

Figure 5.2: Energy levels of an organic molecule withπ-electron structure. There are two possible configurations, singlet and triplet states. Electronic states are denoted by the first number of the index, the vibrational states are denoted with the second number. Possible transitions from one level to another are indicated with arrows. This figure was drawn following the description by [Bir64]

typical time constant is approximately 10⁻¹¹s. Above considerations lead to the following simplified model of the organic molecule structure: There are basically three quasi-bands made out of the electronic states, a ground quasi-bandS₀⁰ and two excited quasi-bandsS₁⁰ andT₁⁰, which is made of all vibrational energy levels to which excitation is possible. An electron can get excited from S₀⁰ to S₁⁰ by ionising radiation and can be transferred to T₁⁰ via inter-system crossing [Ber71][Win86][Kno00][Kau57].

Next, the evaluation of the distribution of vibrational states in an equilibrium is of interest. The distribution of vibrational states taking part in fluorescence is given by:

n₁ =n₀g₁ g₀exp

− ∆E k_BT

(5.1) The scintillator has a certain number of molecules in the vibrational states 0 and 1, denoted byn0 andn1. Furthermore, the two states have degeneracies g₀ and g₁ respectively. The energy difference between the two states is given by ∆E, and the energy of the system by the temperature T, and Boltzmann’s constant kB.

Equation 5.1 shows that at 300 K electrons which are able to participate in fluorescence are predominantly in the lowest vibrational state.

5.2 Luminescence

Luminescence is radiation emitted in a transition between the two lowest electronic states S₁⁰ or T₁⁰ and S₀⁰. Luminescence of a wavelength range

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between 300nm and 650nm is defined as fluorescence and is caused by the deexcitation from a singlet excited state to the singlet ground state.

Furthermore there is phosphorescence, the deexcitation of a triplet state T₁⁰ to the ground state S₀⁰. For scintillators there is also a division by the type of excitation. Photoluminescence is luminescence after excitation by electromagnetic radiation. Scintillation is luminescence following excitation by ionizing particles in general. The time-dependent emission of light by a scintillator, given by the intensityI, dependent on the time t, for one of the lifetime parameter τ, is given by [Ber71] [Win86]:

I(t) =I(0) exp

− t τ

(5.2) Emitted light has less energy than the one with which it was excited.

This effect is called Stokes shift. By considering the electronic structure the processes involved become clear.

The governing selection rules for possible transitions from S₁₀ or a longer-lived vibrational state of S₁ to S_0m are the Franck-Condan factors, with m denoting a vibrational state of S₀. The setting is demonstrated in Figure 5.3. Two electronic states are considered with their vibrational levels. For each vibrational state the wave function is illustrated. A transition from one electrical state to another via photons is determined by the wave functions of the vibrational states. This is because a transition’s probability is given by the overlap of the wave functions of the transfer levels.

Rapid transition occurs between states of the same multiplicity. In short, if the amplitudes of the wave functions of the transfer states are both at a maximum, it is much more likely that a transition occurs, than if there are at knots of the wave function [Kau57] [Ber71] [Bir64] [Win86].

The band structure effectively gives a four-level system, allowing for a continuous light output. The frequency spectrum is given by a Lorentz distribution. If enough thermal energy is present, the Lorentz distribution transforms to a Gaussian distribution [Win86].

In addition to fluorescence, there is another luminescence process: Phos- phorescence is a transition fromT₁₀toS₀⁰. This inter-combination radiation has a lifetime orders of magnitudes larger then fluorescence, because it is a quantum-mechanically suppressed process. As shown in Figure 5.2, phosphorescence only occurs when there are inter-system crossings. Once an electron has found its way into a triplet state, there are basically two types of phosphorescence:

(i) α-phosphorescence, or delayed fluorescence, in which the electron gains energy from phonons, lattice vibrations of the material, and is able to decay via fluorescence after an inter-system crossing from T₁⁰ to S₁₀. (ii) β-phosphorescence with a direct transition from T₁₀ toS_0m. In both cases, the electron stays in the excited state orders of magnitudes longer then in the fluorescence case [Ber71].

After considering the mechanism of luminescence, it is worthwhile to have a short look at the structure of the molecules again. It has been demonstrated that the peak wavelength of the fluorescence is bigger, if the

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CHAPTER 5. GENERATION OF SCINTILLATION LIGHT Page 23

Nuclear Coordinates

Energy

Energy Plane E₁

Energy Plane E₀ ν’ = 0

1

ν = 0 2

3 4 5

6

1 2

3 4

5 6

Figure 5.3: Franck-Condon diagram displaying the electric potential of a ground state E₀ and an excited stateE₁ and the levels and wave function of the vibrational states. The transitions, shown in the figure by the green and blue arrow, illustrate the overlap of the vibrational wave functions needed for a transition.

In contrast, the grey arrow shows an unprobable transition between two knots of vibrational wave functions [Sam14].

non-localized π-system of a particular class of compounds is made longer by substitution. This means a two-benzene ring structure emits at a longer wavelength range then a one-benzene structure. This can be validated by comparing the fluorescence of PEN and PET [Ber71].

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Chapter 6 Light Detection Methods

Light and its detection play a major role in many fields of science. Albeit the applications are numerous, the fundamental principle of detection is mostly the same: A photon flux is converted into an electric signal which can be digitised. In the following, three light detection systems which are frequently used in science, including for this thesis work, are described.

6.1 Working Principle of a Spectrometer

The dispersion of light is the cornerstone of the field of spectral analysis, first conducted by Kirchhoff and Bunsen. These pioneers developed an apparatus with which it is possible to distinguish different wavelengths of light emitted by a medium. Through a thin slit light enters into the apparatus. The light rays are made parallel by an optical element. The parallelised light beam meets another optical element splitting the light into beams of different wavelengths. The dispersed light can then be analysed.

In the following, the working principle of the spectrometer used for this thesis work and its tool for detecting spectra are described [EGH⁺78].

6.1.1 Spectrometer

The spectrometer used in this thesis was a Czerny-Turner spectrograph.

This configuration uses two concave mirrors for aligning the light and a grating plate for splitting up the light into its wavelengths. As illustrated in Figure 6.1, the first mirror aligns the incoming light into parallel beams.

The dispersive element is a plane grating which reflects light of different wavelengths by different angles. The range of wavelengths detected can be varied by rotating the plane grating angle to select the required diffracted images. At last the second mirror focuses light on the sensor, a 1024 channel CCD camera which is detecting the spectra [Tec].

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Figure 6.1: Scetch of a Czerny-Turner Spectrometer [Tec]

6.1.2 CCD Physics

Charge-coupled devices are silicon wafers divided into geometric light sensitive regions. These regions are metal oxide semiconductor (MOS) capaci- tors, operated as photodiodes. They are the fundamental light sensing unit, called pixels. The pixels are defined in the silicon matrix by an orthogonal grid of gates. The pixels capture image information as localised electrical charges, varying with light intensity. The dependency on charge and photons is linear because a single electron hole pair is produced per photon, but quantum efficiency is varying for different wavelengths. The produced single electron per photon is called photoelectron. The MOS capacitor wells have a maximum capacity referred to as full well capacity. The full well capacity determines the maximum signal detectable. The most important noise source is shot noise due to the Poissonian distribution of the number of photoelectrons coming from the the quantum nature of radiation [SFD17].

6.2 Working Principle of a PMT

The working mechanism is as follows: If a single photon hits the photocath- ode, an electron is emitted through photoelectric effect and accelerated to the first dynode of the electron multiplier section. This electron starts an avalanche or cascade of electrons produced in the dynode structure. Each electron hitting the dynode causes the emission of several electrons which are again accelerated to the next dynode. Cathode, dynodes and anode effectively constitute a potential ladder with the cathode as its start and the anode as the charge collector providing a detectable current. In order to obtain enough amplification, PMTs are operated with high voltage [Leo87].

PMTs are prone to an intrinsic count rate not induced by photons, the

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CHAPTER 6. LIGHT DETECTION METHODS Page 27

so called dark current and dark count [Leo87].

6.3 Working Principle of a SiPM

A silicon photomultiplier is a solid state photo detector. In response to a photon electron-hole pairs are created. Accelerated by the bias voltage, these create an avalanche that produces a current pulse which is several nanoseconds long and has a gain of approximately 10⁵ to 10⁶. SiPMs are divided into pixels. Each pixel, or microcell, is an avalanche photodiode (APD) connected in series with a quenching resistorR_Q, which can be seen in the circuit diagram in Figure 6.2 on the right side. All microcells (pixels) are connected in parallel. Effectively, this gives an anode and a cathode for the SiPM and the output signal is given by the sum of all pixels.

Figure 6.2: Structure of a silicon photomultiplier (SiPM) [Pia17].

When a pixel fires, the resulting current causes a voltage drop overRQ. If R_Q is chosen properly, the voltage drops below the breakdown voltage leading to a stop of the charge avalanche.

In an experiment a voltage drop is being measured. It is given by the number of electrons N released in a pixel, the elementary charge e, and the time ∆t, giving Equation 6.1:

U = N eR

∆t (6.1)

Since N, ∆t and R are constants of the system, the voltage signal for a single pixel is always the same.

The gain g is dependent on the capacity C_J of the APD’s avalanche region, the breakdown voltage of the SiPMV_B, and the supply voltageV_S, and given by:

g = (V_B−V_S)C_J

e (6.2)

Since V_B is known to be temperature-dependent, this is also true for the gain. Fortunately, following Equation 6.2, the gain can be kept constant by adjusting V_S such that the overvoltage is constant: (V_B−V_S) = const [Pia16].

The quantum efficiency (QE) of the SiPM is effectively temperature- dependent as it also depends on the overvoltage. The quantum efficiency

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gives the probability that an incident photon can produce a charge carrier capable of triggering a Geiger discharge. The likelihood of a Geiger discharge is dependent on three factors: i) the strength of the electric field,ii) the avalanche regions width, andiii) collisional cross section of charge car- riers with phonons and the specific history of these interactions. The first two factors depend on the(V_B−V_S). That means by adjustingV_S to keep the gain fixed, one also fixes the QE of the SiPM, which is an important property used in Chapter 9.

Noise sources in the SiPM are dark current and shot noise as described in the previous sections [Pia17].

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Chapter 7 Light Spectrum Measurements

In the development of custom-made PEN samples it is essential to evaluate how the moulding process influences the scintillation properties of the material. An important property of a scintillator is the emitted light spectrum when hit by ionising radiation: In particular, the peak wavelength and the integrated value of the spectrum within the wavelength range to which the used detectors respond. The integrated value will from now on be denoted as light output. First the customisation process of PEN is descriped. Af- ter that, a description of the setup is given. Finally the results and the corresponding analysis and method are presented.

7.1 Moulding Parameters to be Investigated

Because it is planned to use custom-made PEN as a scintillator material, understanding the production process of arbitrary shapes via injection moulding is essential. The production process has a great influence on the crystallinity, and crystallinity effects the light output, because the less amorph a polymer is the less transparent it gets, as the light passing through is scattered more. For scintillator purposes the transparency de- fines how much of the produced light is emitted from the material. At the Fraunhofer-Institut für Chemische Technologie (ICT) scintillator tiles were produced with injection moulding with varying moulding parameters. The parameters considered are:

• the humidity of the granulate

• the injection speed

• the tool temperature

• the in-tool cooling time of the tile

• the cooling time with and without water

For the injection speed, the effects of different tool temperatures and the humidities were also studied. There are 23 additional parameters for the

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injection moulding process. Most of them are dependent on the moulding parameters stated above and are not considered further, as the goal was to obtain a better understanding of the overall parameter space, especially which of the above parameters influence the light output and in which way they do so. A similar study was done at the Technische Universität Dort- mund (TUD), with a screening plan to investigate the following moulding parameters:

1. Melt temperature 2. Mould temperature 3. Injection speed 4. Packing pressure 5. Packing time 6. Cooling time

The parameter investigation supervised by TUD included 12 samples, presented in Table 7.1, which were tested for transparency. The most transparent samples were then compared to the ones produced at ICT during this thesis.

Table 7.1: Parameters as stated in the text for the parameter investigation at TUD.

Parameter #: 1 in ^◦C 2 in^◦C 3 in cm³/s 4 in bar 5 in s 6 in s

Experiment: 1 280 30 10 1000 2 20

Experiment: 2 280 30 10 1000 2 40

Experiment: 3 280 30 50 1600 7 20

Experiment: 4 280 80 10 1600 7 20

Experiment: 5 280 80 50 1000 7 40

Experiment: 6 280 80 50 1600 2 40

Experiment: 7 310 30 50 1600 2 20

Experiment: 8 310 30 50 1000 7 40

Experiment: 9 310 30 10 1600 7 40

Experiment: 10 310 80 50 1000 2 20

Experiment: 11 310 80 10 1600 2 40

Experiment: 12 310 80 10 1000 7 20

In order to understand the influences on the light output the tiles where then measured in a spectrometer setup, one measurement for each of the 30 mm×30 mm surfaces, in order to understand possible influences of the tool used for the production. Results of the study of the ICT tiles are presented and compared to tiles produced at the Technische Universität Dortmund (TUD) in Section 7.7.2.

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CHAPTER 7. LIGHT SPECTRUM MEASUREMENTS Page 31

7.2 Experimental Idea and Boundary Criteria

The idea of the setup, for measuring the light spectrum of PEN, is sketched in Figure 7.1. It is to place a scintillator tile in front of a photo-spectrometer and to excite this tile with a ionising source in an environment of low light background.

Dark Box

Photo-Spectrometer Entrance

Slit Scintillator

Tile

Excitation Source

Figure 7.1: Spectrometer setup design sketch

The design of the setup needs to meet the following boundary conditions:

• To be fit for the 30 mm×30 mm×3 mm standard tiles from ICT and TUD.

• A well understood, stable, and safe-to-use ionising source.

• A photo-spectrometer which is optimised for wavelengths of 400 nm to 600 nm.

• Optimised arrangement for maximum light collection and minimal optical defects

• Temperature stability.

• Reproducibility of below 0.5 % uncertainty.

7.3 Experiment Setup

The scintillation light is measured by an Czerny-Turner spectrograph (An- dor Shamrock 303i) equipped with a CCD-camera (Andor DV420A OE) and a grating plate which allows to measure a wavelength interval of 405

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nm to 542 nm with a resolution of 0.135 nm. The light enters the photo- spectrometer via a slit that is adjustable in width from 10 µm to 2.5 mm and height of 40 mm.

The CCD is cooled down to−60^◦C in order to reduce the thermal noise of the sensor and is capable of detecting wavelengths in the range of 200 nm to 1050 nm.

Both the spectrometer and the camera are connected to a computer via USB; the computer is used to control the two devices and to record data.

The samples can be mounted on a two-dimensional mount adjustable in height and distance with respect to the spectrometer with a precision of 0.1 mm. The positioning of mount and tile is shown in Figure 7.2. The distance between the entrance slit of the spectrometer and the sample can be between 6 mm and 30 mm.

Spectrometer ^CCD-Cam

Scintillator Tile Entrance Slit

2D Mount UV Lamp Fiber

UV Lamp

PM POE Switch

RS232 Lakeshore PT 100

Light Power Sensor

Figure 7.2: Spectrometer setup, with all relevant technical components. The USB connections are indicated by blue lines. Furthermore, the Ethernet connections, in green, are used to power devices and control them. The UV light is coupled in a splitting fibre (the blue line in the picture). The beam lines of the UV are indicated in pink, one of them is measured by a power meter, denoted as PM. Red circles illustrate PT100 sensors, measuring the temperature. They are connected to the readout, indicated as red lines.

A fibre-coupled Ultra Violet (UV) light source (Ocean Optics LLS-255 LED) can be used to excite the scintillating tiles, with a centre wavelength of 255 nm and a full width half maximum (FWHM) of 10 nm.

The UV light is splitted evenly using a splitting fibre, in order to use one half of the light to excite the scintillation tile and the other half to monitor the stability of light output by a photo sensor (Newport 918D-UV-OD3R), read out by a power meter (Newport 1919R). The optical fibre can be positioned with a precision of 0.1 mm

The temperature of the setup is monitored with up to 4 PT100 sensors, each connected with a four-core cable to a Lakeshore readout device.