LSO-BASED LIGHT YIELD AND TIMING MEASUREMENTS

Every measurement involved the data acquisition of 2·10⁵ coincidence events which were post-processed with Matlab. Assuming perfect linearity of the PMT, the amount of detected scintillation photons N_Det can be considered to be proportional to the integral of the PMT voltage U_PMT over the entire pulse,

N_Det =C

 τ 0

U_PMT(t) dt , (4.32)

with C being a constant depending on the PMT gain and the signal the amplification. The integration limit τ was set to 300 ns. The correlation between energy and detected photons was calibrated through locating the abscissaN_Det,PPcorresponding to the 511 keV photopeak in the histogram of all N_Det-values. Afterwards, the energyE_γ(i) of thei-th registered event could be derived from the corresponding amount of detected photons N_Det(i) through

E_γ(i) = N_Det(i)

N_Det,PP·511keV. (4.33)

For the evaluation of the CRT, the signal of the last dynode stage of each PMT was used.

After selecting only events with 430 keV< Eγ <650 keV, walk correction was applied through

Optical grease PMT entrance window

Optical grease PMT entrance window

Glass chip Glass chip

Plain SiPM Cr grid on glass chip mimicking SiPM

Microcell Inactive

area PMT

entrance window

Cr grid sample on PMT

Epoxy Glass chip with

Cr grid Glass chip

Si-Concentrator SiPM equipped

with LC LC on on glass chip LC sample on PMT

a b c

d e f

Figure 4.23: Illustration of replacing SiPMs with PMTs to evaluate the impact of LCs.

Instead of using an actual plain SiPM (a), a glass-chip coated with a Cr grid is coupled to the entrance window of a PMT to mimic the geometric fill factor of the SiPM (b). The experimental realization of this uses the Cr grid sample consisting of two glass chips bonded via epoxy (c) described in Sec. 4.2.1.4. Similarly, the experiments involving an SiPM equipped with an LC (d) are replaced through the LC placed on a PMT (e) using the samples shown in (f).

CHAPTER 4 MATERIALS AND METHODS

PMT LSO Teflon

Optical grease Black tape

Encapsulated sample PMT window 511 keV

photon

Test PMT Reference setup

68Ge source

68Ge source

a b

Figure 4.24: a) Drawing of setup involving an LSO crystal wrapped in Teflon, an LC sample encapsulated between two glass chips, and a PMT. The components are optically coupled using grease. b) Photograph of the coincident measurement setup consisting of a reference system at the top, a ⁶⁸Ge gamma source in the center, and the test setup from (a) on the bottom. Note that no sample or LSO crystal is mounted on the test tube in this photograph and the distance between the source and bottom PMT is larger than during the actual experiments. The color impression of the photograph is due to the yellow light used in the laboratory.

multiplication of the signal with 511 kev / Eγ. Next, a time stamp was generated through the application of an LED, which used a threshold voltage of 10 mV for the reference PMT R9779. For the test PMT, i.e. the XP20D0, the LED threshold was varied from 2 mV to 30 mV to obtain timing curves CRT(U_LED) and determine the minimum possible CRT. The CRT of the entire systems results from the contributions of the two PMT setups according to

CRT = ^δt²_Test+δt²_Ref, (4.34) wherein δt_Test is the timing jitter of the test setup consisting of XP20D0 tube with LC or Cr grid sample. The second contribution δt_Ref of the reference system was determined as 140 ps in previous experiments. This allowed calculating δt_Test from the measured CRT to study the impact of the LC and Cr grid samples on the timing. To capture the pulse shape of the XP20D0 PMT for different samples and LSO crystals, the anode signal of the tube was recorded with the digitizer for another 80·10³ coincidence events.

Each measurement consisted of i) positioning the sample and LSO crystal on the PMT; ii) acquisition of the data for timing and pulse shape; iii) detaching of sample and LSO crystal from the setup; iv) removal of the optical grease and cleaning of all interfaces to prepare 82

4.4 LSO-BASED LIGHT YIELD AND TIMING MEASUREMENTS

for the next measurement. This procedure was repeated five times for each sample/LSO combination. The resulting pulse shapes and timing curves were averaged and the standard deviation of the five datasets was used as the error of the measurement.

The difference in light yield obtained for the LC and the Cr grid sample N_Det^LC/N_Det^Cr was compared to its simulated value. To this end, the test setup was reproduced within the optical simulation framework including models of the LC and Cr grid samples based on microscopy images. Computing the CRT based on these optical simulations was not feasible, as the simulation tool simulating the timing resolution was not applicable for this PMT-based detector configuration.

4.4.2 Incompatibility with Photonic Crystal Samples

Using this experimental setup for measuring the effect of PhCs was not possible since it was beyond the scope of this work to fabricate PhC films on actual scintillator surfaces. Also, the PhC gratings realized on glass chips could not be integrated into the measurement setup explained above, as there was no suitable coupling agent available that had an RI in the range of nLSO. Therefore, the studies of PhCs were limited to the validation of the transmission characteristics and the MC simulations.

Chapter 5 Results

This chapter first presents the results of the reference PET detector simulations. Next, the works regarding LCs are summarized, including their optical properties derived from MC simulations, the fabricated samples with their measured transmission characteristics, a MC study regarding the impact of LCs on the light yield and timing of the reference detector, as well as the LSO-based measurements. After that, this chapter presents the results obtained with PhCs. These efforts comprise the implementation of PhCs into the optical MC simulations, a discussion of their optical properties, the characterization of EBL-fabricated samples regarding their transmission characteristics, and the evaluation of a PhC-enhanced PET detector. Further, the concepts of LC and PhCs are combined and their impact on the PET detector performance is studied. Finally, the results of direct nano imprinting of PhC samples are presented, their transmission properties are evaluated, and their implementation in the PET detector module are discussed.