Lab Nuclear and Particle Physics 1 Lab Nuclear and Particle Physics 1
(PHY221) (PHY221)
Fall 2020 Fall 2020
Olaf Steinkamp,
Stefanos Leontsinis, Dario de Simone
Formalities
Lab course is a separate module: PHY221
→ compulsory for Physics major 180 & 150
→ enrol by Friday this week if you haven’t done so yet You’ll do one experiment and write one report
→ one week Monday 9h-18h + Thursday 13h-18h Fill in your availability by Friday this week at https://www.doodle.com/poll/mez2n3gbt3zgx3ya
→ mark as many slots as possible, we’ll do our best to fulfill all wishes You’ll work in teams of two,
→ if you know with whom you would like to work together, fill the doodle once with both your names
The Team
Olaf Steinkamp olafs@physik.uzh.ch
36-J-05 Dario de Simone
dadesi@physik.uzh.ch 36-J-94
Stefanos Leontsinis steleo@physik.uzh.ch
36-J-86
Covid 19
//www.uzh.ch/cmsssl/en/about/coronavirus/safetyconcept.html
Covid 19
Typically 2 students + 1 assistent per room (rooms have windows and will be aired) We will all have to wear masks throughout
(masks and disinfectants will be provided by us) We won’t have to wear gloves,
except when handling radioactive sources Stay at home if you have symptoms or feel sick.
Inform us (email to olafs@physik.uzh.ch) as early as possible, we’ll look for a replacement date (e.g. swap with another team).
If you have any questions or concerns, if you belong to a high-risk group:
Experiment: Positronium Lifetime
β+ decay:
a proton inside the nucleus decays through weak interaction
u d
u d d
e+ νe W+
u
The 22Ne nucleus is produced in an excited state
→ goes to the ground state by emitting a 1275 keV photon
Experiment: Positronium Lifetime
Encase the sodium source in aluminium:
→ the emitted positron (e+) annihilates almost instantaneously with an electron (e–) in the material
→ two 511 keV photons are emitted
Experiment: Positronium Lifetime
Measure the time difference between
→ the 1275 keV photon from the decay chain and
→ one of the 511 keV photons from the e+e– annihilation, Expect to see a sharp peak
→ the width of the peak is determined by physics processes and by the time resolution of our measurement setup
Experiment: Positronium Lifetime
If the source is encased in the right type of material (we use POM, Polyoxymethylene)
the e+ can form a bound state with an e– , called positronium
Two possible spin orientations of the e+ and the e–:
↑↑ = “ortho-positronium”
↑↓ = “para-positronium”
Experiment: Positronium Lifetime
The two positronium states have finite lifetimes In vacuum:
τ↑↓ = 1.25 × 10-10 sec τ↑↑ = 1.42 × 10-7 sec
(reason for these very different lifetimes: exercise session) In material:
→ collisions with atoms in the material
→ ortho-positronium (↑↑) can change into para-positronium (↑↓) → τ↑↑ much shorter than in vacuum
(value depends on material)
Experiment: Positronium Lifetime
Measure the time difference between
→ the 1275 keV photon from the decay chain and
→ one of the 511 keV photons from the e+e– annihilation, Extract τ↑↓ and τ↑↑ from a fit to the data
Setup
Scintillator + Photomultiplier
Scintillator + Photomultiplier
Source
Photon detection
Step 1: Scintillator
Photon interacts with detector material:
at ~ 1 MeV mostly Compton scattering Excited atoms fall back into ground state
by emitting light in the blue-UV range This light can be detected if the detector material is transparent Various materials, we use BaF2
Amount of light is proportional to energy deposited by the γ → energy measurement
Signals are fast (ns)
Photon detection
Step 2: Photomultiplier
Photon (blue-UV) releases electrons from photo-cathode by photo-electric effect
Electrons accelerated in electric field, release secondary electrons in dynodes
Typically 10-12 stages, HV ≈ 1-2 kV, amplification factor ≈ 106
Energy Spectrum
1275 keV 511 keV
“Compton edge”
“noise”
Energy Spectrum
Detector 2 Detector 1
Signal processing
Set up a logic to
→ select events with two photons in the correct energy range
→ measure the time difference between the two photon signals
→ digitize the time information and fill a histogram ( also: time calibration measurements )
We’ll use electronics modules like these Each module does one thing, e.g.
→ amplify/shape (analog in, analog out)
→ discriminate (analog in, logical out)
→ coincidence (logical in, logical out) See full list on last slides ...
Signal processing
Set up a logic to
→ select events with two photons in the correct energy range
→ measure the time difference between the two photon signals
→ digitize the time information, fill a histogram ( also: time calibration measurements )
We’ll combine these modules to assemble our setup.
At each step, we will look at the (analog or logic) signals on the scope to understand
exactly what is going on.
Signal processing
Set up a logic to
→ select events with two photons in the correct energy range
→ measure the time difference between the two photon signals
→ digitize the time information, fill a histogram ( also: time calibration measurements )
We’ll combine these modules to assemble our setup.
At each step, we will look at the (analog or logic) signals on the scope to understand
exactly what is going on.
For illustration only
is is actually from another experiment)
Experiment: Angular Correlation
β– decay:
neutron inside the nucleus decays through weak interaction
u d
u d u
e− νe W−
d
The 60Ni nucleus is produced in an excited state
→ goes into ground state via a cascade
emitting two photons (1172 keV and 1333 keV)
Experiment: Angular Correlation
By measuring the angular distribution between the two photons, we can determine the quantum numbers of the excited states
Use the same type of electronics modules,
similar measurement programme as in positronium experiment
Report
(1) Short physics motivation (2) Description of the setup
→sketch of the logic
(with short description of the modules you used)
→ list of important settings
(e.g. high voltage, discriminator thresholds, signal delays)
→ oscilloscope screen shots
(3) List of data files with description of what they contain (4) Histograms of the data you collected
(5) Analysis of the data
→ fit functions, fit framework (ML/LS?)
→ results WITH UNCERTAINTIES!
Report
One team = one report
Deadline for submission: 2 weeks after the experiment (e.g. experiment on October 5 and 8 → deadline October 23)
Any questions, any problems → contact us (contact details: slide 3)
That’s it ...
Enrol if you haven’t done so yet
Fill in your availability by Friday this week :
https://www.doodle.com/poll/mez2n3gbt3zgx3ya
The doodle shows the Monday of the week, you do the experiment on Monday
AND on Thursday afternoon of the same week!
Acknowledgement
Most of the plots and pictures in these slides are from lab reports
Many thanks to the students !
Electronics Modules
Amplifier
→ amplify and shape the output signals from the PMT’s Discriminator
→ generate logical (YES/NO) signal if the amplitude of the analog signal is
above (or below) a chosen threshold Coincidence units
→ generate a logical output signal
if two (or more) logical input signals at the same time
Electronics Modules
Time-to-Analog Converter
→ generate an analog signal with an amplitude that is proportional to the time difference between
to logical input signals
Analog-to-Digital Converter
→ convert an analog input signal to a digital signal Data acquisition
→ transmit the digital information to a computer and store it there
(in our case, we just store a histogram)
Electronics Modules
“Fan out”
→ to produce two identical copies of a logical signal Level adaptors
→ to change between different logical signals Delay units
→ to adjust timing between signals,
compensate for different propagation times
