N11 / PHYS820 Radiation Shielding 2023 Practical 1
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N11 / PHYS820
Radiation Shielding 2023
Practical 1
PRACTICAL 1:
An introduction to Neutron detectors
Record below information on the detector and its performance.
A. He-3 detector
Model/serial number |
Dimensions |
Gas pressure |
Bias voltage |
Amplifier shaping time |
N/A |
1m x 2inch |
2 atm |
1.2kV |
12µsec |
Sketch or paste a copy of the energy spectrum provided (He-3.csv) into the box below. Ensure you have calibrated the energy on the x-axis correctly.
Spectrum:
Digital Rise-time = 12µsec Flat top = 0.8µsec
Comment on the clarity of the structure in the spectrum, for example, how clearly are wall effects visible? What is the origin of these wall effects?
Moderator, moderator thickness and cadmium screening
This practical is done with the Am/Be neutron source in a water tank with a cadmium screen between the source and the detector to maximise the fast neutron component.
A fixed source-to-detector distance is used, and the count rate in the detector is recorded as moderating material is placed first, between the end of the tank and the detectors, then on the far side of the detector.
A An unshielded detector [Polythene moderator]
A.1 Moderator between source and detector (Cadmium between tank and moderator).
The detector is most sensitive to thermal neutrons, but will have some sensitivity to epi- thermal neutrons.
Use your notes to sketch the setup used to collect the data showing relevant dimensions:
Detector used |
Moderator |
He-3 |
Polythene |
Use the experimental data provided to extract the neutron count rate versus moderator thickness. The “P1aMod” files are the ones required here. You can use either the Excel or Prospect format files to do this. The moderator slabs are each 2cm thick.
Determine the total count rate with (i) no moderator (ii) moderator slabs added one by one
Moderator thickness (cm) |
Count time |
Count ± error |
Count rate ± error |
Ratio relative to zero moderator ± error |
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Plot count-rate ratio against moderator thickness below (or attach another plot e.g. Excel)
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Explain the result:
A.2 Moderator on thefar side of the detector
Use the provided experimental data to extract the neutron count rate versus moderator thickness. The “P1bMod” files are the ones required here. You can use either the Excel or
Prospect format files to do this. The moderator slabs are each 2cm thick.
Sketch the setup showing dimensions:
Determine the total count rate with (i) no moderator (ii) moderator slabs added one by one
Moderator thickness (cm) |
Count time |
Count ± error |
Count rate ± error |
Ratio relative to zero moderator ± error |
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Plot count-rate ratio against moderator thickness below (or attach another plot e.g. Excel)
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Explain the result:
B An unshielded detector [Borated Polythene]
B.1 Borated Polythene between source and detector (Cadmium between tank and moderator).
Use the provided experimental data to extract the neutron count rate versus moderator thickness. The “P1cMod” files are the ones required here. You can use either the Excel or
Prospect format files to do this. The borated moderator slabs are each 1cm thick. Sketch the setup showing dimensions:
Determine the total count rate with (i) no moderator (ii) moderator slabs added one by one
Moderator thickness (cm) |
Count time |
Count ± error |
Count rate ± error |
Ratio relative to zero moderator ± error |
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Plot count-rate ratio against moderator thickness below (or attach another plot e.g.Excel)