Chapter 1- Introduction

Link it to medical physics – neutron in cancer center and neutron production in LINAC linear accelerator

1.2 Aim of the thesis – provide reference for gamma neutron discrimination at low energy level
We will use time of flight technique because pulse shape technique doesn’t work with low energy – our focus about fast neutron

Neutron field always contaminated with gamma ray. So, neutron detector should be either insensitive to gamma or have capabilities to discriminate between neutron and gamma

In this way pulse shape methods working with organic scintillator (liquid- plastic) are the standard choice. These detectors offer the pulse shape discrimination property and there has been research on pulse shape discrimination method. However, pulse shape discrimination fails at low energy range

So, the aim of this project is to produce a reference method for neutron gamma discrimination at low energy and by using simply available Am/Be Americium-Beryllium neutron source the method is based on the time of flight measurements between tow organic detectors.

1.3 Thesis Structure
Steps what we have done

Chapter 2 – radiation interaction with matter
2.3 Interaction of Radiation with Matter
2.3.1 Interaction of gamma with Matter
2.3.2 Interaction of neutron with Matter.
2.4 Neutron Detection – our focus in this experiment about fast neutron
2.4.1Fast neutron and fast neutron detectors
2.4.2 slow neutron and slow neutron detectors.

Chapter 3 – Neutron Gamma Discrimination- why we need to discriminate?
3.1 technique for neutron measurements- briefly describe both and the difference between them
3.1.1 pulse shape
3.1.2 time of flight – general information
* why we need to measure the time of flight? Mention example of the application
Time of flight used for measurements of neutron energy also it has been used for neutron gamma discrimination but in accelerator because in accelerator much easier
The highlight of my measurement (this project) using Am/Be Americium-Beryllium neutron source for time of flight measurements

Time of flight for n/g discrimination – new section added
Discus the theoretical background behind the simple Equations – the basis of my measurements
For photons (Gamma ray):
Distance=Speed × Time (Eq. 1)
D= V × T
T= D/V
Where:
D: distance
V: speed of light
T: time

For Neutron
E=1/2 MV^2 (Eq. 2)
V= √(2 E/M)

Where:
M: Neutron mass
V: speed of light

Discussion and calculation:
for example, for 1 MV OR 2 MV neutron this is the time
And for gamma ray this is the time
So, if we measure the time we should be able to discriminate between neutron and gamma rays
Then Talk about time difference for gamma and neutron

Chapter 4 – Experimental Setup and Measurements
4.1 Radioactive sources
4.1.1 neutron sources – Am/Be Americium-Beryllium source placed in a water tank- talk about the source in general- there is two source we used in this experiment – higher and lower activity

higher activity = 18 MBq
lower activity = 11 MBq

useful note from our lab script:

This figure from the internet – similar to the one we work with in our experiment.
Note: in some parts of the experiment we joined the two sources together – the lower source on the top of the higher source – reason why: to save time and get more events for better results

4.1.2 gamma sources – source used for calibration – Cs 137 and Na 22

4.2 Detector – we used two detectors in this experiment briefly explain each
4.2.1 liquid – we chose it to be the start detector in our experiment
4.2.2 plastic – stop detector

4.3 Electronics – Briefly talk about them:
High-voltage power supply (HVPS)
Amplifier
Oscilloscope
Multichannel analyzer (MCA)
Constant Fraction Differential Discriminator (CF DIFF)
Nanosecond Delay
4.4 Detectors characterization
To check if the detectors we will use for this experiment are working properly before we start: we set the voltage – gain – shaping time for both detectors as in the table – best setting we observed
Liquid Plastic
Voltage + 1300 – 1300
Gain 20 20
Shaping time 0.5 0.5

We calibrate both detectors with gamma sources – see fig 2
We calibrate both detectors with neutron source

With gamma: we get spectrum energy for both detectors – Talk about the shape of the spectrum we expect to get for both
for example: for gamma source – no photoelectric events (no photon peak) only Compton edge due to the atomic number and density
Include block diagram of the basic setup
see spectrum run 9-10-11-12

Fig 2: with gamma source

with neutron source: we take two spectrums one with lead shield under the detector and other spectrum without lead shield – see fig 3
reason why – without lead shield the spectrum will include both neutron and gamma – once we put the lead shield gamma ray will be blocked.
See difference between both spectrum – run 13 -14 for the liquid detector – run 15 for plastic

Fig 3 : with lead shield

4.5 Measurements
Setup for time measurements:
We checked the time measurements for both detectors with gamma source – time resolution concept
Align both detector at the same level – for more efficiency
Both detector used with the gamma source closer to one of the detector (we chose the liquid detector as start) for higher efficiency – because when we put the source close to the plastic detector we couldn’t see a clear peak
We check the signal with oscilloscope
We start with different distance between both detector from smaller distance to higher distance
See fig 4 for the setup
See block diagram of the basic setup

Fig 4 : right 100 cm – left 10 cm

Block diagram for the basic experimental setup used to collect the data

Then for time calibration we changed the delay time- run 27- see the spectrum – we should see clear peak moving position – three peaks
Peak Channel Delay
1st 62.38 7 ns
2nd 129.64 7 + 16 ns
3rd 198.68 7 + 32 ns

Time difference between the fist channel and the second is 16 nanoseconds and the difference in the peaks channel is 67.26. so, 1 channel = time/ difference in channel
1 channel = 16 ns / 67.26 = 0.238 ns (time calibration)

Then we do calculation and compare it with measurements – to make sure we did the right thing – we obtained good results the number very close
Measurements:
Run Peak channel Position – cm
1st 226.96 5
2nd 226.90 10
3rd 229.01 20

For example: the difference in channel between the fist run and the third is 2.04 channel
We convert it to time: 2.04 × 0.238 = 0.476 ns

Calculations:
the difference in distance between the first run and the third is 15 cm, then we used eq. 1 to calculate the time
T= 0.15 m3 × 〖10 〗^(8 ) m/s = to convert it to nanosecond we multiply by 〖10 〗^(9 )
Time = 0.5 ns

Next step we start the measurement over the neutron tank
We put the detector on the top of the neutron tank with known distance between them – liquid as start detector and plastic as stop
We used plastic tube (air pipe) over neutron source like the figure above – This figure from the internet – similar to the one we work with in our experiment.
We used gamma source as reference – we put the source close to the liquid detector – to check the place of gamma events in the spectrum – so we know that the events come after is neutron – see run 28
We calculate the time for example, for 1 MV OR 10 MV neutron by using eq.2 to find the velocity (Speed) and then we used eq.1 to calculate the time
E=1/2 MV^2 (Eq. 2)
V= √(2 E/M)

Where:
1 MeV = 1.6022 ×10-13 J
mass of Neutron = 1.675×10-27 kg
1 MeV neutron has a speed of 1.383 x 107 m/s

10 MeV = 1.602 ×10-12 J
mass of Neutron = 1.675×10-27 kg
10 MeV neutron has a speed of 43,739,338 m/s

To calculate the time T= D/V (Eq. 1)

Chapter 5 – Results and Discussion
After we know the gamma events location in the spectrum we run the spectrum again
Starting with 5 cm between detectors and we put lead shield under the plastic detector to shield gamma and reduce chance coincidence
Explain briefly chance coincidence: means that gamma ray may reach both detectors at the same time – that’s why we used lead shield so gamma ray will reach only the start detector (liquid) – result run 32 – peaks overlapped – we should increase the distance to increase the flight path. – See fig 5

Fig 5

Then we increased the distance to 30 cm and we used both neutron sources on the top of each other – result run 33- fig – the spectrum was good but we can get better

Other way to get better result:
Repeat the same procedure as before (run 33) but increasing distance to 40 cm extra shielding – result run 34
For comparison: Repeat the run with same distance (40 cm) but with extra shielding under the start detector to block gamma- result run 35

Comparison between run 34 and 35 – there is massively decrese in gamma events and little decrease in neutron events due to lead shield under the start detector – good result but we can improve

Next step for improvement:
We increased the distance to 50 cm but only we used the higher source we couldn’t use both sources – lower source was in use by someone else – we used gamma source as reference run 36
then repeat the run with extra shield for the area between both detector also on the top of the detectors to block gamma scatter– result run 37 – see fig

Still need improvement:
we managed to get the tow sources together again – both higher and lower
we increased the distance to 65 cm
used gamma as reference – result run 38
then repeat the run with extra shield and we used lead shield as collimator for both detectors – result run 39 -the neutron and gamma peaks were separated and gamma counts were reduced- see the figure

we can compare between run 37 and run 39 – in excel graph – to explain the difference when we increased the flight path from 50 cm to 65 cm we get better results.
Also, comparison between run 32 – distance 5 cm and run 39 – distance 65 cm – explain the difference for the first run the peaks were overlapped for the second run the peak were separated

another step for improvements and comparison with run 39:
We run the spectrum again with the same distance 65 cm and using collimators but the difference: we removed the lead shield under the first detector (liquid) and we add wax sample (polyethylene) instead – this used as a moderator to slow fast neutrons to thermal energies
By using Neutron shielding like polyethylene, Fast neutrons will Slow down rapidly by scatter in this hydrogenous material, shielding turns fast neutrons into thermal – result run 41 – see fig – there was chance coincidence may be from gamma scatter – we could improve the results by adding more shielding in the side of or on the top of the detector
Also, we can improve that the spectrum we got in run 41 include chance coincidence from gamma scatter we can put lead shield between both detector this will block both gamma and neutron and we will get only gamma scatter in the spectrum
We decide to run the spectrum again like run 41 but by adding more shielding in the side of or on the top of the detector – result run 42
result run 42: still there is gamma events, may be our theory is wrong and they are not chance coincidence and they are true gamma events from direct way between both detectors. to prove that experimentally: we run the spectrum again with neutron shiled under the start detector and we put lead shield between both detector to block the direct way by this we block both gamma and neutron – result run 43
for comparison: we run the spectrum again for same time as run 43 but without the neutron shield – we expect to get more neutron events – result run 44
then we can compare by calculation for both run 43 and run 44 : number of counts for whole spectrum / time

My last measurement:
we used maximum distance we can: 71 cm (the maximum distance we can – reason limited space inside the cage where the neutron tank placed)
we tried to shield the whole area as possible to block gamma scatter
we put lead shield on the top of the detectors
we used lead shield as colorimeters
we used gamma source as reference for gamma location on the spectrum
then we run again for the maximum time we can
see the fig below for the setup
see the excel graph

talk about:
effect of distance – when we increase the distance
effect of shielding
increase the time – if we have more time probably we get very good results

Message to the reader:
We can discriminate between neutron and gamma ray with this flight path using Am/Be Americium-Beryllium neutron source
We managed to discriminate between neutron and gamma, we faced problems and we tried to sort out those problems by: Increase distance use shielding, use colorimeter, increase the time and if we want better results give more time with increasing distance

Chapter 6 – Conclusion and Suggestions for Future Work
6.1 Conclusion
6.2 Suggestions for Future Work

Reference List
Appendix