This document details the study of thermal neutron flux distribution using a BF3 counter for an Am-Be source, including the determination of source strength. It covers the discovery and properties of neutrons, detection methods, and the setup required for experiments. The results indicate an exponential decrease in thermal neutron flux with increasing distance from the source.

1. Thermal neutron flux
distribution by using BF-3
counter
Miss Neha Mannewar
Msc-I (Medical Physics)
Dean
Dr.C.D.Lokhande
Guide
Dr .Manisha Phadtare

2. Aim
 To determine thermal flux distribution flux for Am-Be source
and source strength for same source with BF3 counter
Purpose
 To determine thermal flux distribution for Am-Be source
moderated with water.
 To calculate source strength for same source

3. Outline:
 Discovery of Neutrons
 Properties of Neutrons
 Detection of neutron flux & strength using BF3 detector.
 Equipment
 Procedure
 Conclusion
 Result

4. Neutron Discovery
 In 1930 two German physicist Walther and Bothe found
that, when beryllium was bombarded with an alpha particle
a highly penetrating radiation was emitted.
 This radiation was capable of traversing through a thick
layer of lead and was unaffected by electric and magnetic
fields.

5.  In 1932 James Chadwick discovered that the emitted radiations
consists of particle of mass nearly equals to mass of protons and it
has no charge i.e it is uncharged particles.
 He called them as Neutrons.

7. Properties of Neutron
 Constituent particle of all Nuclei except Hydrogen
 Neutral particle with no charge.
 They are not deflected by a magnetic field as well as electric
field.
 Mass slightly greater than that of Proton.
 Stable inside the nucleus but unstable outside the nucleus.

8.  Free neutron decay with proton and an electron with antineutrino i.e
nothing but as Negatron decay with half life with 13 minutes.

9.  They can easily penetrate.
 Classified according to their kinetic energy as
1.Slow Neutrons
2.Fast Neutrons
 Both are capable of penetrating a nucleus causing artificial
transmutation of nucleus.
 Slow Neutron :Energy from 0-1000 eV
 Fast Neutron: Energy from 0.5 to 10 MeV
 Neutrons with an average energy about 0.025ev in thermal
equilibrium are called thermal neutrons
 In nuclear reactors fast neutrons are converted into slow neutrons
using moderators.

10.  There are two key aspects to effective neutron detection
1.Hardware
2.Software
 Detection hardware refers to kind of neutron detector used and
electronics is used in the detection setup.
 Detection software consist of analysis tools that perform tasks such
as graphical analysis to measure the number and energies of
neutrons striking the detector.

11. Types of Neutron detector:
1.Gas proportional detectors.
 3He gas-filled proportional detectors
 BF3 gas-filled proportional detectors
 Boron lined proportional detectors
2.Scintillation Neutron Detectors.
 Neutron-sensitive scintillating glass fiber detectors
 LiCaAlF

12. 3. Semiconductor neutron detectors
4. Neutron activation detectors
5. Fast neutron detectors

13. BF3 Detector
 As elemental boron is not gaseous, neutron detectors
containing boron may alternately use boron trifluoride (BF3)
enriched to 96% boron-10 (natural boron is 20% 10B,
80% 11B).
 In this detector, BF3 gas acts as both a proportional gas
and a neutron detection material.

14. • Because of the larger cross section of the 10B (n,α) reaction, the
bare BF3 counter has a high sensitivity for slow neutrons with the
well known energy dependence of 1/v.
While,when the counter is covered with a suitable moderating
medium,it makes a sensitive detector for fast neutrons.

15.  A typical BF3 detector consist of cylindrical aluminum (brass or
copper) tube filled with BF3 gas at pressure of 0.5 to 1.0
atmosphere.
• The boron gas accomplished two things:
1. It function as proportional counter
2. It undergoes an neutron alpha (n,a) interaction with
thermal neutron.

16.  To improve detection efficiency, the BF3 enriched in B-10.
 Aluminum is typically used as the detector (cathode) well
because of its small cross section for neutrons.
 The anode is almost always a single thin wire running down the
axis of tube.

17. Equipments:
 BF-3 detector
 Amplifier
 Pre-amplifier
 Power supply
 Multi channel analyzer (MCA)
 Am-Be source

18. Neutron Detection
 Unmodified neutron detectors (e.g BF3 or He-3)
usually respond to slow or fast neutrons, but not
both.
 Slow neutron detectors are far more common
and can be modified so that it responds to both
slow or fast neutrons.
 It can even be modified so that it only responds
to fast Neutrons.
 BF3 and He-3 tubes operate in the proportional
counting mode.
He-3 tube

19.  Surrounding a slow neutron detector with an appropriate
thickness of a moderator (eg.polyethylene) will slow some of the
fast neutrons down to energies that the detector can respond to.
 The moderator increases the detector response to fast neutrons,
but reduces the response to slow neutrons.

20. Reaction
B-10 + n ----→ Li-7 + a

21. Procedure:
 Set up experiment as shown in a diagram.
 Take a counts from MCA for a live time 50 sec by varying
source to detector distance.
 Calculate for thermal flux for each distance.
 Plot a graph count verses distance.

22. Experimental diagram of Bf-3

23. Practical Arrangement

24.  High voltage- 1200V
 Amplifier Coarse gain-110
 Amplifier fine gain-Minimum (10)
 Amplifier shaping constant-1µsec
 Amplifier input polarity- (+)
 Amplifier output type-unipolar
 MCA input size:8192 channel
 Detector:BF31x
Experimental set up

25. Formula
•Where,
•R=Counting rate (reactions per second)
•N=number of 10B atoms per unit volume
•V=volume of counter
Φ=neutron flux (m-2 s-1 )
σ=cross-section of the (n,α) reaction for neutron energy
•This equation will be used for determining neutron flux in the
experiment .
•In this equation, V, σ0 and v0 are known .
•N can be calculated from the ideal gas model, P=NRT
•v can be determined from the relation:

26.  Here vp is the most probable speed and can be obtained from
most probable energy Ep as;
Vp =1728.87 m/s
= 1951.31 m/s
N=0.0286
V=πr2 h=90.04
σ=3840 barns
V0 = 2200m/s
Example= φ5cm=
(228.82*1951.31)/(0.0286*90.04*3840*2200)
=0.0205 neutrons cm-2 s-1

28. Observation Table
Distance Integral Area Counts/min R=Integral/60
5 13729 153 228.82
8 7384 88 123.07
11 3583 45 59.72
14 2128 30 35.47
17 1251 22 20.85
20 507 12 8.45

29. Graph

30. Result
The thermal neutron flux for an Am-be
source decreasing exponentially with
distance S.

31. Thank You !!!!!!