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