Neutron dosimetry

1. Neutron dosimeter personal and area
monitoring
Presenter – Hirakjyoti Bezbaruah
( intern medical Physicist)
moderator - Dr. M.N.Singh
( Medical Physicist cum RSO)
Dr. B . Borooah Cancer Institute

2. CONTENTS
 INTRODUCTION
NEUTRON CLASSIFICATION
NEUTRON SOURCES
NEUTRON DETECTION BASED ON NUCLEAR REACTIONS
NEUTRON DETECTION DEVICES ( PERSONAL MONITORING AND AREA
MONITORING)
PERSONAL MONITORING DEVICE ( TLD , OSLD, FILM BASED)
AREA MONITORING ( REM COUNTER, BONNER SPHERE SPECTROMETRY)

3. INTRODUCTION
James Chadwick discovered Neutron in the year of 1932 .
Rest mass of neutron = 1.0086 amu = 1.67 x 10-27 kg = 939.566 Mev
Half life of neutron outside the nucleus is 10.4 minutes .
Mean life of neutron is 14.76 minutes .
Decay scheme of neutron is
1
n0 → 1p 1 + 0 β-1 + antineutrino
Neutron have two down quarks and one up quark (one up quark have charge
2/3 coulomb) and one down quark (one down quark have charge -1/3 coulomb) .
Net charge of neutron = 2 x (-1/3) + 1 x (2/3) = 0 C

4. Neutron classification
Neutrons are classified according to their kinetic energy –
 Cold neutron (0.0<E<0.003eV)
 Slow thermal neutron (0.003<E<0.4eV)
 Slow epithermal neutron (0.4<E<1000eV)
 Intermediate neutron (1000eV<E<0.5MeV)
 Fast neutron (0.5<E<10MeV)
 High energy neutron (E>10MeV)

5. Neutron source
 Alpha- neutron source .
 Photo-neutron source .
 Spontaneous fission neutron source .
 Reactions from accelerated charged particles

6. Alpha-neutron source :
Here an alpha comes out from radioactive nuclei such as 241Am, 210Po,
226Ra, 116In, 124Sb, 238Pu mixed with low z material such as 9Be4, 6Li3
and produced neutron.
α + 9Be4 → 13c6* → 12c6 + 1n0 + 4.44 MeV
α + 6Li3 → 10B5 + 1n0
α + 13c → 17O8
* → 16O8 + 1n0 + gamma ray

8. Photo neutron source :
Some gamma ray emitters can also be used to produce neutrons when combined
with an appropriate target material . Mostly used targets in photo neutron sources
are 9Be and deuterium .
Both nuclear reactions are endothermic ( i.e energy need to supply to procced the
reaction).

10. Spontaneous fission neutron source :
The most common spontaneous fission neutron source is
252Cf ( half life of 2.65 years).
252Cf98 → 140Xe54 + 108Ru44 + 4 1n0
252Cf98 → 140Cs55 + 109Tc43 + 3 1n0

12. Reactions from accelerated charged particles:
Here , deuteron is accelerated by a potential of about 100 to 300 KV and
bombarded to deuteron and Tritium .
The columbo barrier between the incident deuteron and the light target nucleus
is relatively small.
All the neutrons produced in these reactions have the same energy .
 deuterons will produce about 109 n/s from thick deuterium target and about
1011 n/s from Tritium target .

13. Neutron detection based on nuclear reactions
10B (n, ) reaction : This is the most popular reaction for the conversion of
slow neutrons into directly detectable particles

14. 6Li(n, ) reaction:
3He(n , p) reaction:

15. Neutron detection devices:
NEUTRON DOSIMETER
PERSONNEL MONITORING AREA MONITORING
TLD REM COUNTER
OSLD BSS
FILM DOSIMETRY

16. TLD Dosimeter:
TLD materials are -
Lithium Fluoride (TLD-100, TLD-600, TLD-700)
Calcium fluoride Dysprosium ( TLD-200)
Aluminium oxide (TLD-500)
Calcium sulphate Dysprosium ( TLD-900)
Calcium fluoride Manganese (TLD-400)
Lithium Fluoride (TLD-600) contains 95.62% 6Li isotope in LiF material .
6LiF has a large cross section for neutron and will be more sensitive to neutrons
than to gamma radiation .

17. How does TLD work

18. Glow curve

19. OSLD(Optically stimulated luminescence device )
OSLD consists of thin layer of aluminum oxide and additional CR-39 for neutron
detection .
It has three filters Aluminum , Tin , Copper
(Each filter offers small , moderate and high attenuation to radiation respectively.)
High energy radiation gives luminescence under copper , whereas medium and
low energy radiation corresponds to Tin and Aluminum respectively.
CR-39(Poly allyl diglycol carbonate) is kept in contact with 1mm polyethylene
radiator .
CR-39 is most suited for neutron detection due to higher number of hydrogen
atoms available for elastic collision with incident fast neutrons .
The energy range for neutron detection is 40Kev - 35Mev

21. • White dots are the damage tracks in the CR-39 by neutron as seen under a
microscope .

22. Film dosimeter
The film is partially covered with the cadmium filter .
Neutrons captured by cadmium and emits gamma rays .
113Cd48 + 1n0
113Cd48 + gamma -ray of energy 5Mev
Uncovered portions of the film corrects for gamma background .

23. REM Counter
BF3 proportional counter is most widely used for neutron detection .
Neutron detection assembly and electric circuit is present in this instrument .
The electrical unit used in rem counter are
i. High voltage supply
ii. Pulse amplifier
iii. Discrimination
iv. CRM (Rate meter)
The nuclear reaction inside the neutron rem counter is
n + 10B5
7Li3 + alpha particle

24. REM COUNTER
• Fig: Ludlum Model 12-4 neutron Meter and its construction

25. Neutron Rem counter:
fig: construction of Rem counter

26. Neutron Rem counter
Fig: (a) Response of a BF3 counter having typical dimension , (b) Energy spectrum
Obtained from a very large volume BF3 counter

27. In this model BF3 or HE-3 surrounded by 9 inch (22.9 cm )Cadmium loaded
polyethylene sphere .
• BF3 counter cannot be detected directly to detect fast neutrons .
• To detect fast neutrons , moderator is used such as polythene , cadmium
• its readout express in millirem per hour .
• Dose rate varies from zero to 10,000 mrem per hour or 0 – 100 mSv per hour .

28. BONNER SPHERE SPECTROMETETRS(BSS)
BSS consists in an array of thermal neutron detectors (He-3 or BF3
proportional counter),each placed in a spherical high density polyethylene
(HDPE) moderator of different diameter .
Higher the neutron energy ( the faster), the larger the sphere must be for
the neutrons to be slowed down and detection .
By the comparison between the counts detected by each sphere , is
possible to determine the incident neutron energy.

29. BONNER SPHERE SPECTROMETRY
Fig a : Bonner sphere of different sizes Fig b: geometrical view of Bonner
sphere

30. REFERENCE
• Radiation detection and measurement by GLENN F.KNOLL
• Direct fast neutron detection – A.J.Peurrung , R.R.Hasen

31. THANK YOU……