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Is Your Neutron Meter Reading Accurately?
James P. Menge PE, CHP

2. Basic Physics of Neutrons
Charge: Neutrons are neutral particles and do not create ionization
Mass: The mass of the neutron is AMU. Detection can only be inferred via indirect methods due to collisions with nuclei or electrons.
Reactions: Neutrons react with a number of materials through
Elastic Scattering producing a recoiling nucleus. (Kinematic)
Inelastic Scattering producing an excited nucleus, or absorption with transmutation of the resulting nucleus. (Kinematic and Nuclear)
Energy Ranges of Neutrons
Slow
(0.5eV – thermal)
Intermediate
(thermal – 100 keV
Fast
(0.1 MeV - >15MeV)
The primary energy region of interest is less than 0.5eV (the Cadmium energy cutoff) - WMD

3. Neutron Interactions Inelastic Scattering of Neutrons
In collision with protons, neutrons lose half their energy on average. This reaction makes hydrogenous materials (materials rich in protons) good shields (e.g. concrete, wax, water, and various plastics).
This reaction is very important from a health physics standpoint, since it is responsible for most of the tissue dose from fast neutrons.

4. Detection Principles
Indirect Reactions with nuclei generate prompt energetic particles
Protons
Alpha Particles
Target Material – Convert neutron
Cross Section – f(x) of neutron energy
Conversion (i.e. Detection of secondary particles) achieved by conventional methods
Proportional Detectors (Gas Detectors e.g. 3He and BF3)
Scintillation Detectors (e.g. 6Li, CLYC)
THE PRIME MECHANISM FOR DETECTION OF NEUTRONS INVOLVES COLLISIONS WITH NUCLEI
The nature of physics in neutron detection is approx. dose based on conservative measurements for safety
Typical tolerance of neutron reading -50% - +20% of reading

5. REM Balls
Hankins REM Ball - standard in nuclear Industry. Goal is to simulate detection properties of human tissue in a reproducible fashion.
A single Bonner sphere (typically 9 inches in diameter) cadmium loaded will approximate the dose rate across a range of neutron energies.
To reduce the instrument’s over-response to low
energy neutrons, they incorporated a borated
plastic sleeve (7.6 cm diameter) around the
detector tube and inner moderator.
Bonner spheres are used to determine the energy spectrum of a neutron by using different spheres optimized for different neutron energies

6. REM Ball Specifications
Detection Range
Thermal to approximately MeV
Detector
2 Atm BF³ or ³He tube
Moderator
22.9 cm (9 in.) diameter cadmium-loaded polyethylene sphere
Sensitivity
Typically 100 cpm/mREM/hr (241AmBe)
Gamma Rejection
Typically 10 cpm or less through 100 mSv/h (10 R/hr) (137Cs) Note value - less for ³He tube

7. BF3 Gas-Filled Proportional Detectors
Detection of Thermal and Fast Neutrons
"Bare" BF3 detectors respond almost exclusively to slow (low energy) neutrons  
To detect fast  neutrons, the BF3 tube can be surrounded by a suitable moderator. The thickness of the moderator (e.g., polyethylene) may range from 1 to 6 inches dependent on the neutron energy spectrum to be detected. 
Gamma Detection
Relatively small pulses in comparison and easily rejected by threshold. Typically >10R/Hr
BF3 and He-3 tubes operate in the proportional counting mode.
The kinetic energy of the alpha particle and recoil Li-7 nucleus is deposited in the detector gas. The pulse height spectrum therefore shows two peaks: a large one at 2.31 MeV (the lithium nuclei were left in an excited state) and a small one at MeV (the lithium nuclei were left in the ground state

8. BF3 Gas-Filled Proportional Detectors (cont’d)
10B is irradiated with low energy or thermal neutrons to yield highly energetic helium-4 (4He) nuclei (i.e., alpha particles) and recoiling Lithium-7 (7Li) ions. 
3He gas-filled proportional detectors
The neutron is converted through the nuclear reaction
n + 3He → 3H + 1H MeV
Cross Section is 5330 barns
into charged particles tritium (T, 3H) and protium (p, 1H) which then are detected by creating a charge cloud in the stopping gas of a proportional detector  
the smaller pulses of the He-3 detectors make them poorer than BF3
detectors at rejecting gamma rays.

9. Dose Equivalent Rate Measurements
The dose equivalent (H) per neutron fluence varies with the neutron energy because both the absorbed dose (D) and the quality factor/radiation weighting factor (QWr) vary with neutron energy:
H = D Q = D Wr
DO YOU NEED TO SAY (WITH WORDS) WHAT THE QUALITY FACTOR IS (ALWAYS DANGEROUS TO ASSUME THAT ALL KNOW THESE THINGS I FIND)
Rule of Thumb: Fluence Rate to Dose
8.0 neutrons/sec/sq.cm./mREM for 2.45 MeV neutrons

10. Technical Question
Does your REM Ball respond to Neutrons (<0.5 eV)?
Answer – NO*
Slow neutrons (<0.5 eV) cannot penetrate cadmium, surrounding the moderator with cadmium means that the detector will only respond to neutrons above 0.5 eV)
* Manufacturer dependent if closed shell or honeycomb shell was used which allow low energy to pass

11. Field Question
Question – Why do 2 neutron detector indications (different instrument types) only agree with each other some of the time?
Standard REM Ball vs light weight neutron meter
Measurement field (i.e. 0.2MeV)
The measurement results from the two detector types differed by a factor of three.
Both instruments were calibrated in a Californium-252 field, with its roughly two (2) MeV neutrons, and comparison was good

12. Field Question (cont’d)
A REM Ball is sensitive to neutrons with energies ranging from thermal to about 10 or 12 MeV (manufacturer dependent)
REM Ball instrument response = significant over response as neutron energies move toward thermal range. From Studies Rem Balls typically over respond by factor 2-5 in lower neutron fields especially in NPPs.
The Fuji Electric detector has a relatively fluence response over the thermal to 15 MeV range as per ICRP 21.

13. Dose Equivalent Rate Measurements
Reference 1

14. Neutron Meter Normalized to Unity w/ AmBe Cal
Reference 2

15. Field Use of Instrument Discussion
The Energy spectrum is moderated by the concrete, the REM Ball would over respond by factor of 2 (on side)
Will the instrument provide correct response if calibrated against an AmBe source?
What is the minimum distance from the cask that allows for correct measurement ?
Reference 3

16. Challenges with REM Ball Type Instruments
The instrument’s accuracy depends on the neutron spectrum.
Scattered neutrons areas represent a significant component (10 to 100 keV).
Dose Equivalent Rate Measurements
< 50 keV over-response can be expected to generate a factor of 3.
> 6 MeV under-response to neutrons (e.g., by a factor of 2 for 10 MeV neutrons).
REM Balls are heavy, awkward to carry and can be dangerous to carry in areas where movement is limited, i.e.., on ladders.

17. RP Detection Challenges
The radiobiological hazard is challenging to properly assess the neutron dose equivalent.
The conversion from neutron fluence to dose equivalent is energy dependent.
The response is highly sensitive to the directional characteristics of the neutron field. An over-response >3x can occur in an isotropic field such as nuclear reactor drywell.
REM Ball Detector Response varies with neutron energy
Over response for < 100keV
Under Response >6 MeV

18. Characteristics of Light Weight Neutron Survey Meter
Key Features
Light Weight 7 lbs.
Energy Range:
Thermal – >15 MeV
No He3 or BF3 Detector
Capable of measuring H*(10)

19. Energy Characteristics
Mixed Gas Proportional Detector
Neutron energy [MeV]
Neutron sensitivity per reference neutron flux [count/(n/cm2)]
Lightweight
Good Gamma Rejection >1 R/hr
Dose Equivalent Response
Energy Characteristics
· Thermal neutron
14N(n,p) 14C reaction of Nitrogen
· Fast neutron
Elastic scattering of Hydrogen
(Recoil proton)
Organic Mixed Gas

20. NSN3 Detection Principles
Proportional gas counter consisted of methane gas of 3.94 atm and nitrogen gas of 0.98 atm. The detector is almost a spherical shape of approximately 13- cm dia.
Effective volume is approximately 1400 cm3. The weight of this detector is approximately 720 g.
The neutrons are measured using the mixed gas.
Fast: elastic scattering reaction of hydrogen of the methane gas
Slow/Thermal Neutrons - using the 14N(n, p)14C reaction of nitrogen gas. The proton energy in this reaction is 626 keV.
By using these two reactions the neutron ambient dose equivalent can be obtained from thermal neutrons up to about 15 MeV.

21. Key Points
Knowing the energy spectrum of the neutron fields at your facility/site is important.
Neutron Energy Spectrums must be characterized to accurately determine Dose Equivalent
Instrument response - varies with the neutron energy
In many cases we can state we are conservative due to over response
Dose rate measurement typically /+20% of reading or better
Newer Technologies for portable neutron detection are Lightweight.
WHAT IS POINT 3???

22. References
NRC ML11229A H122 Basic Health Physics- 27 Neutron Detectors 10/1/2010
DW Rogers; “Why Not to Trust A Neutron Ratemeter”, Feb 1979
J.K. Shultis; “RADIATION ANALYSIS OF A SPENT-FUEL STORAGE CASK “ Jan
G Knoll; “Radiation Detection and Measurement” 3rd Ed 2000
ANI Information Bulletin dated July 2012

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