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1 SME Associates LLC Providing Innovative Solutions IS YOUR NEUTRON METER READING ACCURATELY? JAMES

1 SME Associates LLC Providing Innovative Solutions IS YOUR NEUTRON METER READING ACCURATELY? JAMES P. MENGE PE, CHP

Basic Physics of Neutrons Charge: Neutrons are neutral particles and do not create ionization

Basic Physics of Neutrons Charge: Neutrons are neutral particles and do not create ionization Mass: The mass of the neutron is 1. 009 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. 5 e. V – thermal) • Intermediate (thermal – 100 ke. V • Fast (0. 1 Me. V - >15 Me. V) The primary energy region of interest is less than 0. 5 e. V (the Cadmium energy cutoff) - WMD 2

 Neutron Interactions Inelastic Scattering of Neutrons Elastic Scattering of Neutrons 3

Neutron Interactions Inelastic Scattering of Neutrons Elastic Scattering of Neutrons 3

Detection Principles Indirect Reactions with nuclei generate prompt energetic particles Protons Alpha Particles Target

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. 3 He and BF 3) Scintillation Detectors (e. g. 6 Li, CLYC) 4

REM Balls Hankins REM Ball - standard in nuclear Industry. Goal is to simulate

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. Bonner spheres are used to determine the energy spectrum of a neutron by using different spheres optimized for different neutron energies 5

REM Ball Specifications Detection Range Detector 22. 9 cm (9 in. ) diameter cadmium-loaded

REM Ball Specifications Detection Range Detector 22. 9 cm (9 in. ) diameter cadmium-loaded polyethylene sphere Sensitivity 2 Atm BF³ or ³He tube Moderator Thermal to approximately 10 -12 Me. V Typically 100 cpm/m. REM/hr (241 Am. Be) Gamma Rejection Typically 10 cpm or less through 100 m. Sv/h (10 R/hr) (137 Cs) Note value - less for ³He tube 6

BF 3 Gas-Filled Proportional Detectors 7 Detection of Thermal and Fast Neutrons "Bare" BF

BF 3 Gas-Filled Proportional Detectors 7 Detection of Thermal and Fast Neutrons "Bare" BF 3 detectors respond almost exclusively to slow (low energy) neutrons To detect fast neutrons, the BF 3 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 >10 R/Hr

 BF 3 Gas-Filled Proportional Detectors (cont’d) 10 B is irradiated with low energy

BF 3 Gas-Filled Proportional Detectors (cont’d) 10 B is irradiated with low energy or thermal neutrons to yield highly energetic helium-4 (4 He) nuclei (i. e. , alpha particles) and recoiling Lithium-7 (7 Li) ions. 8

Dose Equivalent Rate Measurements 9 The dose equivalent (H) per neutron fluence varies with

Dose Equivalent Rate Measurements 9 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 Rule of Thumb: Fluence Rate to Dose 8. 0 neutrons/sec/sq. cm. /m. REM for 2. 45 Me. V neutrons

Technical Question 1) Does your REM Ball respond to Neutrons (<0. 5 e. V)?

Technical Question 1) Does your REM Ball respond to Neutrons (<0. 5 e. V)? Answer – NO* Slow neutrons (<0. 5 e. V) cannot penetrate cadmium, surrounding the moderator with cadmium means that the detector will only respond to neutrons above 0. 5 e. V) * Manufacturer dependent if closed shell or honeycomb shell was used which allow energy to pass 10

Field Question – Why do 2 neutron detector indications (different instrument types) only agree

Field 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. 2 Me. V) 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) Me. V neutrons, and comparison was good 11

Field Question (cont’d) A REM Ball is sensitive to neutrons with energies ranging from

Field Question (cont’d) A REM Ball is sensitive to neutrons with energies ranging from thermal to about 10 or 12 Me. V (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 thermal to 15 Me. V range as per ICRP 21. 12

13 Dose Equivalent Rate Measurements Reference 1

13 Dose Equivalent Rate Measurements Reference 1

14 Neutron Meter Normalized to Unity w/ Am. Be Cal Relative Response per m.

14 Neutron Meter Normalized to Unity w/ Am. Be Cal Relative Response per m. REM 6 5 4 3 2 1 0 20 50 75 100 150 200 300 Neutron Energy (ke. V) 400 500 ICRP 21 600 1000 NCRP 38 Reference 2

Field Use of Instrument Discussion • The Energy spectrum is moderated by the concrete,

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 Am. Be source? • What is the minimum distance from the cask that allows for correct measurement ? Reference 3 15

Challenges with REM Ball Type Instruments The instrument’s accuracy depends on the neutron spectrum.

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 ke. V). Dose Equivalent Rate Measurements < 50 ke. V over-response can be expected to generate a factor of 3. > 6 Me. V under-response to neutrons (e. g. , by a factor of 2 for 10 Me. V neutrons). REM Balls are heavy, awkward to carry and can be dangerous to carry in areas where movement is limited, i. e. . , on ladders. 16

RP Detection Challenges The radiobiological hazard is challenging to properly assess the neutron dose

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 >3 x can occur in an isotropic field such as nuclear reactor drywell. REM Ball Detector Response varies with neutron energy Over response for < 100 ke. V Under Response >6 Me. V 17

Characteristics of Light Weight Neutron Survey Meter Key Features • Light Weight 7 lbs.

Characteristics of Light Weight Neutron Survey Meter Key Features • Light Weight 7 lbs. • Energy Range: • Thermal – >15 Me. V • No He 3 or BF 3 Detector • Capable of measuring H*(10) 18

 Mixed Gas Proportional Detector • Lightweight • Dose Equivalent Response Neutron sensitivity per

Mixed Gas Proportional Detector • Lightweight • Dose Equivalent Response Neutron sensitivity per reference neutron flux [count/(n/cm 2)] • Good Gamma Rejection >1 R/hr Energy Characteristics Neutron energy [Me. V] · Thermal neutron 14 N(n, p) 14 C reaction of Nitrogen Organic Mixed Gas · Fast neutron Elastic scattering of Hydrogen (Recoil proton)

NSN 3 Detection Principles Proportional gas counter consisted of methane gas of 3. 94

NSN 3 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 cm 3. 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 14 N(n, p)14 C reaction of nitrogen gas. The proton energy in this reaction is 626 ke. V. By using these two reactions the neutron ambient dose equivalent can be obtained from thermal neutrons up to about 15 Me. V. 20

Key Points Knowing the energy spectrum of the neutron fields at your facility/site is

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 Dose rate measurement typically -/+20% of reading or better 21 Instrument response - varies with the neutron energy In many cases we can state we are conservative due to over response Newer Technologies for portable neutron detection are Lightweight.

References 1. NRC ML 11229 A 713 -0751 -H 122 Basic Health Physics- 27

References 1. NRC ML 11229 A 713 -0751 -H 122 Basic Health Physics- 27 Neutron Detectors 10/1/2010 2. DW Rogers; “Why Not to Trust A Neutron Ratemeter”, Feb 1979 3. J. K. Shultis; “RADIATION ANALYSIS OF A SPENT-FUEL STORAGE CASK “ Jan 2000 4. G Knoll; “Radiation Detection and Measurement” 3 rd Ed 2000 5. ANI Information Bulletin 11 -02 dated July 2012 22

23 Thank You SME Associates LLC Providing Innovating Solutions

23 Thank You SME Associates LLC Providing Innovating Solutions