hps neutron meter presentation(updated)

© Fuji Electric Co., Ltd. All rights reserved.© Fuji Electric Co., Ltd. All rights reserved.

2014 HPS PDS Is Your Neutron Meter Reading Accurately?

Fuji Electric Corp. of America Providing Innovative Solutions for the Nuclear Industry

James P Menge PE, CHP

© Fuji Electric Co., Ltd. All rights reserved.

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 (0.5eV – 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


Neutron Interactions

Inelastic Scattering of Neutrons

Elastic Scattering of Neutrons


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)


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


REM Ball Specifications

• Detection Range– Thermal to approximately 10-12 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


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 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.


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

Rule of Thumb: Fluence Rate to Dose 8.0 neutrons/sec/sq.cm./mREM for 2.45 MeV neutrons


Technical Question

1) 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


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 high-energy neutron generator (i.e. 14MeV)

– 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


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 decreases significantly as neutron

energies exceed six (6) MeV.

• The other neutron detector had a relatively flat energy response over the thermal to 15 MeV range

© Fuji Electric Co., Ltd. All rights reserved.© Fuji Electric Co., Ltd. All rights reserved.Reference 1

Dose Equivalent Rate Measurements

© Fuji Electric Co., Ltd. All rights reserved.© Fuji Electric Co., Ltd. All rights reserved.

20 50 75 100 150 200 300 400 500 600 10000

1

2

3

4

5

6

ICRP 21 NCRP 38

Neutron Energy (keV)

Rela

tive

Resp

onse

per

mRE

M

Reference 2

Neutron Meter Normalized to Unity w/ AmBe Cal


Question Re: Field Use of Instrument

Question: Is the neutron spectra encountered during the various phases of cask loading, sealing and storage the same?

Answer– Phase 1 - Spent fuel in steel canister, filled with water, and inside the steel transfer

cask?– Phase 2 - Water is drained?– Phase 3 - Spent fuel canister is placed in the concrete cask?– Your REM Ball is calibrated to AmBe


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


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.


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


Different Neutron Technologies

Tissue Equivalent Proportional Counter (TEPC)

• Proportional Detector technology combined with a 256 channel multichannel analyzer.

• The instrument applies the appropriate ICRU/P quality factors to each neutron event which results in a true REM response.

• Measures gamma and neutron dose independently

• Highly Sensitive

Prescila

19

• Proton recoil type detector

• Lightweight

• Gamma Rejection up to 100 mR/hr.


Mixed Gas Proportional Detector

Organic Mixed Gas

Energy Characteristics

· Fast neutron Elastic scattering of Hydrogen (Recoil proton)

· Thermal neutron 14N(n,p) 14C reaction of Nitrogen

Neutron energy [MeV]

Neu

tron

sens

itivi

ty p

er re

fere

nce

neut

ron

flux

[cou

nt/(n

/cm

2 )]

• Lightweight

• Good Gamma Rejection >1 R/hr

• Dose Equivalent Response


Domino Neutron Detector• Thin Form Factor• Adjustable Detection area • End-to-end pluggable Domino™ with 4 cm2 detection area, 20% thermal

neutron detection efficiency, a <3 mW power consumption, and digital output per module.

• Digital outputs may be tied together in the tiled format.

MSND TechnologyThe semiconductor-based detector utilizes microstructured semiconductor neutron detector (MSND) technology with 6Li conversion to yield a thermal neutron detection efficiency greater than 20%.


Cs2LiYCl6:Ce (CLYC) Features

• High sensitivity for Neutrons• 1 cm of 95% 6Li enriched CLYC ~80% efficiency for thermal neutron

detection

• Pulse Shape Discrimination (PSD) for γ-Rays and Neutrons• Rise & decay times different for n and γ • Pulse height discrimination (PHD)

• Good Proportionality• gamma-ray energy resolution 25-30% better than NaI(Tl)

• Fast Neutron Detection Due to Presence of 35Cl

• Potential Material for Combined n-γ Detection

Reference 6


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 -50 to +20% of reading

Newer Technologies for portable neutron detection are improving.


References

1. NRC ML11229A713-0751-H122 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” 3rd Ed 2000

5. ANI Information Bulletin 11-02 dated July 2012

6. Robert McKenzie Nuclear Technology Consultant

7. Hilger Crystals


Thank YouFuji Electric- RADIATION

Bringing Innovating Technology for Customer Solutions


Characteristics of Light Weight Neutron Survey Meter

Key Features

• Light Weight 5.1 lbs.

• Energy Range:• Thermal – >15 MeV

• No He3 or BF3 Detector

• Capable of measuring H*(10)


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.


Mixed Gas Proportional Detector

Detector Gas Energy Characteristics

CH4and N2

Neutron energy [MeV]Neu

tron

sens

itivi

ty p

er re

fere

nce

neut

ron

flux

[cou

nt/(n

/cm

2 )]

Organic Mixed Gas· Fast neutron Elastic scattering of Hydrogen (Recoil proton)

· Thermal neutron 14N(n,p) 14C reaction of Nitrogen

Detection Method

hps neutron meter presentation(updated)

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