Active methods of neutron detection

International Atomic Energy Agency

NEUTRON DOSIMETRY AND MONITORING

Active Methods of Neutron Detection


Active Methods of Neutron Detection - Unit Objectives

The objective of this unit is to present a summary of the active detection mechanisms that can be applied to personal and area monitoring, and calibration instrumentation used for neutron dosimetry.

At the completion of this unit, the student should understand how the detection mechanisms for active methods employed in current neutron monitoring problems function. The student should also have a general understanding of the advantages, disadvantages, and areas of application of each of these methods.


Active Methods of Neutron Detection - Unit Outline

Introduction Gas Filled Detectors

Ionization chambers Proportional counters

Scintillators Thermal neutron detection Fast neutron spectrometry

Semiconductor Detectors Silicon Diode Based Detectors Direct Ion Storage Detectors

Superheated Emulsion (Bubble) Detectors


INTRODUCTION


Neutron detection Neutrons can be detected only indirectly by

charged particles from nuclear reactions.

For spectrometry applications, the energy of these charged particles must be related to the energy of the neutron.

Two kinds of reactions can be used in neutron detectors: Exothermic nuclear reactions Elastic scattering of neutrons with

detector nuclei


Exothermic reactions

Result in secondary charged particles, e.g. 3He(n,p)3H + Q (Q = 764 keV).

A neutron of energy En produces an electric signal at the detector output, the height of which is proportional to En+Q.


Elastic scattering

Elastic scattering of neutrons with nuclei of the filling gas, i.e. production of recoil protons in hydrogen (or a hydrogen-containing gas such as CH4) or of alpha particles in 4He filling.

The maximum energy transfer (in the case of a head-on collision) from the neutron to the recoil nucleus of mass M is given by

Emax = [4M/(M + 1)2] En


GAS FILLED DETECTORS


Gas filled detectors

Voltage supply

Electric current or

pulse measuring

device

Fill gas, e.g. air, CH4, etc.

Incidentradiation

Anode+

Cathode-


Gas filled detectors

Lo

g d

etec

ted

ch

arg

e, Q

0p+

I II III IV V VI

e-

Voltage

Region Process*

I Recombination

II Ionization

III Proportional

IVLimited

proportionality

V Geiger

VI Breakdown

* Red indicates useful for neutron applications


Ionization Chambers


Ionization chambers

The ionization current indicates exposure rate. Very rapid response time. Dual chambers used for neutron measurement.

Air equivalent walls and air fill gas for photons

A-150 Tissue Equivalent plastic walls and T.E. fill gas for neutrons + photons

Difference = neutrons Relatively insensitive Neutron applications - Used mainly for

calibrations.


Ionization chambers


Proportional Counters


Proportional counters

Pulse height is proportional to the number of ions resulting from a charged particle interaction.

Proton and alpha particles produce larger pulses than a beta particle or photon.

Discriminator can reject photons and betas.

BF3 and 3He fill gases used for thermal measurement.

H2, CH4, 4He, etc. used for spectrometry.


Pulse-height spectrum with BF3 gas


Pulse-height spectrum with 3He gas counter


SP2 proportional proton recoil counter

Proportional counters

Operated as recoil detectors Filled with H2 or CH4, using elastic (n,p)

scattering, or 4He gas resulting in (n,α) scattering.


Proton recoil proportional counters

Use the (n,p) scattering cross section: Well known and changes monotonically with

energy. Isotropic in the centre-of-mass system for

neutron energies less than at least 5 MeV. Described by simple scattering theory -

calculations are relatively straightforward. Reasonably large absolute value so that the

counters have a useable efficiency. Recoil protons have a constant mean energy loss

of W = 36 eV/ ion pair produced > 3 keV.


Spherical proton recoil counter


Processes occur in a proportional counter

The incident neutron, if scattered by a hydrogen nucleus, produces a recoil proton.

Electrons produced along the proton track due to ionizations drift towards the anode wire along an electric field line.

Near the wire, the electron gains enough energy for the gas atoms to be ionized.

Ionization electrons can produce further electrons and thus an avalanche is produced (gas amplification).


Processes in a proportional counter

Charges are collected at the anode and a signal is produced.

Signal is proportional to the amount of ionization and thus to the energy of the recoiling proton (not of the incident neutron).

For each neutron energy En, proton energies Ep in the range 0 ≤ Ep ≤ En can be obtained depending on the scattering angle between the neutron and the proton.


Spherical proton recoil counters

Spherical detector response is nearly independent of the incident neutron direction.

Fill gas purity and constant electric fields are important.

Electric field constancy depends on: Diameter of the insulator, Diameter of the anode wire, Diameter of the wire holder, Length of the anode wire holder and Distance between the insulator and the end

of the anode wire.


Cylindrical proton recoil detectors

Not as complicated in their construction.

Can be manufactured with large volumes for increased sensitivity and energy range.

The active volume of the detector can be exactly defined with special precautions at both ends of the anode wire – “field tubes”.

Disadvantage – anisotropy.

Problems in multidirectional neutron fields.


Attributes of SP2 proportional counters*

High energy resolution (ΔE/E in the order of a few per cent) for neutron spectrometry.

Isotropic response. Work in high thermal and epithermal fields. Tried and tested counters - expertise in their

use is available. They cover the 50–1500 keV energy range

where fluence to dose equivalent coefficients vary rapidly with energy.

With electronic n/γ discrimination lower energies can be measured.

* Characteristic of most, well made spherical proportional counters


Proton recoil proportional counter disadvantages

May be highly microphonic. Solution - Enclosing counters in a firm

metal box for acoustic noise damping with minimum neutron attenuation.

May be made of aluminum or cadmium. A sheet of lead reduces gamma rays.

Since 2 or 3 counters may be used in succession to cover the energy range, longer measuring times are required.

Low efficiency due to the low fill gas density compared with solid scintillators.


Ideal response function of a hydrogen recoil counter to monoenergetic neutrons

For a monoenergetic neutron fluence of energy E, the proton recoil energy distribution P(E) would ideally have the characteristic rectangular shape.


Response of a hydrogen recoil counter

Not the case because of distortion effects - the number of ions collected at the anode does not give a measure of the proton recoil energy:

Not all recoil protons lose their entire energy within the counter before hitting the wall - wall distortion effects - and

Gas amplification is not constant over the entire volume (electric field strength drops at the ends of the anode wire) - gas amplification distortion effects.


SP2* counter response to 144 keV neutrons

* 100 kPa


Proportional counter response functions*

* Type SP2

Type SP2


n/γ discrimination


Typical SP2 application energy ranges


Proton recoil counter maximum sensitivity

Maximum fluence and dose rate limits usually of the order of 5000 to 10,000 counts s-1(cps).

Pile-up rejection and dead-time correction must be made correctly.

Given a 1% proportional counter efficiency - the corresponding integral fast neutron fluence rate of (0.5 - 1) x 106 cm2 s-1 dose equivalent rate of 500 - 1000 mSv h-1, respectively, is an acceptable upper limit.


Proton recoil counter limit of detection

Determined by the statistical uncertainty and acceptable measuring time.

Assume a required number of counts is at least a few tens of thousands of events.

Necessary count rate for a 2 h measurement is about 5 cps: Fast neutron fluence rate 5x102 cm-2 s-1, or Dose equivalent rate of 500 Sv h-1.

Definite values cannot be given.


SCINTILLATION DETECTORS


Scintillation detectors

Radiation enters detector material.

Interaction causes light flash (scintillation).

Scintillation detected by photomultiplier.

Signal processed by electronics

Pulse height proportional to energy deposited.


Scintillation detection

Scintillator

Source

γ

Neutron

β+

β-

α

p

Window

U.V. photons produced from local excited states following ionization

Dynode (secondary electron emission)

Anode

Reflector

Photocathode

P.M. tube

Photoelectron from photocathode


Thermal Neutron Detection


Thermal neutron detector – 6LiI(Eu) Used for moderated detector (multisphere)

measurements.

Reaction energy - 4.780 MeV - shared by the resulting alpha particle and triton.

Appears in a pulse-height spectrum as a broad quasi-Gaussian full-energy peak.

6LiI crystal usually connected to a photomultiplier with a light pipe.

Crystal sizes typically: 4 mm x by 8 mm, 8 mm x 8 mm, or 12.7 mm x 12.7 mm.


6LiI pulse-height spectrum

4mm x 4mm detector

a-b. Range of fitted data

c. Gaussian peakd. Photon

backgrounde. Sum of fitted

componentsf. Lower

discriminator limit


Fast Neutron Detection


Organic scintillation detectors

Organic scintillators are best neutron spectrometry at higher energies (>1 MeV).

Scintillation materials include: Plastic Anthracene Stilbene Liquid scintillators


Organic scintillation detectors

High detection efficiency due to: High n-p scattering cross section, and Higher density of scintillation detectors

compared with gas counters. Neutron response well calculated from cross

sections, up to 20 MeV. All organic scintillation detectors are equally

sensitive to photons and neutrons. Photon energies of up to 10 MeV must be

taken into consideration in mixed fields.


Organic scintillator characteristics Plastic scintillators not suitable for mixed

fields – no n/γ discrimination. Good for neutron time-of-flight spectrometry -

excellent sub-nanosecond time resolution. Stilbene crystals have excellent n/γ

discrimination. Light production by the secondary charged

particles depends on the ion direction. neutron response functions depend on

angle of incidence - must be determined for the actual neutron directional distribution.


Liquid scintillator characteristics

NE213 or BC501A liquid scintillators do not have directionality drawback.

n/γ discrimination properties are also good.

Xylene is the basic liquid, so container must be carefully prepared.

Concerns: Chemical properties Rather large xylene expansion coefficient.


Liquid scintillator characteristics

Aluminum capsules in polyethylene expansion tubes - can be used from 5°C to 35°C.

Any size and shape can be constructed for coupling to one or two phototubes as appropriate for optimal response.

Liquid scintillators encapsulated in aluminium are well suited for most applications.

NE213 scintillators no longer available

BC501A (model MAB-1F) are identical to NE213 in design and chemical composition.


Charged particle ranges organic scintillators


NE213 organic scintillator assembly

n/γ field

NE213scintillator

Plexiglas lightguide

Light emitting diode (LED) Photomultiplier


Calculated NE213 response function

10 MeV neutrons


Calculated NE213 response functions


SEMICONDUCTOR DETECTORS


Silicon Diode Based Detectors


Detection mechanisms for SSD

Use same principles as passive dosimeters. Detect charged particles in detector, or Use converter layers (e.g. polyethylene).

Silicon diodes.

Direction ion storage.


Semiconductor based detectors

Semiconductor detectors detect charged particles generated in neutron-induced nuclear reactions:

In the detector itself, or

Charged particles generated in converter layers mounted close to the detector.

Conventional semiconductors will not detect neutrons below 1 MeV since they do not contain hydrogenous material.


Detection of low energy neutrons

Radiators such as 6LiF or 10B can be used.

Albedo neutrons can be detected with this type of converter.

Converters are layers upon or incorporated into charged particle detectors.

Secondary charged particle energy deposition allows discrimination against intrinsic noise and photons.


Detection of higher energy neutrons

Above several tens of keV recoil protons from elastic scattering in hydrogen play the important role in generating dose equivalent in tissue.

Recoil protons from hydrogenous converters can be detected in this energy range.

A 20mm (CH2)n converter will provide an acceptable dose equivalent response.


Illustration of a Si diode neutron dosimeter


Cross section of a silicon detector

Silicon substrate

Radiator

Air gapDead layer

SiO2

≥2 mm

~2 mm

~4 μm

~300 μm

n n nγγ

Positive traps

Holes

ElectronsComptonelectrons

Protons


Energy dependence

Range of low-energy recoil protons is short. Detector “dead layer" between converter and

sensitive layer reduces the proton response. Dead layers (~ 50 nm to 300 nm) result from:

Construction of junction devices requiring a surface electrode, or

Basic physics of devices which may result in a surface undepleted layer.

Typical noise levels correspond to energy depositions of 10 keV to 20 keV in silicon.


Energy dependence

At low proton energies, pulse energy is similar to that deposited by photon interactions.

Typical sensitive layer thicknesses are of a few tens to a few hundreds of m.

Silicon ranges of 50 keV and 100 keV electrons are, respectively, 24 mm and 78 mm LETs are 1.2 keV mm-1 and 0.76 keV mm-1.

Result many non-neutron induced pulses of a few tens of keV.


Neutron-photon discrimination

Two-diode devices can be used to subtract the photon component with paired detectors.

With S.S. detectors, the fast neutron detection threshold via recoil protons can be reduced to ~200 keV using just an electronic threshold.

Pulse shape analysis can be used for photon discrimination, but needs special electronics.


Alternate approach to n/γ discrimination

Use of very small volumes ⇒ neutron events confined to smaller volumes than γ events.

Small volumes: Arrange strip or pixel structures ~ few μm

dimensions as arrays on one silicon chip. Anti-coincidence between neighboring

elements suppresses photons. Threshold should be reduced to ~ 100 keV, with

acceptable noise level. Charge-coupled devices (CCDs) would have

smallest volumes and low noise, but dead layers may cause problems.


Example of a silicon based detector


Direct Ion Storage Detectors


DIS detectors are small ion chambers

Information stored as charge trapped on the floating gate of a MOSFET transistor.

Charge in each memory cell can be made fully variable.

Result memory cell used to store analog information. Control gate

Source Drain

Si

Silicon oxide

Floating gate

OxideElectron tunneling pathsAnalog-EEPROM

memory cell


DIS detectors are small ion chambers

Charge on floating gate set by tunneling electrons through the oxide layer.

Charge is permanently stored on the gate. Stored information is read without disturbing

the charge stored, by measuring the channel conductivity of the transistor.

Radiation incident on the oxide layer produces electron-ion pairs but most of the free charge is neutralised before it has a chance to cross the metal-oxide interface.


Cross section of DIS

Source Drain

Si

Opening Floating gate

Fill gas

Modified transistor with ion chamber

OxideElectron tunneling path


DIS detectors for neutron dosimetry

Ion chambers can be made sensitive to neutrons and photons.

DIS for neutron dosimetry requires two chamber system. One chamber with high neutron sensitivity. One chamber with low neutron sensitivity.

Signals must be differentiated.

Photon energy dependence of the chambers must be almost equal.


Dual ion chamber DIS system

A-150 plastic with BN

photons

particles

thermal neutrons

fast neutrons

protons electronselectrons photons

Graphite or Teflon

Ion chamber with Teflon or graphite

Ion chamber with A−150/PE containing BN/LiNO3

Neutron sensitive detector Neutron insensitive detector


SUPERHEATED EMULSION (BUBBLE) DETECTORS


Bubble Damage Polymer Detector

Superheated droplets are suspended in a firm elastic polymer.

Neutrons trigger droplets giving rise to formation sites.

Number of bubbles is a measure of the neutron dose.


Bubble formation steps in superheated emulsions


Halocarbons used in superheated emulsions

Chemical nameEmpiricalformula

Boilingpoint

Tb (°C)*

Criticalpoint

Tc (°C)

1,2-dichlorotetrafluoroethane C2Cl2F4 3.65 145.7Octafluorocyclobutane C4F8 -6.99 115.22Dichlorofluoromethane CCl2F2 -29.76 111.81,1,1,2-tetrafluoroethane C2H2F4 -26.07 101.2Hexafluoropropylene (HFP) C3F6 -29.40 85.0Monochloropentafluoroethane C2ClF5 -39.17 79.9Octafluoropropane C3F8 -36.65 71.95

* At atmospheric pressure (101 kPa)


Trapped Bubbles ~1 mm diam.

SuperheatedLiquid Drops~0.025 mm diam.

Elastic Polymer

(Gel)

Cap

Noise AcousticalTransducer

Anti-Coincidence Circuitry

Counting and Display Circuitry

EventAcousticalTransducer

EventAcousticalTransducer

Superheated Drop detectors use different detection mechanisms

APFEL Liquid MatrixTM

Superheated Drop Detector

Bubble TechnologyBubble Dosimeter

Glass or PlasticTube


Bubble damage polymer detector

Passive readout – optical bubble detection.

Active readout – acoustical detection of bubble formation.

Extremely sensitive to neutrons (in µSv range).

Completely insensitive to gamma rays.

Can be made with neutron energy thresholds from <20 keV to several MeV.


References

BURGESS, P.H., MARSHALL, T.O., PIESCH, E.K.A., The design of ionisation chamber instruments for the monitoring of weakly penetrating radiation, Radiat. Prot. Dosim. 39, No. 3 157-160 (1991) .

D’ERRICO, F. AND MATZKE, M., Neutron Spectrometry in Mixed Fields: Superheated Drop (Bubble) Detectors, Radiat. Prot. Dosim. 107, Nos 1–3, pp. 111–124 (2003).

INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment of Occupational Exposure Due to External Sources of Radiation, Safety Guide RS-G-1.3 (1999).

INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR OFFICE, Occupational Radiation Protection, Safety Standards Series No. RS-G-1.1, IAEA, Vienna (1999).

INTERNATIONAL ATOMIC ENERGY AGENCY, Calibration of Radiation Protection Monitoring Instruments, Safety Series No. 16 (2000).

INTERNATIONAL ATOMIC ENERGY AGENCY, Neutron Monitoring for Radiological Protection, Technical Reports Series No. 252, IAEA, Vienna (1985).

INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Measurement of Dose Equivalents Resulting from External Photon and Electron Radiations, Report No. 47, ICRU, Bethesda, MD (1992).

INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Quantities and Units in Radiation Protection Dosimetry, Report No. 51, ICRU, Bethesda, MD (1993).

INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, General Principles for the Radiation Protection of Workers, Publication No. 75, Pergamon Press, Oxford and New York (1997).


References

KLEIN, H., Neutron Spectrometry in Mixed Fields: Ne213/BC501A Liquid Scintillation Spectrometers, Radiat. Prot. Dosim. 107, Nos 1–3, pp. 73–93 (2003).

KLEIN, H. AND NEUMANN, S. Neutron and photon spectrometry with liquid scintillation detectors in mixed fields. Nucl. Instrum. Methods A476, 132–142 (2002)

KNOLL, G. F. Radiation Detection and Measurement, 3rd edition (New York: John Wiley) (2000).

Nakamura, T., Nunomiya, T. and Sasaki, M., Development of active environmental and personal neutron dosemeters, Radiat. Prot. Dosim. 110, Nos 1-4, pp. 169-181 (2004).

TAGZIRIA, H. AND HANSEN, W., Neutron Spectrometry in Mixed Fields: Proportional Counter Spectrometers, Radiat. Prot. Dosim. 107, Nos 1–3, pp. 95–109 (2003).

Active methods of neutron detection

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