Neutron Detectors

10/13/2010
General
Challenges 

• Neutron Energy, Quality Factor and Dete

Response Detector efficiency, as well as the neutron 

factor (radiation weighting factor) vary

General

factor (radiation weighting factor), vary

significantly with neutron energy. Neutron fields typically include a wide ranneutron energies.
Challenges 

• Gamma Rays

Where there are neutrons, there are alwagamma rays. 

General

Some neutron detectors can respond to grays if they are not set up properly (e.g., uhigh an operating voltage).
Challenges 

• Pulsed Neutron Fields

Neutron fields around accelerators are uspulsed.

General

Pulsed neutrons can be impossible to mewith some types of active detectors systePassive monitors (e.g., dosimeters) are commonly used to evaluate pulsed neutr
Detector Response and Neutron Energy 

• Unmodified neutron detectors (e.g., BF

3 o

usually respond to slow or fast neutrons, bboth. 

General

• Slow neutron detectors are far more comm• A slow neutron detector can be modified 

responds to both slow and fast neutrons. 

• It can even be modified so that it only resp

fast neutrons! 
Detector Response and Neutron Energy 

• Surrounding a slow neutron detector with 

appropriate thickness of a moderator (e.gpolyethylene) will slow some of the fast nedown to ener

gies that the detector can res

General

g 

• The moderator increases the detector res

fast neutrons, but reduces the response to neutrons.

• If the moderator is too thick, all of the neu

be moderated and absorbed before they creach the detector.
Detector Response and Neutron Energy 

• Since slow neutrons (<0.5 eV) cannot pen

cadmium, surrounding the moderator with cadmium means that the detector will only res

pond to fast neutrons!

General
Detector Response and Neutron Energy 

Bare detector Moderator Cadmiu

General

Responds 

only to slow 

neutrons

Responds to 

fast and slow 

neutrons

Respon

only to f

neutron
Detector Response and Neutron Energy 

• The thickness of moderator that produces 

highest count rate depends on the neutrospectrum.

General• A detector with a thin moderator has a hig

detection efficiency for low energy neutro

• A detector with a thick moderator has a h

detection efficiency for high energy neutro
0.01 MeV

1.0 keVThermal10 MeV1.0 MeV0.1 MeV

Air Polyethylene mode
Detector 

0.01 MeV1.0 keVThermal

Air Polyethylene mode

10 MeV1.0 MeV0.1 MeV
Detector 

0.01 MeV1.0 keVThermal

Air Polyethylene mode

10 MeV1.0 MeV0.1 MeV
Detector 

0.01 MeV1.0 keVThermal

Air Polyethylene mode

10 MeV1.0 MeV0.1 MeV
Detector Types
General

There are many different types of neutron detectors. In part, this is because many of them have specialized (i e limited) applicationDetector Typesspecialized (i.e., limited) application.
General

Neutron detectors are designed to measure the following:

1. Fluence rate (e.g., n/cm2 s)

Detector Types

2. Dose equivalent rate (e.g., mrem/hr, m3. Energy spectrum4. Dose equivalent (e.g., mrem, mSv)
1. Detectors to Measure Fluence Rate

The following might read out in n/cm2/s

• Boron-lined proportional counters• Fission counters

S lf dd t t

Detector Types

• Self-powered detectors

• Compensated ion chambers• Proton recoil scintillators• Long counter• Lithium iodide scintillators• Lithium glass scintillators
2. Detectors to Measure Dose Equivalen

The following might read out in mrem/hr or 

• Rem ball• Snoop

y

Detector Types

• Leake detector• Dineutron• REMbrandt• Tissue equivalent proportional counte
3. Detectors for Neutron Spectroscopy

These detector systems are used to measuenergy of the neutrons:

• Bonner spheres

ROSPEC

Detector Types

•

• Threshold activation foils• Bubble detector spectrometer
4. Neutron Dosimeters

The following can be used to measure the dpersonnel in mrem or mSv:

• TLDs

• Track Etch

Detector Types

Track Etch

• Neutron track emulsion• Film• Bubble/superheated drop dosimeters• Proton recoil detectors• Fission track dosimeters• Electronic (diode) detectors
BF3 and He-3 Detectors
General

• Many of the detector types listed in the p

section (Detector Types) employ:

1. BF3, or 

BF3 and He-3 Detectors

2. He-3 tubes.

• BF3 and He-3 tubes operate in the propo

counting mode.
1. Boron Trifluoride (BF

3) Detectors

• A typical BF

3 detector consists of a cylind

aluminum tube filled with a BF

3 fill gas. 

• The boron trifluoride:

BF

3 and He-3 Detectors

• The boron trifluoride:

- functions as a proportional fill gas- undergoes an n-alpha interaction with 

neutrons: B-10 (n, 

α) Li-7 

B-10 + n 

ÿ Li-7 + 

α
1. Boron Trifluoride (BF

3) Detectors

10BF

3

A slow neutron combines with the B-10 atom of a BF3 gas molecule

BF

3 and He-3 Detectors

n

B-10

10BF

3

molecule.
1. Boron Trifluoride (BF

3) Detectors

The reaction results in a Li-7 atom and an alpha particle heading off in opposite

BF

3 and He-3 Detectors

Li-7

heading off in opposite

directions. As they travel through the BF3 gas, they ionize it to 

create ion pairs.
2. He-3 Detectors

• He-3 detectors are more popular than BF

detectors because the gas is less hazardAmong other things, this facilitates shippi

BF3 and He-3 Detectors

• Unfortunately, the smaller pulses of the H

detectors make them poorer than BF3 de

rejecting gamma rays.

• A typical He-3 detector consists of a cylin

aluminum tube filled with helium at a presseveral atmospheres. 
2. He-3 Detectors

The helium gas accomplishes two things: 

• it functions as the proportional fill gas

BF3 and He-3 Detectors

• it undergoes an n p interaction with th

neutrons: He-3 (n, p) H-3 

He-3 + n ÿ H-3 + p
2. He-3 Detectors

3He

A slow neutron combines with the He-3 atom. 

3He

BF3 and He-3 Detectors

n

3He

3He
2. He-3 Detectors

The reaction results in a H-3 atom and a proton heading off in opposite directions. 

BF3 and He-3 Detectors

H-3

As they travel through the He-3 gas, they ionize it to create ion pairs.
Neutron Dose Equivalent R

Measurements
General

• Several different detector types can be em

to measure the neutron dose equivalent. presentation considers:

• Moderator-type Rem detectorsDose Equivalent Rate Measurem

type Rem detectors

• Tissue Equivalent Proportional Counters

- REM 500- Single detector: Hankins-type

Anderson-BraunLeake-type

- Dual detector: Dineutron
General

• The dose equivalent (H) per neutron fluen

(i.e., rem per neutron) varies with the neuenergy because both the absorbed dose the quality factor/radiation weighting facto

)

Dose Equivalent Rate Measurem

wR vary with the neutron energy.

H = D Q = D wR

• Due to a combination of factors, the dos

equivalent per unit fluence (e.g., rem or sper neutron/cm2) as a function of neutron 

energy shows the following relationship.
Unit Fluence

Dose Equivalent per Unit Neutron Fluen

per neutron) as a Function of Ener

ICRP 21 Recommended C

Neutron Energy (MeV) 

Relative H per U

0.001 0.01 0.1 1 10 
General

• It turns out that it is possible to design a 

moderated detector whose counting effic(counts per neutron) varies with neutron ein the same way that the dose equivalent 

Dose Equivalent Rate Measurem

y q

unit neutron fluence varies with neutron e

• Such an instrument can be calibrated to r

in units of dose equivalent (or rate). 

• This requires a judicious choice of moder

thickness, and possibly the incorporation neutron absorbers to “tweak” the respons
General

• The two most widely used moderated neu

rem meters are the:

• Hankins Rem Ball

Dose Equivalent Rate Measurem

• Andersson-Braun rem meter (“sno

• Even though they operate in the pulse m

they can be used to measure dose equivrates in the pulsed fields around accelerbecause the neutron moderation broadeduration of the pulses.
Hankins Rem Ball

• Hankins’ original design consisted of a 

cadmium loaded 9 inch polyethylene modhousing a small BF3 detector. Its respons

(counts per neutron) as a function of neut

Dose Equivalent Rate Measurem

(p )energy approximated the neutron fluence dose equivalent conversion factors. 

• Such an instrument could be calibrated to 

indicate neutron dose equivalent without need to determine the neutron spectrum.
Hankins Rem Ball

• The cadmium loading consisted of a 0.00

cm thick cadmium foil covering a 6 cm innpolyethylene sphere.

Dose Equivalent Rate Measurem• Current versions of the Hankins design:

• Ludlum Model 12-4• Eberline NRD detectors
Ludlum Model 12-4 Neutron Counter

• Detector: BF3 or He-3 surrounded by 9 inch (22

cadmium loaded polyethylene sphere.

• Weight: 21 lbs.• Readout: mrem/hr• Dose rate: 0-10,000 mrem/h 

Dose Equivalent Rate Measurem

0-100 mSv/h

r

• Sensitivity: ca. 30 cpm per mrem/hr.

ca. 50 cps per mSv/hr.

• Gamma response: <10 cpm at 10 R/hr.

<10 cpm at 0.1 Sv/hr

• Temperature range: -20 to +50°C.• Battery life: 600 hours.
Eberline E-600/NRD

• Detector: BF3 or He-3 surrounded by 9 inch (22

cadmium loaded polyethylene sphere.

• Weight: 18 lbs.• Energy range: 0.025 eV-10 MeV• Readout: mrem/hr mrem

Dose Equivalent Rate Measurem

Readout: mrem/hr, mrem

• Dose rate: 0-10,000 mrem/h 

0-100 mSv/hr

• Sensitivity: ca. 45 cpm per mrem/hr.

ca. 75 cps per mSv/hr.

• Gamma response: rejection up to 500 

R/hr.

• Temperature range: -20 to +50°C.
Hankins “Rem Ball”

Dose Equivalent Rate Measurem

Ludlum 12-4 

Neutron Counter
Meter electr

Cadmium ca

Hankins “Rem Ball”Dose Equivalent Rate Measurem

Cadmium 

surroundin

spherical mo

(6 cm diame

detect

Outer polye

spherical m
“Snoopy” Andersson-Braun “Rem Mete

• In 1963, Andersson and Braun designed 

neutron rem meter with a cylindrical polyemoderator - 21.6 cm diameter by 24.4 cm (Model NRC III). These devices employ a 

Dose Equivalent Rate Measurem

cylindrical BF

3 or He-3 tube. 

• To reduce the instrument’s over-response 

energy neutrons, they incorporated a borplastic sleeve (7.6 cm diameter) around tdetector tube and inner moderator.
Andersson-Braun “Rem Meter”

• Current versions of the Andersson-Braun 

“snoopy” design:

Al (St d ik) 2202

Dose Equivalent Rate Measurem

• Alnor (St

u

dsvik) 2202

• NE Technology NM1, NM2• Eurisys Mesures NG2A
Eurisys Mesures/Canberra/APTEC/NRC 

• Neutron Detector: BF

3 or He-3 detector with cyl

polyethylene moderator. 

• Gamma detector: GM• Weight: 25 lbs• Readout: mrem/hr, mrem, Sv/hr, Sv, uR, uR/hr 

D t 10 10 /h (0 1

S /h 0 1

Dose Equivalent Rate Measurem•

Dose ra

te: 10 urem

– 10 rem/h (0.1 u

S

v/hr 

–

0.1

• Energy Range: 0.025 ev to 15 MeV (neutrons)• Sensitivity: ca. 6000 counts per mrem (600 coun• Gamma response: no response up to 500 R/hr (• Temperature range: -20 to +50°C• Battery life: 40 hours• GM detector gamma response: 0.1 uR/hr – 10 R

1 nSv/hr – 0.1 S
Andersson-Braun Rem Meter

Dose Equivalent Rate Measurem

EURISYS MESURES “Snoopy” NG-2A 
BF3 or He-3 Tube

Perforated borated plastic Polye

sleeve
B

Andersson-Braun Rem MeterDose Equivalent Rate Measurem

Inner polyethylene 

moderator

End section of perforated 

borated plastic sleeve
Andersson-Braun Rem Meter

Dose Equivalent Rate Measurem
Leake Rem Meter

• In 1966 Leake described a rem meter tha

some of the features of both the AnderssoBraun detector and the Hankins detector.

Dose Equivalent Rate Measurem• The original version employed a 20.8 cm 

diameter spherical polyethylene moderatoa lithium iodide scintillator.
Leake Rem Meter

• Later versions replaced the lithium iodide 

detector with a spherical He-3 detector.

• The detector is surrounded by an inner (5Dose Equivalent Rate Measurem

diameter) and outer (20.8 cm diameter) spherical polyethylene moderator. A 1 mm cadmium shell perforated with holes is locbetween the inner and outer moderators. 
20.8 cm 

polyethylene 

sphere

Pe

cadm

S

H

Leake Rem MeterDose Equivalent Rate Measurem

Polyethylene
Leake Rem Meter

Dose Equivalent Rate Measurem

Polyethylene moderator Spherical He-3 Detec
• Current versions of the Leake-type neut

detector:

• Berthold LB 6411

Leake Rem Meter

Dose Equivalent Rate Measurem
Berthold LB 6411 Leake-type Meter

• Detector: cylindrical He-3, with 

25 cm spherical polyethylene moderator and perforated cadmium moderator.

Dose Equivalent Rate Measurem

• Energy range: 50 keV – 10 

MeV (±30%).

• Sensitivity: 3 counts/nSv 

44 cpm per uSv/hr.

• Designed to measure H*(10) as recommended • Lower over-response than standard Leake dete

intermediate energy neutrons, with increased s
Problems with Typical Instruments

• The instrument’s accuracy depends on th

neutron spectrum.

• The manufacturer’s specifications for accDose Equivalent Rate Measurem

are not always accurate. 

• They can be very heavy, awkward and ev

dangerous to carry in areas where movemlimited, e.g., on ladders.
Problems with Typical Instruments

• In fields dominated by scattered neutrons

significant component of the dose might bintermediate energy neutrons (10 to 100 

where many of these detectors over

-resp

Dose Equivalent Rate Measurem

where many of these detectors over resp

• Maximum over-response can be expecte

keV. Over-responses seen by a factor of to 30 keV neutrons.

• Significant under-respond to neutrons ab

MeV (e.g., by a factor of 2 for 10 MeV ne
e per Unit Fluence 0.4 Relative Fluence Response ICRP 21 Recommended C

Dose Equivalent Rate Measurem

Neutron Energy (MeV) 

Relative Response

0

0.2

0.001 0.01 0.1 1 10 

Rembal

”Snoopy” re
Problems with Typical Instruments

Newer versions of these rem-meters (WENLINUS) have largely eliminated the over-resat the lower energies and the under-responhi h iDose Equivalent Rate Measuremhi

g

her energies.

The latter was accomplished by adding a hemetal (lead or tungsten) to the detector modThe high energy neutrons interact with the (n,2n) and (n, 3n) reactions to produce loweneutrons that are detected with greater effic
Problems with Typical Instruments

Prescila is a neutron scintillator rem-meter developed at Los Alamos and commercializLudlum. It has better energy response (up tM V) th th t d d A d B Dose Equivalent Rate MeasuremM

e

V

) than the s

tan

dar

d

A

n

derssonBraun a

Hankins design and is much lighter. Its gamma rejection is not as good as that odetector. In addition, it can have problems evaluating pulsed neutron fields.
Angular Dependence Problems with “Sno

• Highest reading when irradiated from the 

the cylinder.

• Angular response is not as good as with s

Dose Equivalent Rate Measurem

Angular response is not as good as with s

detectors (moderator-type or TEPC). In thcase, an under-response of 35% was noteMeV neutrons.
e per Unit Fluence 1.0

Angular Response of a “Snoopy

Andersson-Braun Meter

15

11

Orientation 

Relative Response

0

0.5

90 75 75 60 60 45 0 

30 30 15 15 45

1 
Angular Independence:

Good Andersson

Dose Equivalent Rate Measurem

Better

Best

Hankins

Leake
Eurisys Dineutron Meter

• The neutron energy spectrum is characte

analyzing the ratio of the counts obtained moderators of varying thickness.

Dose Equivalent Rate Measurem• Once the spectrum has been characterize

effective quality factor (radiation weightincan be determined. The measured absordose (D) can then be converted into the dequivalent (H).
Eurisys Dineutron Meter

• Detector: Two He-3 detectors. One is surrounde

spherical 2.5 inch diameter polyethylene modera

other is surrounded by a 4.2 inch diameter sphemoderator.

• Weight: 3.5 kg.

Dose Equivalent Rate Measurem• Energy range: 0.025 eV – 15 MeV (±30%)• Readout, Sv, Gy, rem, rad, also quality factor• Dose rate: 0.01 - 9999 mrem/h

0.1 uSv/hr - 99.99 mSv/hr

• Dose: 0.0001 - 9999 mrem

1 nSv – 99.99 mSv

• Temperature range: -10 to +55°C• Battery life: 25 hours
• The calculation of dos

equivalent involves the the count rates in the tdetectors.

Eurisys Dineutron Meter

Dose Equivalent Rate Measurem

• The ratio is referred to 

spectral index (I):

Where C1 is the count rate from the smaller c

C

2 is the count rate from the larger ch
Eurisys Dineutron Meter 

• The dose equivalent (H) is calculated as 

The constant K is a function of the spect

Dose Equivalent Rate Measurem

• The constant

K

h is

a function of the spect

• The constants a and b were determined 

empirically by measuring the spectral indthe dose equivalent in neutron fields at npower plants.
Far West Technology REM 500

Dose Equivalent Rate Measurem
• Detector: Rossi-type spherical tissue equ

proportional counter (TEPC), 2.25 inch indiameter – it mimics a 2 um sphere of tiss

For details about TEPCs see Appendix

Far West Technology REM 500Dose Equivalent Rate Measurem

For details about TEPCs, see Appendix.

• Wall: A 150 TE plastic, 1.2 mm thick• Housing: aluminum can, 0.065 inch thick• Fill gas: propane• Internal 0.3 uCi Cm-244 alpha source (lic

required) 
• Dose rate: 0.1 mrem/hr to 100 rem/hr

1 uSv/hr to 1 Sv/hr

• Response: ca. 480 counts per mrem

48 counts per uSv

E 70 k V t 20 M V

Far West Technology REM 500Dose Equivalent Rate Measurem

•

Energy response: 70

k

e

V

to 20

M

e

V

• Angular dependence: ± 10% through 270 

degrees

• Battery life: ca. 100 hrs.• Weight: 5 lbs, 2 oz (without batteries)• Temperature response: ± 10% 15 to 45 °C

± 20% -15 to 50 °
• After a count has been performed, the pu

height distribution (a spectrum) is determ

• Pulse height is related to the “lineal energ

energy deposited by a single particle in a

Far West Technology REM 500Dose Equivalent Rate Measurem

energy deposited by

a single particle in

a

specified volume divided by the average length of the particles through that volum

• Lineal energy is related to the quality fact• The larger the pulse, the larger the lineal 

the greater the quality factor and the highchannel number into which the pulse is so
• Employs 256 PHA. Pulses in channels 1-

assumed to be due to photons or noise.

• Pulses from an internal Cm-244 alpha so

are set in channel 90 to calibrate the s st

Far West Technology REM 500Dose Equivalent Rate Measurem

are set in channel 90 to calibrate the

s

yst

with lineal energy of 90 keV/um.

• 10 keV – 1 MeV neutrons have maximum 

energy around 85 – 90 keV/um. Above 1 the lineal energy decreases.
• Equation used to calculate dose equivale

Far West Technology REM 500Dose Equivalent Rate Measurem

Where K is the calibration factor

TC is a time constant (depends mode of operation, e.g.,1 for 10 second integration step)Q, the quality factor, is a functiothe channel number
• Examples of quality factors employed by 

REM 500 to calculate dose equivalent (H

Ch.# Q

1 0.4

Ch.# Q

70 15.3

Ch.# 

140 2

Far West Technology REM 500

Dose Equivalent Rate Measurem

10 3.220 5.730 7.840 9.950 11.860 13.6

80 17.090 18.3

100 19.6

110 20.7

120 21.5130 22.4

150 2160 2170 2180 2190 2

>190 2
Neutron Energy Spectrum 

Measurements
Energy Spectrum Measuremen

General• Determining the energy spectrum of the 

is important because the response of nesurvey instruments and dosimeters varieneutron energy.

• Almost all neutron spectroscopy systems 

the use of multiple detectors. The differedetectors respond differently with neutron 

• The most common method is the use of 

Spheres 
Bonner Spheres

• A Bonner Sphere system consists of a Li

BF3 detector in conjunction with a set of 

polyethylene spherical moderators. 

Energy Spectrum Measuremen• A typical set consists of 6 to 8 polyethyle

spheres of different sizes. In some sets, sphere (3”) is covered with cadmium. Typical sizes: 2", 3", 5", 8", 10" and 12
Bonner Spheres

• A count is performed with each of the sph

around the detector.

• The higher the neutron energy (the fasterEnergy Spectrum Measuremen

g gy ( 

larger the sphere must be for the neutronslowed down and detected.

• Larger spheres reduce the detector’s res

to low energy (slow) neutrons because thwill be absorbed by the hydrogen in the sbefore reaching the detector.
Example of Detector Response to Different 

Neutrons with Large and Small Sphere

1000 counts 100 c

100 counts 100

SLOW NEUTRONS FAST NEUTR
Bonner Spheres

• The ratio of the counts obtained with the 

sized Bonner Spheres carries the informabout the energy of the neutrons.

Energy Spectrum Measuremen• Computer Codes such as “BON” must be 

to fully interpret the results and determinneutron spectrum. This is sometimes refeas deconvoluting the spectrum.
Lithium iodide (Li

scintillator detecto

Bonner Sphere

Energy Spectrum Measuremen
Activation Foils

• Threshold activation foils can be used to 

general information about the neutron spThis approach takes advantage of the fathere can be an effective threshold, (i.e., 

Energy Spectrum Measuremen

,( ,

minimum energy) for a given neutron inteto occur. 

• After a set of different activation foils has 

exposed to neutrons, they are analyzed factivation products indicated in the follow
Example Activation Foils

Effective Threshold 

(MeV)

Target 

Material

React

0.5 Indium In-115 (n, n

Energy Spectrum Measuremen

2.0 Zinc Zn-64(n,p) 3.8 Aluminum Al-27(n,p)4.9 Aluminum Al-27(n, alp6.0 Magnesium Mg-24(n,p8.6 Gold Au-197(n,2n
ROSPEC Rotating Neutron Spectromete

• Uses four spherical proportional counters

2" chambers filled with hydrogen at 3/4, atmospheres, and one 6" diameter cham

with argon

-methane at 5 atmospheres

Energy Spectrum Measuremen

with argon methane at

5 atmospheres. 

• The useful energy range (50 keV to 4.5 M

typical of the degraded spectra encountenuclear power plants and other nuclear f

• The detectors are rotated about a commo

order to “average out” the field. 
ROSPEC Rotating Neutron Spectromete

• Pulse height analysis is 

performed on the signals from all four detectors and the data is unfolded b

y the 

Energy Spectrum Measuremen

ROSPEC software to produce a neutron spectrum.

• The software also calculates 

the dose rate, KERMA, dose equivalent and ambient dose equivalent (H*(10)). 
Neutron Dose Equivalen

Measurements

(dosimeters)
General

• Most dosimeters are of the albedo type - t

on the body to moderate and reflect neutr

• An albedo dosimeter should be worn closDose Equivalent Measurements

body. Wearing albedo dosimeters over loclothing so that the body-dosimeter separtoo great is a common problem. 

• When used as area or environmental dos

they must be placed against a thick slab ohydrogenous material e.g., 8" x 8" x 4" luc
Types of Neutron Dosimeters

• Thermoluminescent dosimeters (TL• Track etch• Nuclear Track Emulsions (NTA)• Film

Dose Equivalent Measurements

• Bubble dosimeters (superheated dr• Proton recoil detectors• Fission track detectors• Electronic (diode) detectors
Thermoluminescent Dosimeters

• In the Panasonic badge employed at man

nuclear power plants, the response of a csulfate element (responds only to gamma subtracted from the res

ponse of a lithium 

Dose Equivalent Measurements

element (responds to neutrons and gamm

• The difference in their response is multipl

factor that accounts for the effect of the faneutron spectrum on the dosimeter respo
Thermoluminescent Dosimeters

• In the Harshaw badge, the TLD-700 elem

(enriched in Li-7) only responds to gammThe TLD-600 element (enriched in Li-6) rto both 

gammas and neutrons. The res

po

Dose Equivalent Measurements

g p

the former is subtracted from the responslatter to determine a net response to neut

• The difference in their response is multipl

factor that accounts for the effect of the faneutron spectrum on the dosimeter respo
Track Etch

• The dosimetry method employed by Land• The dosimeter consists of CR-39 plastic (

diglycol carbonate) in contact with a “radi

Neutrons interacting in the radiator produ

Dose Equivalent Measurements• Neutrons interacting in the radiator produ

particles that create damage tracks in the 

• Following a neutron exposure, the CR-39 

etched in sodium hydroxide for 15 hours tthe damage tracks visible under microsco

• The number of damage tracks produced 

neutrons in the CR-39 is related to the do
Track Etch

Dose Equivalent Measurements

CR-39

Radiator

• For fast neutron dosimetry, the radiator is polyeth

recoil protons from the radiator produce the damain the CR-39.

• For slow neutron dosimetry, the radiator employs 

loaded Teflon. The alphas produced by interactionboron produce the damage tracks in the CR-39.

Appearance of damage 

Cr-39 as seen under a 

Arrangement of radiator and Cr-39.
Track Etch

Dose Equivalent Measurements

Fast neutrons

B L dd

Slow neutron

p+

Radiator

p+ CR-39 "+ CR-39 

Boron Loaded

Radiator
Nuclear Track Emulsions

• Not used nearly as much today as in the p•The most common nuclear track emulsion 

film is Kodak Type A film.

Dose Equivalent Measurements• Neutrons strike the hydrogen nuclei in the 

emulsion. The recoil protons (nuclei) travethrough the emulsion transferring energy tsilver bromide crystals. 
Nuclear Track Emulsions

• When the film is processed, the image of t

tracks become visible under a microscopenumber of tracks per unit area of film is determined and related to the dose e

quiva

Dose Equivalent Measurements• Relatively insensitive to neutrons below 0• They tend to over respond to neutrons ab

MeV, e.g., the over response to 14 MeV necan be as much as 100%! 
Film

• Landauer manufactures a film dosimeter 

measure the dose due to thermal neutronfilm is partially covered with a cadmium ficapture of thermal neutrons by the cadmi

Dose Equivalent Measurements

p y

results in the emission of prompt gamma expose the film. Uncovered portions of thused to correct for the gamma backgrounspecific application of this dosimeter mighmeasure personnel exposures in the vicingraphite moderated neutron source. 
Film

n

Dose Equivalent Measurements

Respon

gamm

Responds to gamma 

rays and neutrons

Cadmium

Film

Capture gammaLandauer Neutron Film Dosimeter
Bubble Dosimeters/Superheated Drop De

• Not widely used, in part due to limited she• Bubble dosimeters consist of a few mls o

gelatinous substance inside a plastic/glas

Dose Equivalent Measurements• Thousands of tiny droplets of an organic 

(e.g., Freon) are suspended in the gelatinmatrix. The pressure inside the vial is kethe vaporization point of the fluid. 
Bubble Dosimeters/Superheated Drop De

• High energy neutrons scatter off hydroge

to produce recoil protons which transfer tenergy to the liquid droplets. 

Dose Equivalent Measurements• To increase the system’s sensitivity for low 

neutrons, an element (often chlorine) is athat produces a charged particle when it iwith these low energy neutrons, e.g.,

Slow neutron + Cl-35 

→ p + 
Bubble Dosimeters/Superheated Drop De

• When energy is transferred to the liquid d

the latter are converted into a gas. This rethe formation of tiny (ca. 1 mm) bubbles imatrix. 

Dose Equivalent Measurements• The number of bubbles, which can be det

manually or with an automated counter, is to the absorbed dose. Various sensitivitieavailable ranging from about 0.5 to 25 bumrem (50 to 2500 bubbles per mSv)
Bubble Dosimeters/Superheated Drop De

Dose Equivalent Measurements
Using the Bonner Sphere 9” to 

Count Ratio to CorrectCount Ratio to Correct

Dosimeter Response
Using the Bonner Sphere 9” to 3” R

Correct Dosimeter Response• The response of many neutron dosimeter

decreases as the energy of the neutrons increases. 

• Some time ago, Dale Hankins suggested 

relatively simple method to correct for the dependence of the dosimeter response othat involves performing counts with a 9 3 inch cadmium covered Bonner sphere.
Using the Bonner Sphere 9” to 3” R

Correct Dosimeter Response• The ratio of the counts performed with the 9 inch 

inch spheres increases with neutron energy:

Energy 

Ratio
Using the Bonner Sphere 9” to 3” R

Correct Dosimeter Response

• On log log paper, 

the dosimeter res

ponse 

plotted 

p p

against the 9 to 3 ratio typically forms a straight line
Appendix 

Tissue Equivalent ProportionTissue Equivalent Proportion

Counters (TEPC)
Proportional Counter vs Ion Chamb

• A proportional counter is essentially the sa

an ionization chamber except that the pulslarger.

• In 

an i

onization 

c

h

a

mbe

r, o

n

e

elect

r

on rea

a o at o c a be , o e e ect o ea

the anode for every primary ion pair produthe gas. In a proportional counter, 100 to electrons reach the anode for every primapair produced in the gas. 
Proportional Counter vs Ion Chamb

• The pulses in an ionization chamber are u

too small to be counted. As such, an ion chamber almost always operates in the cumode, i.e., it measures the current from th

c

h

a

mber.

c a be

• A proportional counter typically operates i

pulse mode, i.e., it counts the number of p

• It is also common to measure the size of t

individual pulses.
Proportional Counter vs Ion Chamb

• By measuring the size of the pulses, the e

deposited in the detector gas by individuaevents (gamma ray or neutron interactionbe determined. This is not possible with a

c

h

a

mber.

c a be

• The larger size of neutron pulses allows th

be distinguished from the smaller pulses produced by gamma rays. This is not poswith an ion chamber.
Tissue Equivalent Proportional Cou

General • TEPCs have been used for many years i

microdosimetry wherein the dose or dose equivalent to small volumes of tissue (e.g

is measured.is measured.

• TEPC use in routine radiation protection 

more recent development.

• A big advantage is that a TEPC can sepa

dose due to gamma rays and neutrons bheight discrimination in mixed fields.
Tissue Equivalent Proportional Cou

General • However, pulse height discrimination for 

neutrons below 100 keV is not very satis

• The main reason is that the range of low 

The main reason is that the range of low 

recoil protons (hydrogen nuclei) in the dewall is short.
Tissue Equivalent Proportional Cou

Typical Construction - Rossi Chamber• The Rossi Chamber is a spherical chamb

employing tissue equivalent walls and a equivalent fill gas. It operates as a propo

counter

.

• The sphere’s response is more or less 

independent of the direction from which tradiation comes (i.e., angularly independ

• Using a wire running along the diameter 

sphere complicates this.
Tissue Equivalent Proportional Cou

Typical Construction - Rossi Chamber• The electric field can become distorted to

the ends of the anode where it gets close spherical detector wall (the cathode). Bethis distorted field, gas multi

plication is 

,g p

decreased for the ionizing events that octowards the ends of the anode. 

• To prevent this and create a cylindrically 

symmetrical electric field along the length anode, the Rossi chamber surrounds the with a fine helical wire. 
Ross

Gas Inlet

Helix

A d

Gas

Outlet

Anode

Anode contact

Cathode

contact
Tissue Equivalent Proportional Cou

Typical Construction - Rossi Chamber• A problem with this design is the fact that 

is susceptible to vibration

S h ib ti lt i i hi i 

•

Suc

h vibration results in microp

honic nois

spurious counts. Since cylindrical chamberequire a helix, they are more rugged as wbeing easier to construct. 
Tissue Equivalent Proportional Cou

How They Measure Dose Equivalent • The size of a pulse reflects the ionization 

produced in the gas by the particle. As sureflects the absorbed dose to the gas per 

(e.g., per neutron).
Tissue Equivalent Proportional Cou

How They Measure Dose Equivalent• By measuring the size of each pulse, we c

measure the absorbed dose (D) per particthe gas. As such, we could calculate the dequivalent (H) per particle if we could esti

th lit f t (Q)the quality 

fac

tor (Q).

• One way to estimate Q is to calculate the 

the particle – Q is derived from the LET.

• Because of the following relationship, we 

determine the LET of the particle if we knepath length in the detector gas.
Tissue Equivalent Proportional Cou

How They Measure Dose Equivalent• The energy transferred (imparted) to the g

particle is:

, = LET x l or LET = 

,/

, is the energy imparted to the detector g

particle (e.g., keV).LET is the average linear energy transfer particle over the path length (e.g., keV/uml is the particle path length (um).
Tissue Equivalent Proportional Cou

How They Measure Dose Equivalent• While the energy transferred (imparted) to 

per particle (

,) can be measured, we don’

the path length (l) so we can’t determine t

• However, if the detector chamber has a kn

geometry, we can calculate the average p

length of the particles traversing the cham

• The average path (chord) length of particl

traversing a sphere is easy to determine. 
Tissue Equivalent Proportional Co

How They Measure Dose Equivalent

The average path (chord) length for a sphere is: l

1

l

5

lave = (4/3) R

Where R is the sphere radius

1

l

2

l

3

l

4

l

7
• Knowing the average path length of part

traversing a spherical detector chamber, knowing the energy transferred to the gaparticle, allows us to calculate a quantity 

Tissue Equivalent Proportional CoHow They Measure Dose Equivalent

related to the LET: the lineal energy.

• The lineal energy (y) is:• The lineal energy (y), unlike the LET, ca

measured easily.
• According to ICRU 40, “the lineal energy 

quotient of

, by lave, where 

, is the energ

imparted to matter in a volume by a singenergy-deposition event and lave is the m

Tissue Equivalent Proportional CoHow They Measure Dose Equivalent

chord length in that volume”

• Unlike LET, lineal energy (y) is a stochas

quantity – for a given type of particle and energy, the lineal energy varies (due to vpath lengths).
• Even though the value assigned to Q is 

conventionally based on the LET, it is rethat there is good reason to employ the energy to calculate Q.

Tissue Equivalent Proportional CoHow They Measure Dose Equivalent• According to ICRU 40, lineal energy “sho

be more closely related to the biological radiation [than the LET]” and “for purporadiation protection, radiation quality [i.eshould be based on lineal energy in a 1 diameter sphere of ICRU tissue.”
Relationship Between the Quality F

and the Lineal Energy ( ICRU 40)
• Since the energy deposited in the gas by 

particle (

,) is directly related to lineal en

the pulse size is also directly related to y

• Pulse hei

ght anal

ysis on the TEPC out

p

Tissue Equivalent Proportional CoHow They Measure Dose Equivalent

gy p

generates a lineal energy spectrum: the the lineal energy, the higher the channel the pulse is sorted into.

• By using the ICRU or ICRP recommenda

regarding the relationship between the lienergy and the quality factor (Q), a value can be assigned to each channel numbe