Lecture 7 Neutron Detection - MIT OpenCourseWare Counter • Proportional region – Electric field...

Lecture 7Neutron Detection

22.106 Neutron Interactions and ApplicationsSpring 2010

Types of detectors

• Gas-filled detectors consist of a volume of gas between two electrodes

• In scintillation detectors, the interaction of ionizing radiation produces UV and/or visible light

• Semiconductor detectors are especially pure crystals of silicon, germanium, or other materials to which trace amounts of impurity atoms have been added so that they act as diodes

Types of detectors (cont.)• Detectors may also be classified by the type of

information produced:– Detectors, such as Geiger-Mueller (GM) detectors,

that indicate the number of interactions occurring in the detector are called counters

– Detectors that yield information about the energy distribution of the incident radiation, such as NaI scintillation detectors, are called spectrometers

– Detectors that indicate the net amount of energy deposited in the detector by multiple interactions are called dosimeters

Modes of operation

• In pulse mode, the signal from each interaction is processed individually

• In current mode, the electrical signals from individual interactions are averaged together, forming a net current signal

Interaction rate

• Main problem with detectors in pulse mode is that two interactions must be separated by a finite amount of time if they are to produce distinct signals– This interval is called the dead time of the system

• If a second interaction occurs in this interval, its signal will be lost; if it occurs close enough to the first interaction, it may distort the signal from the first interaction– Show figure p.120

Dead time

• Dead time of a detector system largely determined by the component in the series with the longest dead time– Detector has longest dead time in GM counter

systems– In multichannel analyzer systems the analog-to-digital

converter often has the longest dead time• GM counters have dead times ranging from tens

to hundreds of microseconds, most other systems have dead times of less than a few microseconds

Paralyzable or nonparalyzable

• In a paralyzable system, an interaction that occurs during the dead time after a previous interaction extends the dead time

• In a nonparalyzable system, it does not• At very high interaction rates, a

paralyzable system will be unable to detect any interactions after the first, causing the detector to indicate a count rate of zero

τ

τ

Dead Live

Dead Live

Paralyzable

Nonparalyzable

Time Events in detector

Image by MIT OpenCourseWare.

Interaction Rate

50,00040,00030,00020,00010,0000

Cou

nt R

ate

0

2000

4000

6000

8000

10,000

IdealNon-paralyzableParalyzable


Current mode operation

• In current mode, all information regarding individual interactions is lost

• If the amount of electrical charge collected from each interaction is proportional to the energy deposited by that interaction, then the net current is proportional to the dose rate in the detector material

• Used for detectors subjected to very high interaction rates

Spectroscopy• Most spectrometers operate in pulse mode• Amplitude of each pulse is proportional to the

energy deposited in the detector by the interaction causing that pulse

• The energy deposited by an interaction is not always the total energy of the incident particle or photon

• A pulse height spectrum is usually depicted as a graph of the number of interactions depositing a particular amount of energy in the spectrometer as a function of energy

700600500400

Energy (keV)

3002001000

Num

ber o

f Int

erac

tions

32 keV

662 keV

Energy (keV)

7006005004003002001000

100

Rel

ativ

e nu

mbe

r of p

hoto

ns32 keV

662 keV


Detection efficiency

• The efficiency (sensitivity) of a detector is a measure of its ability to detect radiation

• Efficiency of a detection system operated in pulse mode is defined as the probability that a particle or photon emitted by a source will be detected

efficiency Intrinsic efficiency Geometric Efficiencydetector reachingNumber

detectedNumber

emittedNumber detector reachingNumber Efficiency

emittedNumber detectedNumber Efficiency

×=

×=

=

A

aS

Ω

d


Gas-filled detectors• A gas-filled detector consists of a volume of gas

between two electrodes, with an electrical potential difference (voltage) applied between the electrodes

• Ionizing radiation produces ion pairs in the gas• Positive ions (cations) attracted to negative

electrode (cathode); electrons or anions attracted to positive electrode (anode)

• In most detectors, cathode is the wall of the container that holds the gas and anode is a wire inside the container

• Outer shell is a metal cylinder made of either Steel or Aluminum– Filled with a gas

• Inner wire is gold plated tungsten– Tungsten provides strength to the thin wire– Gold provides improved conductivity

Incidentchargedparticle

Anode (+)

Cathode (-)

Battery or power supply

Meter

+ + + + + +-

-- - -

-


Types of gas-filled detectors• Three types of gas-filled detectors in common

use:– Ionization chambers– Proportional counters– Geiger-Mueller (GM) counters

• Type determined primarily by the voltage applied between the two electrodes

• Ionization chambers have wider range of physical shape (parallel plates, concentric cylinders, etc.)

• Proportional counters and GM counters must have thin wire anode

Ionization Counter

• Ionization region– All electrons are

collected– Charge collected is

proportional to the energy deposited

– Called ion chambers

1012

1010

108

106

104

102

10005000

Puls

e H

eigh

t (A

rbitr

ary

Uni

ts)

Applied Voltage

Geiger-MullerRegion

Proportional Region

Ionization ChamberRegion

RecombinationRegion


Ionization chambers• If gas is air and walls of chamber are of a

material whose effective atomic number is similar to air, the amount of current produced is proportional to the exposure rate

• Air-filled ion chambers are used in portable survey meters, for performing QA testing of diagnostic and therapeutic x-ray machines, and are the detectors in most x-ray machine phototimers

• Low intrinsic efficiencies because of low densities of gases and low atomic numbers of most gases

Proportional Counter

• Proportional region– Electric field is strong

enough to create secondary ionization

– Further increases create more ionization in the gas

• Avalanche ionization

– Charge collected is linearly proportional to energy deposited

1012

1010

108

106

104

102

10005000

Puls

e H

eigh

t (A

rbitr

ary

Uni

ts)

Applied Voltage

Geiger-MullerRegion

Proportional Region


RecombinationRegion


Avalanche Ionization

Anode wire

25µm


GM Counter

• Geiger-Mueller region– Saturation of the

anode wire– Avalanche over the

entire wire– Cannot be used for

high count rate applications

1012

1010

108

106

104

102

10005000

Puls

e H

eigh

t (A

rbitr

ary

Uni

ts)

Applied Voltage

Geiger-MullerRegion

Proportional Region


RecombinationRegion

Cathode

Cathode

Anode WireUV Photon

UV Photon

Individualavalanches

e

Images by MIT OpenCourseWare.

GM counters• GM counters also must contain gases with

specific properties• Gas amplification produces billions of ion pairs

after an interaction – signal from detector requires little amplification

• Often used for inexpensive survey meters• In general, GM survey meters are inefficient

detectors of x-rays and gamma rays• Over-response to low energy x-rays – partially

corrected by placing a thin layer of higher atomic number material around the detector

GM counters (cont.)

• GM detectors suffer from extremely long dead times – seldom used when accurate measurements are required of count rates greater than a few hundred counts per second

• Portable GM survey meter may become paralyzed in a very high radiation field – should always use ionization chamber instruments for measuring such fields

Neutron Detection

• Neutral particles– Detection is based on indirect interactions

• Two basic interactions– Scattering

• Recoiling nucleus ionizes surrounding material• Only effective with light nucleus (Hydrogen,

Helium)– Nuclear reactions

• Products of the reaction can be detected (gamma, proton, alpha, fission fragments)

Neutron Detection• Since x.s. in most materials are strongly

dependent on neutron energy, different techniques exist for different regions:– Slow neutrons (or thermal neutrons), below

the Cadmium cutoff– Fast neutrons 105

104

103

102

101

100

10-1

10-110-210-310-410-510-610-710-810-9 100 101

Energy (MeV)

Cro

ss S

ectio

n (b

)


Thermal Neutron Detectors• Two important aspects to consider

– Find a material with large thermal x.s.– Find an arrangement that allows you to

distinguish from gamma rays• High Q-value of neutron capture will make it easier

104

103

102

10-2 10-1 1 101 102 103 104 105 106 107

10

1

10-1

Neutron Energy (eV)

Cro

ss S

ectio

n (b

arns

)

He-3 (n, p)B-10 (n, α)Li-6 (n, α)


10B(n,alpha)• Equations• 94% of the time in excited state

– Q-value of 2.310 MeV– Other 6%, Q-value of 2.792 MeV

B-10

*Li-7

α

n

(excited)

B-10

Li-7

α

n

Alpha kinetic energy = 1.78 MeVLi-7 kinetic energy = 1.02 MeV

Alpha kinetic energy = 1.47 MeVLi-7 kinetic energy = 0.84 MeV


BF3 Tube

B-10αLi-7

n

B-10α Li-7

n

The Li-7 deposits all its kinetic energyin the gas while the alpha particledeposits only a fraction of itsenergy.

The alpha particle deposits all its kinetic energy in the gas while the Li-7deposits only a fraction of itsenergy.

Pulse Size (energy deposited in detector)

# Pu

lses

2.31 MeV

2.79 MeV

1.47 MeV0.84 MeV

Gamma raypulse, noise, etc.



Efficiency of BF3 Tube

• Intrinsic efficiency– Eff(E) = 1- exp(-Sigma(E) * L)

6Li(n,alpha)

• Equation• Only a ground state exists with Q-value of

4.78 MeV– EHe-3 = 2.73 MeV and Ealpha = 2.05 MeV

• Other possible reactions– 3He(n,p)– 157Gd

Gamma-ray sensitivity

3He (2.5 cm diam, 4 atm)

Ar (2.5 cm diam, 2 atm)

BF3 (5.0 cm diam, 0.66 atm)

Al tube wall (0.8 mm)

4He (5.0 cm diam, 18 atm)

Scintillator (5.0 cm thick)

Al tube wall (0.8 mm)

Thermal Detectors

Fast Detectors

Interaction Probability

Interaction Probability

Thermal Neutron

1 - MeV Gamma Ray

1 - MeV Gamma Ray

1 - MeV Neutron

0.77

0.0

0.29

0.0

0.0001

0.0005

0.0006

0.014

0.01

0.0

0.78

0.001

0.014

0.26

Neutron and gamma-ray interaction probabilities in typical gas proportional counters and scintillators


Fission chambers

• Inner surface of gas tube is coated with a fissile material (U-235)– Current mode in reactor operation– Pulse mode elsewhere

• Efficiency of about 0.5% at thermal energies• Combine fissile and fertile material to avoid loss

of sensitivity (regenerative chambers)• Memory effect when used in high flux regions

– Decay of FPs

Fission Chambers104

103

102

100 20 40 60 80 100 120

N(E

)

Energy (MeV)

104

103

102

100 20 40 60 80 100 120

N(E

)

Energy (MeV)

Deposit thickness = 0.0286 µm - 0.0312 cm2

mgDeposit thickness = 0.714 µm - 0.778

cm2

mg


Neutron Spectroscopy

• Bonner Sphere

2" 3" 5" 8" 10" 12" 12" + Pb

12" + Pbinternal part


Long Counter• Flat response for neutrons of all energy

Paraffin BF3_ Counter (12.2" active length) B2O3

20"

8 holes on a 31/2"diameter pitchcircle 6" deep1" diameter

0.018" Cd cap

14"

8" d

iam

eter

9" d

iam

eter

15"

diam

eter

1.0

0.9

0.8

0.7

0.6

0.5

0.40.02 0.04 0.06 0.1 0.2 0.4 0.6 0.81.0 2 4 MeV 6

E

Rel

ativ

e Se

nsiti

vity

0"0.5"

1.0"

3.0"

Counter Position

2.5"

1"


Fast-Neutron induced reaction detectors

• 6Li(n,alpha)– Q-value of 4.78 MeV– Peaks appear at Q-value + neutron kinetic

energy– Lithium based scintillators

1 x 1041 x

10-1

1 x

101

1 x 105 1 x 106 1 x 107

1

2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9

23

45

67

89

23

45

67

89

Energy [eV]

Cro

ss-s

ectio

n [b

arns

]3He [π, ρ] 6Li [π, α]


• 3He(n,p)– Energy range 20 keV to 2 MeV

• Smooth, nearly flat x.s.• Tube is wrapped in Cadmium and Boron layersto

reduce contribution from thermal neutrons• Lead shield is also added to reduce impact of

photons• Intrinsic efficiency is very low (0.1%)• Full energy peak at En + 765 keV• Thermal neutron capture peak at 765 keV• Elastic scattering spectrum with a max at 0.75En

0.764 MeV En + 0.764 MeV0.75 En E

En

dNdE

Epithermalpeak

Recoildistribution

Full-energypeak


Fast Neutron Recoil Detectors

• Most useful reaction is elastic scattering– Recoil nucleus will ionize medium– Most popular target is hydrogen

• Large elastic scattering x.s• Well known x.s.• Neutron can transfer all of its energy

0 1 2 3 4 5 60.1

1

10

100

Fission spectrum shape

4He

1H

Neutron energy (MeV)Sc

atte

ring

cros

s sec

tion

(bar

ns)

in one collision– Q-value is zero

• Information can be found about the incoming neutron energy


Gas recoil detectors

0.40.6

0.81.0

1.2

1.6

2.0

4He Recoil Pulse Height, MeV

4 He

Wei

ght/M

eV(~

Cou

nts/

Cha

nnel

)

00

10

20

30

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4


4 He

Wei

ght/M

eV (~

Cou

nts/

Cha

nnel

)

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

00 1 2 3 4 5 6

4.0

6.0

8.0

Incident Neutron Energy (MeV)




4 He

Wei

ght/M

eV (~

Cou

nts/

Cha

nnel

)

10.012.0

14.0

00

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 2 3 4 5 6 7 8 9


Scintillators• Desirable properties:

– High conversion efficiency– Decay times of excited states should be short– Material transparent to its own emissions– Color of emitted light should match spectral sensitivity

of the light receptor– For x-ray and gamma-ray detectors, μ should be large

– high detection efficiencies– Rugged, unaffected by moisture, and inexpensive to

manufacture

Scintillators (cont.)

• Amount of light emitted after an interaction increases with energy deposited by the interaction

• May be operated in pulse mode as spectrometers

• High conversion efficiency produces superior energy resolution

Materials

• Sodium iodide activated with thallium [NaI(Tl)], coupled to PMTs and operated in pulse mode, is used for most nuclear medicine applications– Fragile and hygroscopic

• Bismuth germanate (BGO) is coupled to PMTs and used in pulse mode as detectors in most PET scanners

Photomultiplier tubes

• PMTs perform two functions:– Conversion of ultraviolet and visible light

photons into an electrical signal– Signal amplification, on the order of millions to

billions• Consists of an evacuated glass tube

containing a photocathode, typically 10 to 12 electrodes called dynodes, and an anode

+100 volts +300 volts

+400 volts+200 volts

Signal topreamp

Visible lightphoton

Glass tube

Anode+500 volts

Photocathode0 volts Dynodes


Dynodes

• Electrons emitted by the photocathode are attracted to the first dynode and are accelerated to kinetic energies equal to the potential difference between the photocathode and the first dynode

• When these electrons strike the first dynode, about 5 electrons are ejected from the dynode for each electron hitting it

• These electrons are attracted to the second dynode, and so on, finally reaching the anode

PMT amplification

• Total amplification of the PMT is the product of the individual amplifications at each dynode

• If a PMT has ten dynodes and the amplification at each stage is 5, the total amplification will be approximately 10,000,000

• Amplification can be adjusted by changing the voltage applied to the PMT

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22.106 Neutron Interactions and ApplicationsSpring 2010

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