Lecture 7 Neutron Detection

22.106 Neutron Interactions and Applications Spring 2010

Types of detectors

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

• I n 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.)

• D etectors may also be classified by the type of information produced:

– D etectors, such as Geiger-Mueller (GM) detectors, that indicate the number of interactions occurring in the detector are called counters

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

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

Modes of operation

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

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

Interaction rate

• M ain 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

– T his interval is called the dead time of the system

• I f 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

• D ead time of a detector system largely determined by the component in the series with the longest dead time

– D etector has longest dead time in GM counter systems

– I n multichannel analyzer systems the analog-to-digital converter often has the longest dead time

• G M 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

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

• I n a nonparalyzable system, it does not

• A t 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



Dead Live

Paralyzable

Events in detector

Time



Dead Live

Nonparalyzable

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10,000

8000

6000

4000

Ideal

Non-paralyzable Paralyzable

2000

0

0 10,000

20,000

30,000

40,000

50,000

Interaction Rate

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Current mode operation

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

• I f 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

• U sed for detectors subjected to very high interaction rates

Spectroscopy

• M ost spectrometers operate in pulse mode

• A mplitude 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

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Detection efficiency

• T h e efficiency (sensitivity) of a detector is a measure of its ability to detect radiation

• E fficiency 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 

Efficiency 

Number Number Number

detected emitted reaching

detector 

Number emitted

Number detected

Number

reaching

detector

Efficiency  Geometric efficiency  Intrinsic efficiency

A

S

a



d

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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

• P ositive 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

Meter

Incident char ged particle

+

-

-

+

-

+ +

+

-

-

-

+

Anode (+)

Battery or power supply

Cathode (-)

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• O uter shell is a metal cylinder made of either Steel or Aluminum – F illed with a gas

• I nner wire is gold plated tungsten

– T ungsten provides strength to the thin wire

– G old provides improved conductivity

Types of gas-filled detectors

• T hree types of gas-filled detectors in common use:

– I onization chambers

– Proportional counters

– G eiger-Mueller (GM) counters

• T ype determined primarily by the voltage applied between the two electrodes

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

• P roportional counters and GM counters must have thin wire anode

Ionization Counter

10 12

10 10

10 8

Recombination Region

Geiger - Muller Region

10 6

Ionization Chamber Region

10 4

10 2

Proportional Region

0

500

Applied V oltage

1000

• Ionization region

– All electrons are collected

– C harge collected is proportional to the energy deposited

– C alled ion chambers

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Ionization chambers

• I f 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

• A ir-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

10 12

10 10

10 8

Recombination Region

Geiger - Muller Region

10 6

Ionization Chamber Region

10 4

10 2

Proportional Region

0

500

Applied V oltage

1000

• P roportional region

– Electric field is strong enough to create secondary ionization

– F urther increases create more ionization in the gas

• A valanche ioniz a tion

– C harge collected is linearly proportional to energy deposited

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Avalanche Ionization

25  m

Anode wir e

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GM Counter

10 12

10 10

10 8

Recombination Region

Geiger - Muller Region

10 6

Ionization Chamber Region

10 4

10 2

Proportional Region

0

500

Applied V oltage

1000

• G eiger-Mueller region

– S aturation of the anode wire

– Avalanche over the entire wire

– C annot be used for high count rate applications

C a t h o d e

e

A n o d e W i r e

I n d i v i d u a l a v a l a n c h e s

C a t h o d e

Image s by MIT OpenCourseWare.

GM counters

• G M counters also must contain gases with specific properties

• G as amplification produces billions of ion pairs after an interaction – s ignal from detector requires little amplification

• O ften used for inexpensive survey meters

• In general, GM survey meters are inefficient detectors of x-rays and gamma rays

• O ver-response to low energy x-rays – partially corrected by placing a thin layer of higher atomic number material around the detector

GM counters (cont.)

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

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

Neutron Detection

• N eutral particles

– D etection is based on indirect interactions

• T wo basic interactions

– S c a t t e r i n g

• R ecoiling nucleus ionizes surrounding material

• O nly effective with light nucleus (Hydrogen, Helium)

– N uclear reactions

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

Neutron Detection

• S ince x.s. in most materials are strongly dependent on neutron energy, different techniques exist for different regions:

10 5

10 4

10 3

10 2

10 1

10 0

10 -1

10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1

10 0

10 1

Ener gy (MeV)

– S low neutrons (or thermal neutrons), below the Cadmium cutoff

– F ast neutrons

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Thermal Neutron Detectors

• T wo important aspects to consider

– F ind a material with large thermal x.s.

– F ind an arrangement that allows you to distinguish from gamma rays

• High Q-value of neutron capture will make it easier

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• E quations

10 B(n,alpha)

• 94% of the time in excited state

– Q -value of 2.310 MeV

– O ther 6%, Q-value of 2.792 MeV

Alpha kinetic ener gy = 1.78 MeV

Li-7 kinetic ener gy = 1.02 MeV

Alpha kinetic ener gy = 1.47 MeV

Li-7 kinetic ener gy = 0.84 MeV





n

B-10

B-10

Li-7

n

*Li-7 (excited)

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BF3 Tube



B-10

Li-7

n

n

The alpha particle deposits all its kinetic ener gy in the gas while the Li-7 deposits only a fraction of its

ener gy .

The Li-7 deposits all its kinetic ener gy in the gas while the alpha particle deposits only a fraction of its

ener gy .

Li-7 B-10 

2.31 MeV

Gamma ray pulse, noise, etc.

0.84 MeV

1.47 MeV

2.79 MeV

Pulse Size (ener gy deposited in detector)

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Image by MIT OpenCourseWare.

Efficiency of BF3 Tube

• Intrinsic efficiency

– E ff(E) = 1- exp(-Sigma(E) * L)

6 Li(n,alpha)

• E quation

• O nly a ground state exists with Q-value of

4.78 MeV

– E He-3 = 2.73 MeV and E alpha = 2.05 MeV

• O ther possible reactions

– 3 He(n,p)

– 157 Gd

Gamma-ray sensitivity

N e u t r o n a n d g a m m a - r a y i n t e r a c t i o n p r o b a b i l i t i e s i n t y p i c a l g a s p r o p o r t i o n a l c o u n t e r s a n d s c i n t i l l a t o r s

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Fission chambers

• Inner surface of gas tube is coated with a fissile material (U-235)

– C urrent mode in reactor operation

– Pulse mode elsewhere

• E fficiency of about 0.5% at thermal energies

• C ombine fissile and fertile material to avoid loss of sensitivity (regenerative chambers)

• M emory effect when used in high flux regions

– D ecay of FPs

Fission Chambers

1 0 4

1 0 3

1 0 2

1 0

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

E n e r g y ( M e V )

1 0 4

1 0 3

1 0 2

1 0

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

E n e r g y ( M e V )

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Neutron Spectroscopy

• B onner Sphere

2 " 3 "

5 "

8 "

1 0 "

1 2 "

1 2 " + P b

1 2 " + P b

i n t e r n a l p a r t

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Long Counter

• F lat response for neutrons of all energy

20"

14"

8 holes on a 3 1 / 2 " diameter pitch circle 6" deep

1" diameter

0.018 " Cd cap

1"

2.5 "

Paraf fin

_

BF 3 Counter (12.2" active length)

B O

2 3

1.0

Counter Position

0"

0.9 0.5"

0.8

1.0 "

0.7

0.6

0.5

0.4

0.02 0.04 0.06 0.1

0.2

0.4 0.6 0.81.0

2 4 MeV 6

E

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Fast-Neutron induced reaction detectors

• 6 Li(n,alpha)

– Q -value of 4.78 MeV

– P eaks appear at Q-value + neutron kinetic energy

– Lithium based scintillators

1 x 1 0 4

2

3 4 5 6 7 8 9

1 x 1 0 5

2

3 4 5 6 7 8 9

1 x 1 0 6

2

3 4 5 6 7 8 9

1 x 1 0 7

E n e r g y [ e V ]

3 H e [    

6 L i [    

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• 3 He(n,p)

– E nergy range 20 keV to 2 MeV

• S mooth, nearly flat x.s.

• T ube 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%)

• F ull energy peak at En + 765 keV

• T hermal neutron capture peak at 765 keV

• Elastic scattering spectrum with a max at 0.75En

Epithermal peak

dN dE

Full-ener gy peak

Recoil distribution

0.764 MeV

0.75 E n

E n + 0.764 MeV E

E n

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Fast Neutron Recoil Detectors

• Most useful reaction is elastic scattering

– R ecoil nucleus will ionize medium

– M ost popular target is hydrogen

• L arge elastic scattering x.s

• W ell known x.s.

• N eutron can transfer all of its energy in one collision

– Q -value is zero

• Information can be found about the incoming neutron energy

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Gas recoil detectors

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Scintillators

• D esirable properties:

– High conversion efficiency

– D ecay times of excited states should be short

– M aterial transparent to its own emissions

– C olor of emitted light should match spectral sensitivity of the light receptor

– F or x-ray and gamma-ray detectors,  should be large

– h igh detection efficiencies

– R ugged, unaffected by moisture, and inexpensive to manufacture

Scintillators (cont.)

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

• M ay be operated in pulse mode as spectrometers

• H igh conversion efficiency produces superior energy resolution

Materials

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

– F ragile and hygroscopic

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

Photomultiplier tubes

• P MTs perform two functions:

– C onversion of ultraviolet and visible light photons into an electrical signal

– S ignal amplification, on the order of millions to billions

• C onsists of an evacuated glass tube containing a photocathode, typically 10 to 12 electrodes called dynodes, and an anode

Glass tube

+100 volts

+300 volts

V isible light photon

Signal to preamp

+200 volts

+400 volts

Photocathode 0 volts

Dynodes

Anode

+500 volts

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Dynodes

• E lectrons 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

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

PMT amplification

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

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

• A mplification can be adjusted by changing the voltage applied to the PMT

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22.106 Neutron Interactions and Applications

Spring 2010

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