NEUTRON SOURCES

10/13/2010 
General
General

Neutron Properties• Composition: two down quarks and one • Rest Mass: 1.0086649 amu• Energy equivalent: 939.5656 MeV• Electric charge: 0• Half-life: 10.4 minutes (outside the nucleu• Decay scheme:

neutron → proton + beta + an
Uses of Neutrons

Reactor start up Density gaugesMoisture gauges Well logging

General

Activation analysis Gemstone colorizatioRadiography Research (physics, mTriggers for nuclear weaponsInstrument calibration
Neutron Fluence Rate

• The intensity of a neutron source is usual

described by the fluence rate

• This is often and incorrectly referred to as General

This is often and incorrectly referred to as 

• The neutron fluence rate (

N) is the numbe

neutrons that pass through a specified areunit time. Commonly employed units for thquantity are n/cm

2/s (i.e., cm-2 

s-1). The d

of the neutrons is irrelevant.
Types of Neutron Sources

• Alpha neutron sources• Gamma neutron sourcesGeneral• Spontaneous fission neutron sources• Fission reactors• Accelerators
Measuring the Neutron Source Strength

• The neutron emission rates for alpha neut

gamma neutron, and spontaneous fission neutron sources can be determined with tmanganese sulfate bath technique.

General• The source is positioned in the center of a 

filled with a solution of manganese sulfate bath must be large enough to moderate aneutrons). By quantifying the Mn-56 (2.6 hproduction via gamma spectrometry, the nemission rate can be calculated.
Measuring the Neutron Source Strength

The manganese soinside the spherical the source is lowerethe top.

General

Image courtesy of NPL.

The white neutron dthe right side of the used to measure neleakage. This value make corrections to calculated neutron erate.
ALPHA NEUTRON SOURC
Alpha Neutron Sources

General• Alpha neutron sources are the most comm

encountered type of neutron source.

• An alpha emitter is intimately mixed with a 

material, usually Be-9.

Be-9 + 

"

ÿ C-12 + neutron + 4.44 Me

• The source strength is specified by the ac

the alpha emitter. Activities of 0.5 to 40 CGBq to 1.48 TBq) are common although pdensity gauges might employ 10 to 50 mCto 1.85 GBq) sources .
General

• Alpha emitters used in neutron sources in

Am-241, Pu-238, Pu-239, Po-210, Ra-226

Alpha Neutron Sources• The “most important” is Am-241. Plutonium 

sources are also common.

• One possible concern with these sources 

potential for the build-up of pressure due tproduction.
AmBe Sources

• AmBe (“ambee”) sources are a mix of Am

Be-9. 

• Yield: ca. 2.0 to 2.4 x 106 neutrons/sec. p

ca 5 4 to 6 5 x 104 neutrons/sec p

Alpha Neutron Sources

ca. 5.4 to 6.5 x 10 neutrons/sec. p

• Half-life: 432.2 years• Average neutron energy: 4.2 MeV (11 ma• Neutron dose rate: 2.2-2.7 mrem/hr at 1 m

0.59-0.73 uSv/hr at 1m

• Gamma dose rate: 2.5 mrem/hr at 1 m/Ci

0.68 uSv/hr at 1m/GBq
PuBe Sources

• PuBe (“pewbee”) sources are a mix of Pu

Pu-238 and Be-9. 

• Yield: ca. 1.5 to 2.0 x 106 neutrons/secon

ca 4 to 5 4 x 104 neutrons/second 

Alpha Neutron Sources

ca. 4 to 5.4 x 10 neutrons/second 

• Half-life: 24,114 years• Average neutron energy: 4.2 – 5 MeV (11 • Neutron dose rate: 1.3-2.7 mrem/hr at 1 m

0.35-0.73 uSv/hr at 1

• Gamma dose rate: 0.1 mrem/hr at 1 m/Ci

0.027 uSv/hr at 1 m/G
RaBe Sources

• RaBe (“raybee”) source, a mix of Ra-226 • Yield: ca. 15 x 106 neutrons/sec. per Ci

ca. 40 x 104 neutrons/sec. per GB

Alpha Neutron Sources• Half-life: 1,600 years• Average neutron energy: 3.6 MeV (13.2 M• Gamma exposure rates of these sources 

high. There is also the problem of leakagesources have been used in moisture gaugSeaman Nuclear - until recently radium hunregulated by the NRC.
Alternatives to Beryllium 

• Beryllium is the most common low Z mate

used in alpha-neutron sources because orelatively high neutron yield.

Alpha Neutron Sources• Nevertheless, fluorine, lithium and boron h

been used.

• Am-F and Am-Li sources have average ne

energies of 1.5 and 0.5 MeV respectively.
Neutron Yield 

• The neutron yield (n/s) of a particular sou

only be determined precisely by measure

Alpha Neutron Sources• Yield values expressed as n/s/Ci are only 

estimates.

• The actual yield depends on the source 

construction and the beryllium - alpha em
Source Construction 

• The alpha emitter and beryllium must be i

intimate contact, e.g., by mixing powdered beryllium metal with an oxide of the alpha This mixture is then compressed into a c

y

Alpha Neutron Sources

shape for encapsulation. Another approaemploy a metallic alloy of the beryllium analpha emitting actinide.

Typical AmBe sources. Largest pictured is 60 x 30 mm. Image courtesy of NPL..
Source Construction 

• The source is doubly 

encapsulated. The inner and outer capsules are usually 

fabricated of stainless steel (type

Alpha Neutron Sources

fabricated of stainless steel (type

304) and the end caps are TIG welded. Space is left within the inner capsule to allow for the gradual buildup of helium that results from the alpha emissions.
Neutron Energies

The energies usually range up to 11 MeV with an average energyAlpha Neutron Sourceswith an average energy

between 4 and 5 MeV.
GAMMA-NEUTRON SOURC
Gamma-Neutron Sources

General • If the nuclei of H-2 or Be-9 are given suff

excitation energy by a gamma ray, a neutbe ejected from the nucleus. 

• Be-9 + 

(

6 Be-8 + neutron (Q: - 1.6

• H-2 + 

(

6 H-1 + neutron (Q: - 2.
General 

• The major advantage of photo-neutron so

that the emitted neutrons are very close tmonoenergetic.

Gamma-Neutron Sources• Their major disadvantage is the very high 

of the gamma source - only one gamma one million (or so) might produce a neutroresulting gamma exposure rates can possignificant radiological hazard.
Source Construction 

• A “typical” photo-neutron source might co

an inner aluminum-encapsulated gammacore (e.g., 1 inch diameter) surrounded bei

ghth of an inch of the neutron emittin

g t

Gamma-Neutron Sources

g g

• The overall shape of the source might be 

cylindrical or spherical. 

• In the case of an antimony-beryllium sour

core is antimony that has been activated reactor. 
Gamma

Alumin

Gamma-Neutron Sources

-

emitting core Neutro

target (
Sb-Be Source Characteristics

• A mix of Sb-124 and Be-9. • Yield: ca 0.2-0.3 x 106 neutrons/sec. per 

ca. 0.54-0.81 x 104 neutrons/sec. 

• Half life: 60 daysGamma-Neutron Sources

-life: 60 days

• Gamma energy: 1.69 MeV• Neutron energy: 0.024 MeV• Neutron dose rate: 0.18-0.27 mrem/hr at 

0.049-0.073 uSv/hr at 

• Gamma dose rate: 1000 mrem/hr at 1 m/

270 uSv/hr at 1m/GB
SPONTANEOUS FISSION 

SOURCES
Spontaneous Fission Sources

General • A number of high mass even-even alpha e

radionuclides (e.g., Pu-238, Cm-242, Cm-252) also undergo spontaneous fission. 

• Each fission event typically results in the e

of 2 to 4 neutrons. 

• Their neutron spectra are similar to that o

reactor. In addition, they have a relatively gamma output.
Cf-252

• Californium-252 is one of the most import

neutron sources. There are two key reaso

- its neutron energy spectrum is very similar to 

reactor fission spectrum

Spontaneous Fission Sources

reactor fission spectrum

- its high neutron yield per unit mass permit the 

construction of physically small neutron sourc

Cf-252 ÿ 2 fission products + 3 - 4 

• Californium-252 sources can contain Cf-2

has a 13.08 year half-life.
Cf-252

• Alpha decay (97%), spontaneous fission (• Effective half-life: 2.645 yearsSpontaneous Fission Sources• Produced in high-flux reactors (U.S.A, Ru• Specific activity: 532 Ci/g; 19.7 x 101

• Average neutron energy: 2 MeV (10+ Me
Cf-252

• Neutron yield: 2.3 - 2.4 x 1012 n, s-1, g-1

4.4 x 109 n, s-1, Ci-11.2 x 108 n, s-1, GBq-1

Spontaneous Fission Sources• Neutron dose rate: 2.2 - 2.3 x 103 rem, m2

22 - 23 Sv, m2, g-1, h-1

• Gamma dose rate: 1.6 x 102 rem, m2, g-

1.6 Sv, m2, g-1, h-1
Cf-252 Neutron Spectrum

• The neutron spectrum is 

very similar to that of a 

fission reactor

Spontaneous Fission Sources

fission reactor.

• The average neutron 

energy is 2 MeV.
Moderated Cf-252

• For instrument calibrations, californium-25

often moderated with heavy water – this ca “degraded” neutron spectrum more simithat in the areas around reactors where 

Spontaneous Fission Sources

dosimeters and survey meters are used.

• When moderated, the Cf-252 is typically 

centered in a 30 cm diameter steel sphere with the heavy water. In general, the steel covered with a 1 mm cadmium shell.
Moderated Cf-252

• Heavy water is used as the moderator be

doesn’t absorb neutrons. Only 11.5% of thoriginal neutrons are lost and these are tythe thermal neutrons absorbed in the cad

Spontaneous Fission Sources• A problem with moderated sources is thei

size. Among other things, it can be difficuuse shadow cones to account for scatter.

• The average energy of the moderated spe

is 0.55 MeV)
The source (californium oxide or 

californium-palladium alloy) is usdoubly encapsulated in stainless 

Spontaneous Fission SourcesTypical Cf-252 sources. Smallest pictured is 10 x 7Image courtesy of NPL.
FISSION REACTORS

FISSION REACTORS
Fission Reactors

General• Very intense sources (e.g., 1012 to 1015 n 

• Their neutron yields can usually be chang

several orders of magnitude. 

• Research reactors, as opposed to power 

incorporate beam ports that allow neutronescape the reactor core. These ports also samples to be inserted into the core. 
General

• In general, the neutrons are produced as 

of the fission of U-235. During operation, an in-growth of plutonium that will also fis

Fission Reactors

n(th) + U-235 ÿ Fission products + 2.4 neutrons • Neutron yield: 1012 n/s per megawatt (MW

• Average neutron energy: 2.0 MeV• Most probable energy (mode): thermal for 

reactors and a few hundred keV for fast re
Fission Reactor Spectrum

Fission Reactors
ACCELERATORS
Accelerators

Electron Accelerators• e.g., betatron, synchrotron and linear acc• produce bremsstrahlung by bombarding h

targets (e.g., tungsten) with electrons. 

g ( g, g )

• The bremsstrahlung then produces neutr

the (( ,n) reaction in beryllium or other m

• Be-9 + bremsstrahlung 6 Be-8 + ne
Electron Accelerators

• The higher the energy of the electron bea

higher the energy of the bremsstrahlung, higher the energy of the neutrons. Some have ener

gies e

qual to the energy of the 

Accelerators

g q gy

bombarding electrons. 

• Uranium targets produce neutrons by an 

method: the bremsstrahlung also generaneutrons by photofission (

( ,f).
Electron Accelerators

• Neutrons can be an unwanted byproduct 

accelerators that produce high energy x-r

• When x ray energies exceed 8 10 MeVAccelerators• When

x

-ray energies exceed

8 - 10 MeV

neutrons can be produced by a wide rangmaterials and the resulting neutron activathe accelerator components, facility compthe dust and air can present a significant radiological hazard.
Positive Ion Constant Voltage Accelerato

• These devices, often referred to as neutr

generators, are frequently used by reseafacilities and universities. 

Accelerators• By accelerating deuterons, protons, or ot

particles, into low Z targets, relatively sminexpensive accelerators can produce intbeams of monoenergetic neutrons. 

• Constant voltage accelerators, e.g., Van d

and Cockroft Walton accelerators, are oft
Positive Ion Constant Voltage Accelerato

• H-3 + H-2 

6 He-4 + 14 MeV neutron 

Q = 17.6 MeV

Accelerators• H-2 + H-2 

6 He-3 + 2.5 MeV neutron 

Q = 3.26 MeV
Positive Ion Constant Voltage Accelerato

• The D-T reaction, where deuterium ions a

accelerated into a tritium target is the mocommonly employed reaction in neutron 

generators. 

Accelerators• The target is a metal halide film, e.g., tita

scandium or zirconium halide deposited ocopper or molybdenum backing. There arhydrogen atoms (deuterium or tritium) pemetal in the target.
Positive Ion Constant Voltage Accelerato

Accelerators
Positive Ion Constant Voltage Accelerato

• A typical neutron generator has three com

the accelerator itself, a high voltage poweand a control console

Accelerators

Unlike alpha-neutron or spontaneous fissneutron sources, these neutron generatoturned off.
Positive Ion Constant Voltage Accelerato

Ion Source 

Magnet Vacuum 

Envelope

Accelerator 

Electrode 

Exit Cathode

Accelerators

Vsource

(ca 2 – 7 kV)

Vaccele

(ca 80 – 1
Positive Ion High Frequency Accelerator

• High frequency positive ion accelerators 

cyclotron, synchrocyclotron, proton synchand heavy ion linear accelerator) produce beams of ions. 

Accelerators• Unfortunately, any neutron production ass

with these accelerators is also pulsed and neutron detectors cannot function properpulsed beams. 
Positive Ion High Frequency Accelerator

• Common reactions used to produce neut

Be-9 + H-2 

6 B-10 + neutron

Accelerators

Q: 4.36 MeV

Li-7 + p 

6 Be-7 + neutron

Q: - 1.65 MeV
Positive Ion High Frequency Accelerator

• The higher the energy of the ion beam, th

the neutron yield and the wider the range neutron energies

Accelerators• The neutrons produced by such accelera

often an unwanted byproduct of their opeProton beams above 10 MeV produce newhen they strike almost any type of mate
ESTIMATING NEUTRON 

DOSE EQUIVALENT RATEDOSE EQUIVALENT RATE
Estimating Neutron Dose Equivalen

GeneralWe will consider two approaches for estimaneutron dose equivalent rate:1 Using the source activity and the neutro

. Using the source activity and the neutro

constant.

2. Using the neutron fluence rate and a flux 

factor.
1. Using the Source Activity and a Neutr

Constant

The dose equivalent rate is calculated with following formula

Estimating Neutron Dose Equivalen

A is the source activity (e.g., Ci, GBq) or mass (e.gN is the neutron dose rate constant (e.g., mrem, m

2

d is the distance (e.g., m) from the source to the powhich the dose equivalent rate is calculated
1. Using the Source Activity and a Neutr

Constant

This method might be used with an alpha-ba photo-neutron or a Cf-252 source. It wou

k if th t i l t

Estimating Neutron Dose Equivalenwor

k if the neu

tron source is an accelera

to

reactor.As an example, we will calculate the dose equivalent rate at 2 meters from a 50 GBq source using a conservative dose factor. 
1. Using the Source Activity and a Neutr

Constant

Estimating Neutron Dose Equivalen
2. Using the Neutron Fluence Rate

This method can be used if the neutron fluehas been measured or calculated. 

Estimating Neutron Dose Equivalen

If the neutrons are produced by an alpha-bephoto-neutron or Cf-252 source, we can cathe neutron fluence rate with the formula on following page.
2. Using the Neutron Fluence Rate

Estimating Neutron Dose Equivalen

N is the neutron fluence rate (n/cm

2/s)

S is the neutron emission rate (n/s)d is the distance from the source at which thfluence rate is calculated (cm).
2. Using the Neutron Fluence Rate

Example: calculate the fluence rate at 2 mefrom a 50 GBq neutron source.

Estimating Neutron Dose Equivalen

First we calculate the neutron emission ratS = A x 6.5 x 104 neutrons/sec. per G

= 50 GBq x 6.5 x 104 neutrons/sec. p

= 3.25 x 106 n/s
2. Using the Neutron Fluence Rate

Then we calculate the fluence rate as follow

Estimating Neutron Dose Equivalen
2. Using the Neutron Fluence Rate

Once we have the neutron fluence rate, the dose equivalent rate can be estimated as fo

Estimating Neutron Dose Equivalen

F is a factor that indicates the neutron fluencunit dose equivalent (n/cm

2/rem) .

3.6 x 103 converts the number of neutrons p

per second into neutrons per cm

2 per hour.
2. Using the Neutron Fluence Rate

There are various sources for the neutron fper unit dose equivalent (e.g., n/cm

2/rem) fa

e.g., they can be obtained from table 100410 CFR 20.

Estimating Neutron Dose Equivalen

One source of uncertainty: these factors are monoenergetic neutrons whereas neutron exposures inevitably involve neutrons with a of energies.One acceptable approach might be to use tfor the average neutron energy.
2. Using the Neutron Fluence Rate

In our example, the average neutron energy AmBe source is approximately 4.2 MeV.

100 ( ) 2 10 C 20

Estimating Neutron Dose Equivalen

Table 100

4

(

B

).2 in 10

CFR 20 indicates tha

fluence per unit dose equivalent for 5 MeV closest energy listed to 4.2 MeV) neutrons 

neutrons cm-2 rem-1.
2. Using the Neutron Fluence Rate

Estimating Neutron Dose Equivalen