NEUDOS10 Tenth Neutron Dosimetry Symposium Uppsala 12th ... · Nuclear Spectrometry User’s Forum 3 Reasons for making neutron spectrum measurements • Photon spectrometry is performed

Nuclear Spectrometry Users’ ForumNPL 22nd May 2007

Neutron SpectrometryDavid Thomas

Neutron Metrology National Physical Laboratory, UK

2Nuclear Spectrometry User’s Forum

Introduction

• Why do neutron spectra need to be measured?

• Why is it difficult? C.f photon spectrometry

• How neutron spectrometry is done


Reasons for making neutron spectrum measurements

• Photon spectrometry is performed for: radioisotope identification (including absolute measurements), activation analysis, nuclear physics, PIXE, PIGE, etc.

• In general neutron spectrometry is performed for different reasons

• Neutron spectrometry measurements may be made, for example, to validate shielding calculations, for nuclear physics experiments, or to characterise calibration fields, but much of the development of neutron spectrometry has been to enable measurements of neutron spectra for radiation protection


Rationale for neutron spectrometry

10-3 10-2 10-1 100 101 102 103 104 105 106 10710-1

100

101

Personal dosemeters Albedo dosemeter Track etch device

Thermal

Under- and over-read for samples of area survey meters andpersonal dosemeters

Dosemeters.opj

Area survey instruments Leake survey meter LB6411 survey meter

Dos

e eq

uiva

lent

resp

onse

Neutron energy (eV)


Why is neutron spectrometry difficult?C.f. photon spectrometry

Photons interact primarily with electrons -importance of photoelectric effect -electron acquires all the energy of the gamma ray

Neither photons or neutrons are directly ionising. To detect them they need to be converted to directly ionising particles


Neutron spectrometry

• There is no real equivalent of the photoelectric effect for neutrons

• At low energies some reactions with low-Z nuclei can be used to transfer the neutron energy to charged particles: e.g.

• Note: both products are charged and can be detected

Reaction Q value3He(n,p)T 764 keV6Li(n,α)T 4.78 MeV

10B(n,α) 7Li 2.79 MeV


3He or 6Li sandwich spectrometer

Neutron

3Heor

6Li

ChargedParticle

Detectors


3He Sandwich Spectrometer

Complex electronics

Extremely low efficiency

In no way is it a field instrument

But: it works well in the lab


241Am-B spectrum measured with a 3He sandwich spectrometer

0 1 2 3 4 5 60.0

0.1

0.2

0.3

0.4

0.5

37 GBq Am-B

Flue

nce

per u

nit e

nerg

y (c

m-2 M

eV-1)

Neutron energy (MeV)


3He Proportional Counter

Neutron

3Hegas

Proton

Triton


3He Proportional Counter Spectrum for Monoenergetic Neutrons

480 keVmonoenergetic

neutrons


3He Proportional Counter in field with thermal neutrons

0 400 800 1200 1600 2000 2400100

101

102

103

104

105

106

107C

ount

s

Energy (keV)

Pulse height spectrum for a 3He gridded ionisation chamber in a realistic neutron field

764 keV

Pulser peak

Pile-uppeak

Energy region of interest


Use of (n,p) Scattering in Neutron Spectrometry

• One of the most important reactions in neutron spectrometry is (n,p) scattering

• When a neutron collides with a hydrogen nucleus some of the neutron energy is transferred to the recoil proton

• The actual amount depends on the scattering angle

Incident neutron

Recoiling proton

Scatteredneutron



2

o

(cos )

recoil scattering angle, when =0 p n

p n

E E

E E

θ

θ θ

=

= =

If the recoils can be restricted to those where θ ~ 0o get a measure of En

This is the principle of the proton recoil telescope

Chargedparticledetector

Neutron Recoil proton

Radiator (CH2)



• If the restriction to θ ~ 0o is removed get a much higher detection efficiency

• However, this introduces the requirement to unfold the spectrum

• Nevertheless proton recoil spectrometers are one of the most commonly used

• There are two basic types:-



Scintillator

Proton

Scatteredneutron

Proton

Hydrogen-filledproportional

counter


A proton recoil proportional counter

60 80 100 120 140 160 180

0

500

1000

1500

2000

2500

Cou

nts

Energy (keV)

Measured Calculated


Characteristics of proton recoil proportional counter spectrometers

Broad pulse height distribution even for monoenergetic neutrons.

Unfolding needed to derive the neuron spectrum from the pulse height distribution.

Response functions must be known, but can be derived from measurement and calculation.

Need several counters (3 or 4) to cover energy range (roughly 50 keV to 1.5 MeV).

For higher energies can use methane or 4He counters, but scintillators are better.


Scintillator spectrometer e.g. NE213

Similarities to proportional counters, but:more efficient,must have n-γ discrimination, andonly one counter needed to cover 1 MeV to >20 MeV.


Summary for proton recoil devices

High resolution.

Cover an important portion of the neutron energy region where a significant fraction of the dose is often located.

Not particularly easy to use or unfold –often need to combine data from several detectors.

Still to some extent under development, digital signal processing being investigated.


Scintillator measurement of 241Am-Be compared with 3He sandwich spectrometer

NPL 10 Ci241Am-Besource


Energy Range for Neutron Spectra

10-3 10-2 10-1 100 101 102 103 104 105 106 1070.00

0.05

0.10

0.15

0.20

0.25

E.Φ

E o

r H*(

10) (

Nor

mal

ised

to u

nity

)

Neutron Energy (eV)

Neutron fluence per unit lethargy Ambient dose equivalent, H*(10)

Range for nuclearrecoil spectrometers

Range for Bonner sphere spectrometers

Fraction of total H*(10) for reactor spectrum shown

69.6% 10.8%19.6%


Integral spectrometers

Bonner spheres


Principle of operation

Escape

Capture in thepolyethylene

Capture in thethermal sensor

Thermalsensor


Response Functions of Bonner spheres

10-3 10-2 10-1 100 101 102 103 104 105 106 1070

1

2

3

4

5

6

Flue

nce

resp

onse

(cm

2 )

Neutron energy (eV)

Bare 6.0" 3.0" 8.0" 3.5" 10.0" 4.0" 12.0" 5.0" 15.0"


Bonner sphere spectra from comparison exercise

10-3 10-2 10-1 100 101 102 103 104 105 106 1070

5

10

15

20

25

30

35

MCNP Calcn. AECL BfS IPSN-Cad

Flue

nce

per u

nit l

etha

rgy

(cm

-2 α

-1)

Neutron energy (eV)


Schauinsland: 1195 m

Schneefernerhaus: 2650 m

Neutron spectrometry: outside the lab, all Bonner spheresNeutron spectrometry: outside the lab, all Bonner spheres

Outside containment UK South Coast

Inside containment


Characteristics of neutron spectrometers

Property3He

SpectrometersNuclear Recoil

DevicesBonner Spheres

Energy resolution Good

50 keV to ~6 MeV

Low

Bulky

Straightforward

Photons OKThermals problem

Not very isotropic

Reasonable straightforward

Good Poor

Energy range 50 keV to 20 MeV Thermal to >20 MeV

Efficiency Low to medium High

Size & mass Bulky Large

Operation Complex Simple

Photon discrimination

Pulse shape analysis essential

for scintillatorsExcellent

Angle response Near isotropic Isotropic

Unfolding Solvable but complex

Complex and subjective

Bonner spheres have been used for more field measurements than all the other devices put together


Neutron spectrometry

End

Documents

NEUDOS10 Tenth Neutron Dosimetry Symposium Uppsala 12th ... · Nuclear Spectrometry User’s Forum 3 Reasons for making neutron spectrum measurements • Photon spectrometry is performed