Nuclear Spectrometry Users’ Forum
NPL 22nd May 2007
Neutron Spectrometry
David Thomas
Neutron Metrology
National Physical Laboratory, UK
Introduction
• Why do neutron spectra need to be
measured?
• Why is it difficult? C.f photon
spectrometry
• How neutron spectrometry is done
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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
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Rationale for neutron spectrometry
Under- and over-read for samples of area survey meters and
personal dosemeters
1
Dose equivalent response
10
Area survey instruments
Leake survey meter
LB6411 survey meter
0
10
Thermal
Personal dosemeters
Albedo dosemeter
Track etch device
-1
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
Neutron energy (eV)
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10
5
10
6
10
7
10
Dosemeters.opj
4
Why is neutron spectrometry difficult?
C.f. photon spectrometry
Neither photons or neutrons are directly ionising. To detect
them they need to be converted to directly ionising particles
Photons interact
primarily with
electrons importance of
photoelectric
effect electron acquires
all the energy of
the gamma ray
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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.
Reaction
Q value
3He(n,p)T
764 keV
4.78 MeV
2.79 MeV
6Li(n,α)T
10B(n,α) 7Li
•
Note: both products are charged and can be detected
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3He
or 6Li sandwich
spectrometer
n
o
r
ut
e
N
3He
or
6Li
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Charged
Particle
Detectors
7
3He
Sandwich Spectrometer
¾ Complex
electronics
¾ Extremely low
efficiency
¾ In no way is it a
field instrument
¾ But: it works well
in the lab
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241Am-B
0.5
37 GBq Am-B
-2
-1
Fluence per unit energy (cm MeV )
spectrum measured with a
3He sandwich spectrometer
0.4
0.3
0.2
0.1
0.0
0
1
2
3
4
5
6
Neutron energy (MeV)
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3He
Proportional Counter
o
r
t
u
e
N
n
Triton
3He
gas
Proton
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3He
Proportional Counter Spectrum
for Monoenergetic Neutrons
480 keV
monoenergetic
neutrons
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3He
Proportional Counter in
field with thermal neutrons
7
10
3
Pulse height spectrum for a He gridded ionisation chamber in a
realistic neutron field
6
10
764 keV
5
Counts
10
Energy region of interest
Pulser peak
4
10
Pile-up
peak
3
10
2
10
1
10
0
10
0
400
800
1200
1600
2000
2400
Energy (keV)
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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
Recoiling proton
Incident neutron
Scattered
neutron
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Use of (n,p) Scattering in Neutron Spectrometry
E p = En (cos 2 θ )
θ = recoil scattering angle, when θ =0o E p = En
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
Radiator (CH2)
Neutron
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Recoil proton
Charged
particle
detector
14
Use of (n,p) Scattering in Neutron Spectrometry
•
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:-
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Use of (n,p) Scattering in Neutron Spectrometry
Scattered
neutron
Proton
Proton
Hydrogen-filled
proportional
counter
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Scintillator
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A proton recoil proportional counter
2500
Measured
Calculated
2000
Counts
1500
1000
500
0
60
80
100
120
140
160
180
Energy (keV)
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Characteristics of proton recoil proportional
counter spectrometers
¾ Broad pulse height distribution even for
monoenergetic neutrons.
¾ Unfolding needed to derive the neuron spectrum
from the pulse height distribution.
¾ Response functions must be known, but can be
derived from measurement and calculation.
¾ Need several counters (3 or 4) to cover energy range
(roughly 50 keV to 1.5 MeV).
¾ For higher energies can use methane or 4He
counters, but scintillators are better.
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Scintillator spectrometer e.g. NE213
Similarities to proportional counters, but:
¾
more efficient,
¾
must have n-γ discrimination, and
¾
only one counter needed to cover 1 MeV to >20 MeV.
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Summary for proton recoil devices
¾ High resolution.
¾ Cover an important portion of the neutron
energy region where a significant fraction of
the dose is often located.
¾ Not particularly easy to use or unfold –
often need to combine data from several
detectors.
¾ Still to some extent under development,
digital signal processing being investigated.
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Scintillator measurement of 241Am-Be
compared with 3He sandwich spectrometer
NPL 10 Ci
241Am-Be
source
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E.ΦE or H*(10) (Normalised to unity)
Energy Range for Neutron Spectra
0.25
Fraction of total H*(10) for reactor spectrum shown
19.6%
69.6%
10.8%
0.20
Range for nuclear
recoil spectrometers
Range for Bonner sphere spectrometers
0.15
0.10
Neutron fluence per unit lethargy
Ambient dose equivalent, H*(10)
0.05
0.00
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
Neutron Energy (eV)
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Integral spectrometers
Bonner spheres
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Principle of operation
Escape
Capture in the
polyethylene
Thermal
sensor
Capture in the
thermal sensor
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Response Functions of Bonner spheres
6
Bare
3.0"
3.5"
4.0"
5.0"
2
Fluence response (cm )
5
4
6.0"
8.0"
10.0"
12.0"
15.0"
3
2
1
0
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
Neutron energy (eV)
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Bonner sphere spectra from comparison exercise
35
MCNP Calcn.
AECL
BfS
IPSN-Cad
-2
-1
Fluence per unit lethargy (cm α )
30
25
20
15
10
5
0
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
Neutron energy (eV)
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Neutron spectrometry: outside the lab, all Bonner spheres
Inside containment
Schauinsland: 1195 m
Outside containment UK South Coast
Schneefernerhaus: 2650 m
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Characteristics of neutron spectrometers
3He
Spectrometers
Nuclear Recoil
Devices
Bonner
Spheres
Energy resolution
Good
Good
Poor
Energy range
50 keV to ~6 MeV
50 keV to 20 MeV
Thermal to >20
MeV
Efficiency
Low
Low to medium
High
Size & mass
Bulky
Bulky
Large
Operation
Straightforward
Complex
Simple
Photon
discrimination
Photons OK
Thermals problem
Pulse shape
analysis essential
for scintillators
Excellent
Angle response
Not very isotropic
Near isotropic
Isotropic
Unfolding
Reasonable
straightforward
Solvable but
complex
Complex and
subjective
Property
Bonner spheres have been used for more field measurements
than all the other devices put together
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Neutron spectrometry
End
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