A stacked scintillator neutron spectrometer for measuring the … · •Use stacked scintillators...
Transcript of A stacked scintillator neutron spectrometer for measuring the … · •Use stacked scintillators...
A stacked scintillator neutron spectrometer for measuring the fluence of mono-energetic
neutron beams up to 150 MeV
Andy Buffler, Frank Brooks, Saalih Allie, Mark Herbert, Siphiwo Makupula,
Department of Physics, University of Cape Town, South Africa
Ricky SmitiThemba LABS, Cape Town, South Africa
Volker Dangendorf, Ralf NoltePhysikalisch-Technische Bundesanstalt, Braunschweig, Germany
SAIP conference, Stellenbosch 2003
For neutron spectrometry up to ∼150 MeV, liquid scintillators(e.g.NE213 or BC501A) offer:
• high neutron detection efficiency, and • good n-γ discrimination.
Response functions can be simulated by Monte Carlo (e.g. NRESP, SCINFUL, MCNPX, etc.) to determine:
• neutron detection efficiencies; and• neutron energy spectra by unfolding measured pulse height spectra.
Two main difficulties for En > 20 MeV:
• contributions from n-12C interactions cannot be simulated reliably; and • proportion of escaping protons increases with energy.
Solution:• Measure response functions• Use stacked scintillators to control escapes
The Stacked Scintillator Neutron Spectrometer
Two event types accepted:1. A only (“singles”, LB = 0); and2. A + B (“coincidences”)
Summed pulse height spectrum for event types 1 and 2 gives an escape-free response function.
p
p
n
n n
n
A
B
1 2
V
Principle:• Select all events for
which a charged particle is detected in A.
• Veto proton escapes into V.
• (LA / MeVee) = (LB / MeVee)
• Response L = LA + LB
Response function measurements
• Experiments at the neutron time-of-flight facility of iThemba LABS• ns-pulsed proton beam energies of 66, 80, 100, 120 and 160 MeV• 5 mm natLi target• Quasi-monoenergetic
(time-of-flight selected) neutron beam energies selected using 7Li(p,n)7Be(gs+1) at 0o
over a flight path of 6.00 m.
40
30
20
10
0
10k
20k
62.5 MeV
Time-of-flight T (ns)
160120
8040
Pulse height L (MeVee )
66 MeV protons on natLi (5 mm)Singles in detector A
140 120 100 80 60 400
50k
100k
150k
200k
250k
W
10 20 30 40 50 70En (MeV)
Cou
nts
per c
hann
elTime-of-flight T (ns)
62.5 MeV
0 10 20 30 40 50 60 70 80 90 100 1100
1
2
3
4
5
6
7
natLi(p,n): 0° 16°
Φ E /
Nm
on (a
rb. s
cale
)
En / MeV0 20 40 60 80 100
0
1
2
3
4
5
6
natLi(p,n): difference beam
ΦE /
Nm
on (a
rb. s
cale
)
En / MeV
100 MeV protons on a natLi target:Spectral neutron fluence, normalised to the same number of protons
C2
C1
0
4k
2k
4030
2010
0
e
Pulse height L (MeV ee
)Pulse shape
S
d
α
p
Cut C1 excludes γ-rays and escaping protons
En = 62.5 MeVSingles in Detector A
0 10 20 30 40 500
1000
2000
3000
4000
5000
1H(n,n)1H12C(n,x)
LT
Cou
nts
per c
hann
el
L (MeVee)
En = 62.5 MeVSingles in detector A
Experiment
SCINFUL
C2
C1
80
6040
200
e
Pulse height L A (M
eV ee)
Pulse shapeS
A
d
α
p
C4
C3
80
6040
200
Pulse height L B (M
eV ee) Pulse shape
SB
p
A singles
AB coincidences
En = 97.5 MeV
A
BV
Ep = 100 MeV
A
BV
Ep = 100 MeV
A
B
V
Pulse heightcalibration
LBLA
0 1000 2000 3000 40000
500
1000
1500
1320
1795
Cou
nts
per c
hann
el
Pulse height L (A or B) (ADC channel)
(a) ABV coincidence (b) BAV coincidence
(a) (b)
0 20 40 60 800
2000
Cou
nts
per c
hann
el
L = LA + LB (MeVee)
0
2000
0
2000
4000
LT
En = 97.5 MeV
L = LA + LB (MeVee)
SCINFUL
Experiment (a) A singles
(LB = 0)
(b) ABcoincidences
(a) + (b)
60 62 64 66 68 700
50
100
150
200
∆D/D = 2.1/64.2 = 3.3 %
- dN
(D) /
dD
Pulse height D (MeVee)
-dN/dL
L (MeVee)
Pulse height resolution(∆L/L) ≈ 3%
∆L
0 20 40 60 800
2000
4000
6000
Nnp
Dnp
12C(n,x)
1H(n,n)1H
Cou
nts
per c
hann
el
Pulse height D ( MeVee )
Lnp
L
Fluence calculation
( ) { }0 np H H np dead ABN N n σ φ φ φ=
Total counts recorded above pulse height threshold Lnp , where Lnp is set so as to select events associated with n-pelastic scattering only.
Total number of neutrons incident on the spectrometer.
where:
nH : number of hydrogen atoms per unit cross sectional area presented to the beam by the scintillator
σH : total cross section for n-p elastic scattering
φnp : fraction of neutrons detected above pulse height threshold Lnpφdead : correction factor for dead time of counting systemφAB : other factors associated with corrections for threshold, reaction tail
and escape effects
Fluence calculation cont.
The neutron fluence Φ is then
where A is the cross section area of the beam.
1
np
H H np dead AB
Nn Aσ φ φ φ
⎧ ⎫⎪ ⎪Φ = ⎨ ⎬⎪ ⎪⎩ ⎭
For experiments requiring high flux neutron beam irradiationsat iThemba LABS, the general approach used for neutron beam monitoring is to use the stacked spectrometer to calibrate a smaller (less efficient) detector (typically positioned at 8o) using a low intensity beam, which is then used to monitor the high intensity neutron beams used during the irradiation experiments.
Typical fluences measured in this way at the neutron beam facility at iThemba LABS are of the order of
Φ = (1.5 ± 0.1) × 1010 neutrons cm-2
for about 40 hours of running with a 10 µA proton beam (pulse selector off) incident on a 10 mm Be target, with the detector at 6.00 m from the target at 0˚.
150 MeV n
A B C
0 100 200 3000
1000
2000
3000
4000
5000
6000
7000
Cou
nts
per c
hann
el
Pulse height L (ADC channel)
LnpA
ABABC
Full response function
Triple scintillator system
Summary
• The effect of escaping charged particles on response function measurements has been minimized using the stacked scintillator system.
• Full response functions have been measured for quasi-mono-energetic neutron beams of energies between 63 and 150 MeV.
• A method for measuring fluences for these beams has been developed, using the total cross section for n-p scattering as a reference.
• Response function measurements have also been used for determining energy spectra via unfolding analyses (MSc of Siphiwo Makupula … next talk).
For more information see …
“Measurement of neutron energy spectra from 15-150 MeV using stacked liquid scintillators”A. Buffler, F.D. Brooks, M.S. Allie, P.J. Binns, V. Dangendorf, K.M. Langen, R. Nolte and H. SchuhmacherNuclear Instruments and Methods A 476 (2002) 181-185
“High energy neutron reference fields for the calibration of detectors used in neutron spectrometry”R. Nolte, M.S. Allie, P.J. Binns, F.D. Brooks, A. Buffler, V. Dangendorf, J.P. Meulders, H. Schuhmacher, B. WiegelNuclear Instruments and Methods A 476 (2002) 369-373
Measurement of neutron fluence spectra up to 100 MeV using a stacked scintillator
neutron spectrometer
Siphiwo Makupula and Andy BufflerDepartment of Physics, University of Cape Town, South Africa
SAIP conference, Stellenbosch 2003
Introduction
Neutron time-of-flight is the technique most often used to determine the energy spectra of ns-pulsed neutron beams.
For continuous (non-pulsed) beams, alternative methods need to be employed, such as those based on unfolding analyses.
The pulse height spectrum that is recorded from any radiation detector is the convolution of its inherent response function and the energy distribution of the incident radiation.
If the purpose of making the pulse height measurements is to obtain information about the energy spectrum of the incident radiation, as is the present case, then this process involves solving the basic system of linear integral equations:
∫∞
φ=0
)( )( dEEERz ii for i = 1 to m
which represents a model of the measurement.
∫∞
φ=0
)( )( dEEERz ii
zi: the input measurands (the recorded pulse height spectrum). Ri(E): the response functions of the measuring system which include the effects
of limited energy resolution of the spectrometer. φ(E): the average fluence values in the intervals between Ei and Ei+1.
The subscript i is related to the channel number of the measuring system havingm channels in total.
Reliable knowledge of Ri(E) is needed to solve for φ(E).
Problem: Monte Carlo codes (eg. NRESP, SCINFUL, MCNPX) which may be used to simulate response functions Ri(E) for NE213 liquid scintillators, are limited to En below ~20 MeV, since the cross-sections for n-12C reactions above these energies are not well known.
Solution: Measure the response functions Ri(E) required for unfolding analyses.
The previous talk demonstrated how a stacked scintillator neutron spectrometer (S3N) may be used to measure response functions for mono-energetic neutron beams up to about 150 MeV, which not affected by charged particle escape.
Measurements were with the S3N at the neutron time-of-flight facility of iThemba LABS, for neutron beams at 0o and 16o, using targets of natural lithium (3 mm and 5 mm) and graphite (10 mm)
window number
( j )
1412
108
7654
32
1
16
80
120160
40T (ns)
DA
Time-of-flight T versus pulse height DA measured by the S3N positioned at 0o for the neutron beam produced by a 100 MeV proton beam on a 3 mm natLi target.
Time-of-flight windows used to select 16 quasi-monoenergetic response functions between 20 and 100 MeV
En = 97 MeV
Response functions measured by the S3N
(for protons only, selected by pulse shape discrimination)
0 10 20 30 40 50 60 70 800
600
j = 8 (55 MeV)
D (MeVee)
0
600
j = 5 (40 MeV)
j = 6 (45 MeV)
j = 7 (50 MeV)
0
600
j = 1 (20 MeV)
0
600
j = 2 (25 MeV)
0
600
Cou
nts
per c
hann
el
j = 3 (30 MeV)
0
600
j = 4 (35 MeV)
0
600
0
600
1200
0 10 20 30 40 50 6
0
600
16 (97 MeV)
0 70 80
j =
D (MeVee)
0
600
j =
j =
j = 15 (
0
600
13 (80 MeV)
14 (85 MeV)
90 MeV)
= 9 (60 MeV)j
0
600
10 (65 MeV)j =
0
600
Cou
nts
per c
hann
el
j =
0
600
11 (70 MeV)
12 (75 MeV)j =
0
600
0
3000
6000
Response function measured by the S3N for 97 MeV neutrons
0 20 40 60 800
2000
4000
6000
Nnp
Dnp
12C(n,x)
1H(n,n)1H
Cou
nts
per c
hann
el
Pulse height D ( MeVee )
The neutron fluence for each response function was determined using the total cross section of n-p scattering as a reference (previous talk).
Region of pulse height spectrum associated with n-p scattering only
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Fl
uenc
e Φ
(ne
utro
ns c
m-2)
Energy bin j
x 10
Fluence Φ measured for each quasi-monoenergetic response function between 20 and 97 MeV (j = 1 to 16)
The response matrix
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0.25
j = 8 (55 MeV)
D (MeVee)
0.00
0.25
j = 5 (40 MeV)
j = 6 (45 MeV)
j = 7 (50 MeV)
0.00
0.05
j = 1 (20 MeV)
0.00
0.05
j = 2 (25 MeV)
0.00
0.05
Cou
nts
per c
hann
el (n
orm
aliz
ed to
a fl
uenc
e of
1 n
eutro
n cm
-2)
j = 3 (30 MeV)
0.00
0.05 j = 4 (35 MeV)
0.00
0.05
0.00
0.05
0.10
0 10 20 30 40 50 60 70 800.000
0.025
j = 16 (97 MeV)
D (MeVee)
0.000
0.025
j = 13 (80 MeV)
j = 14 (85 MeV)
j = 15 (90 MeV)
0.000
0.025
j = 9 (60 MeV)
0.000
0.025
j = 10 (65 MeV)
0.000
0.025
Cou
nts
per c
hann
el (n
orm
aliz
ed to
a fl
uenc
e of
1 n
eutro
n cm
-2)
j = 11 (70 MeV)
0.000
0.025
j = 12 (75 MeV)
0.000
0.025
0.000
0.025
0.050
Each response function was smoothed, and scaled to the same incident fluence of 1 neutron cm-2 by multiplying each channel by the factor: -21 neutron cm .
(measured)Φ
Unfolding
The equation
∫∞
φ=0
)( )( dEEERz ii
may be transformed (in a number of ways) into the matrix equation
z = R Φ.
which represents a system of linear equations which must be solved for the column matrix Φ = (φ1 … φn)T
where the superscript T indicates transposition.
If data values are known for both:the column matrix z = (z1 … zm)T of the measured pulse height spectrum; and the response matrix R = Rij (i = 1 … m ; j = 1 … n), then z = R Φ may be unfolded to determine Φ.
The unfolding code used for this work was MIEKE (based on a Monte Carlo algorithm) and is available from the PTB, Braunschweig, Germany.
Unfolding ... 2
Tests of the unfolding code were performed by unfolding pulse height spectra “manufactured” from different combinations of the (unsmoothed) response functions making up the response matrix.
The results are not presented here, but were very satisfactory.
It is more interesting to look at the analyses of different neutron beams ...
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hann
el
(a) 3 mm Li, 0o (G)
160 140 120 100 80 60 40 20 0 Tn (ns)
En (MeV) 10 20 30 40 60 100
(b) 5 mm Li, 0o (H1)
(c) 10 mm C, 0o (H2)
(d) 3 mm Li, 16o (H3)
(e) 5 mm Li, 16o (H4)
T (ADC channel)
ToF spectrum measured at 0˚for 3 mm Li target (data used to construct the response matrix)
Four other measurements made at 0o and 16o with Li and C targets
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(d) H4 (5 mm Li, 16o)
Cou
nts
per c
hann
el
(c) H3 (3 mm Li, 16o)
Pulse height D (MeVee)
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(b) H2 (10 mm C, 0o)
Cou
nts
per c
hann
el
(a) H1 (5 mm Li, 0o)
Pulse height D (MeVee)
Pulse height spectra measured (histograms) for 4 neutron beams, together with refolded fits (red curves) resulting from the unfolding analyses.
Spectral fluence measured from unfolding analyses (points) for 4 neutron beams, together with the “expected” spectra (histograms - rebinned ToF spectra).
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(b) H2 (10 mm C, 0o)
Flue
nce
Φ (n
eutro
ns c
m-2)
Energy bin j
(a) H1 (5 mm Li, 0o)
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(d) H4 (5 mm Li, 16o)
Flue
nce
Φ (n
eutro
ns c
m-2)
Energy bin j
(c) H3 (3 mm Li, 16o)
Summary
The stacked spectrometer allows the measurement of response functions for (quasi-) monoenergetic neutrons which are minimally affected by chargedparticle escape.
These response functions have been used to form a response matrix for the stacked spectrometer which has allowed reliable unfolding of measured pulse height spectra.
The spectrometer has thus been shown to be able to measure energy spectrafor continuous (unpulsed) neutron beams.
Future work is aiming at increasing the energy resolution of the unfolding analyses and extending the capability of the spectrometer up to 150 MeV.