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Digital analysis of scintillator pulses
generated by high-energy neutrons.
Jan Novák, Mitja Majerle, Pavel Bém,
Z. Matěj1, František Cvachovec2,
1
Faculty of Informatics, Masaryk University in Brno, Czech Republic
2 University of Defence, Brno, Czech Republic
High energy neutron detection
• High energy neutron detection is generally based on the nuclear
interaction basis.
• Charged particles produced in the n reaction on the nuclei are detected by
usual charged particle detection methods.
• For this purpose, well known (n,p) elastic scattering is frequently used.

pn2


p p2

p n1



2
2
2
p n 1  p n 2  p p 2  p p 2  p n 1  p n 2  p n 1 p n 2 cos 
E n1  E n 2  E p 2
( Ei  mic )  pi c  mi c
2
2
2
2
2
4
Proton Recoil Telescope for Neutron Spectroscopy
209,0 mm
■ The Proton Recoil Telescope (PRT)
is based on the detection of protons
in the n+p scattering on a thin
hydrogenated target.
■ By correcting for the energy
dependent efficiency, the energy
spectrum for the neutrons emitted by
the source is determined from the
detected proton spectrum, on the
kinematics relations basis.
■ Protons are detected at defined
angle by telescope of Si detectors.
CH2 Φ 14,0 mm
m 31.103 mg
12C
Tube, SS, T 2 mm
44,4o
7Li
456,5 mm
CH2(B)
30x30x60 mm3
Fe
30x30x60 mm3
Beam spot on Li target
Φ(FWHM) 4 mm mm
Si-D
Φ 7,0 mm
The PRT is proposed to detect neutron spectrum
at short distances from the source where TOF method is not applicable
3
Simplified reconstruction of neutron spectrum
■ A code for the reconstruction of neutron spectrum from mesured proton recoil spectrum
was developed. In the code, the response function is based on the n+p differential cross section
knowledge (LA-150h library) and the n+p scattering kinematics.
■ The model of response functions was calculated including the realistic geometry arrangement
of the neutron source-radiator-detector set. As a simplified approach to the reconstruction
procedure, the effects of secondary scattered neutrons were neglected.
En=15 MeV Response Function
n/MeV
n/MeV
En=10 MeV Response Function
4,00E-008
4,00E-008
3,50E-008
3,50E-008
3,00E-008
3,00E-008
2,50E-008
2,50E-008
2,00E-008
2,00E-008
1,50E-008
1,50E-008
1,00E-008
1,00E-008
5,00E-009
5,00E-009
0,00E+000
0,00E+000
-5,00E-009
-5,00E-009
1
2
3
4
5
6
Ep (MeV)
4
5
6
7
8
9
Ep (MeV)
■ Response functions were superposed by optimization procedure to fit measured proton-recoil
spectrum. Multiplication factors pertaining to response functions create resulting n spectrum.
4
NE 213 scintillator
•
•
•
In the NE 213 scintillator, protons are scattered by neutrons. Moreover, at higher neutron energy
(over 8 MeV), number of another processes producing charged particles contributes to overall
scintillator response.
Protons and another charged particles excite atomic levels, whose number is proportional to
energy loss of the particle. Therefore, ray caused by the deexcitation has an intensity proportional
to energy lost in the scintillator. The light ray is converted to an electric pulse and amplified by the
photomultiplier base assembly Ortec 265.
Light gain dependency on the proton energy is empirically determined:
Lp(Ep)=0.07269+0.11237 E2
Lp(Ep)= -0.20570 + 0.35260 E + 0.01343 E2 + 0.00250 E3
Lp(Ep)= -0.25999 + 0.34141 E + 0.3303 E2 + 0.0092 E3
Lp(Ep)= -1.43180 + 0.69325 E
The response function shape is determined mainly by the
(n,p) angular distribution and by the scintillator geometry.
dN
dE
E n Max
p
p
(E p ) 

0
dw p
dE
p
(En , E p )
dN
n
dE n
( E n ) dE n
0< E < 1.5 MeV
1.5 < E <3.5 MeV
3.5< E < 8.0 MeV
8.0 < E < 20 MeV
Neutron-gamma discrimination
•
•
•
In the real experiment, gamma photons interact with the scintillator also. It is necessary
distinguish neutron and gamma events.
Gamma photons scatter electrons, therefore the atomic levels are excited by a different way.
n-gamma discrimination using pulse shape analysis is feasible.
0.2
We have two possibilities of the neutrongamma discrimination:
1. Pulse rise time analysis. Pulses originated
from neutrons and gammas are
distinguished on the rise time duration.
2. Pulse shape analysis based on the
comparison of the overall pulse area and
the pulse part area beginning from a
fixed sample. The ratio of two areas:
r 
A0
A1
U[V] 0
-0.2
Sample
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
14
-0.4
-0.6
-0.8
Area of the pulse part A1
-1
-1.2
-1.4
-1.6
-1.8
Overall pulse area A0
Rise time
16
18
20
1. Pulse rise time analysis
Ne 213
Converter
PC
Start
HV
PSA
TAC
Stop
In the past, we performed two-parametrical data processing. Besides the pulse amplitude data
collection, the pulse was processed by a second way. The PSA generates start and stop
pulses, which are derived from 10% and 90% of the pulse amplitude. The TAC converts the
time delay between start and stop pulse to amplitude. Hence, amplified NE 213 pulse
amplitude and the amplitude containing the pulse rise time are together saved in the time
coincidence.
In the optimal conditions, we distinguished neutron pulses for neutron energies from 0.5 MeV.
2. Pulse shape analysis based on the areas
comparison
Present level of the digital signal processing enables us to
process very short pulses by the digital oscilloscope
technique.
Our oscilloscopic card has 10 bits AD converter with maximal
sampling frequency 500 MHz.
The pulse information is given by some tenths of samples,
because the maximal pulse duration is 100 ns.
Ne 213
HV
PC
with
card
At the present time, all samples are recorded to the PC hard disk. Therefore, we are able to collect
pulse samples with relatively low dead time at pulse frequency up to 1000 pulses/s. But now
we try preprocess pulse samples before their transfer by PCI bus and saving on the hard disk.
This way will lead to saving of the markedly lower parameter number. Advantage of this way
will be achievement of the markedly higher pulse frequency. Moreover, the PC hard disc
volume requirements will be decreased.
Neutron-gamma discrimination results
On the figure, the neutron and gamma pulses
of the 252Cf neutron source are separated very
well. Our NE 213 set-up was used, pulses were
digitized by digitizer of Faculty of Informatics,
Masaryk University in Brno.
In our experimental set-up, the PuBe neutron
source was used. The neutron and gamma
pulses are clear separated. The A1 area is
integrated from 8 th channel after pulse
maximum, what is 16 ns after maximum.
However, this method is extremely sensitive to
signal-noise distance mainly due the A1 area
integration.
Count
1600
1400
1200
1000
800
600
400
200
0
0
10
20
30
40
50
Integration ratio
Conclusions and visions
Advantages of the pulse sampling:
•
•
•
•
•
High event frequency (in the mode of the pulse processing before saving).
Possibility of the on-line data analysis for the experimental set-up correction.
Possibility of the processing algorithm development on the saved samples.
Absolute freedom in the processing algorithm choice.
Portability.
Challenges:
•
•
•
To solve the problem with the high neutron-gamma sensitivity to signal-noise distance.
To implement the on-line pulse processing before the data PCI transfer and saving on the
hard disc.
To choose and to handle the optimal method for the neutron pulse spectra deconvolution in
order to neutron spectra obtaining.
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