plications of Monte Carlo Code for a Gamma Resonance System Analysis
L. Wielopolski, A. Hanson, I. Dioszegi, M. Todosow,
Brookhaven National Laboratory,
Environmental Sciences Department,
Upton, NY 11973
POC – Lucian Wielopolski, E-mail: [email protected]
Objective
A Gamma Resonance System
To implement Monte Carlo calculations using MCNP code for the analysis and optimization of a Gamma Nuclear
Resonance Absorption (GNRA) and Gamma Fission (GF) systems. These systems are planned for detection of explosives
and nuclear materials concealed in either small packages or large shipping cargo containers. GNRA is based on element
specific nuclear resonance absorption of high energy gamma radiation, whereas GF is based on a photofission reaction that
occurs in fissionable material at above ~ 6MeV threshold energy. GF results in prompt and delayed neutrons that can be
measured using neutron detectors. The usefulness of the proposed approaches has been demonstrated for GNRA using 9.17
MeV gamma radiation for detection of nitrogen present in the explosives and, for GF, using a 6 MeV Bremsstrahlung
radiation for detection of delayed neutrons in fissionable material.
A GNRA system consists of a proton accelerator equipped with a suitable
target material upon which impinging protons produce a resonance gamma beam
via a proton resonance (p,) reaction. The resonance gamma radiation attenuation
by the nitrogen present in the explosive is measured in the transmission mode
using nitrogen resonance detectors. The nitrogen signature is indicative of presence
of explosive. Alternatively elements can also be measured in nuclear resonance
fluorescence mode by placing regular detectors around the inspected container.
Each element in the system as well as the configuration of the entire system require
engineering-optimization that can be accomplished using MCNP calculations. Two
aspects of nuclear fluorescence yield and gamma energy distribution together with
stand off calculations are presented below.
The main hurdle to overcome in implementing MCNP for GNRA is the lack of photon cross sections libraries
that include photon nuclear resonance interactions. Thus these cross sections need to be prepared on individual basis
for each element of interest and then incorporated into the existing standard MCNP cross section libraries. One
MCNP library has been modified for the 9.17 MeV level in the nitrogen, however, in its current configuration it does
not include the angular correlation given by (1-0.44P2). Although this is not critical for transmission calculations it will
be important to consider for nuclear fluorescence calculations.
Nuclear Absorption versus Nuclear Fluorescence
Nitrogen Cross Sections
10-2
10-2
and at 9.17 MeV Gamma Resonance
104
Gamma Yield (/Sr/source)
10-4
10-5
103
10-7
10-4
10-5
10-6
10-6
10-7
0
2
4
6
8
10
0
2
4
6
8
10
Energy (MeV)
Energy (MeV)
102
Total Attenuation, 
101
Resonance
101
0
10
Compton, 
Photoelectric, 
Pair Production, 6
10-1
-2
10
10-3
10-2
10-1
Incident Gamma on the Detector Surface
at Alpha 0
100
Gamma Yield (/Sr/source)
Attenuation (barns/atom)
Gamma Yield (/Sr/source)
Mass Attenuation of Nitrogen
Stand – off Calculations
10-3
10-3
105
Incident Gamma on the Detector Surface
at Alpha 90
Incident Gamma on the Detector Surface
at Alpha 45
100
101
102
Energy (MeV)
10-1
10-2
10-3
10-4
10-5
2
10
2
4
10
6
8
10
Energy (MeV)
Breit-Wigner Cross Section
at 9.17 MeV in Nitrogen
1
100
10-2
10-2
10-3
abi = 2g
-3
10
10-4
-5
10-2
10-3
10-4
10-5
10
10-6
10-6
10-7
10-4
10-5
-6
10
10-7
0
2
4
6
8
10
0
2
4
9.04
9.16
9.28
9.40
Resonance cross section is given by the
Breit-Wigner formula:
abi =
abi
2g
(E-ER)2
+
2/4
where g is a statistical factor given by:
2J + 1
g=
(2s + 1)(2i + 1)
2.5*10-003
1.5
Nitrogen Angular Correlation for 9.17 MeV Level
1-0.44P2(cosq)
The angular correlation is
maximum at 90° that need to
be considered when
optimizing a stand off system
based on nuclear fluorescence.
Angular Correlation
1.3
1.1
0.9
0.7
0.5
0
30
60
90
120
8
10
A hypothetical spherical detector (in gray) surrounds an explosive with a mass m
that is irradiated with a gamma beam emanating from a proton accelerator. The
incident gamma beam is partially attenuated by the atomic interactions and
partially by nuclear resonance interactions with nitrogen. The energy distributions
of the gamma radiation incident on the detector surface at 0, 45, 90,135, and 180
degrees are shown in the spectra above. These results show that; 1) it is conceivable
to measure gamma radiation resulting from the nuclear fluorescence and 2) the
backward angles are preferable over forward angles due to reduced Compton
background.
Energy (MeV)
150
180
Teta (degrees)
Increased mass of the explosive
increases the yield of nuclear
fluorescence in the backward
angles. This results from
competition between penetration
of the incident resonance
radiation and escape of the
fluorescence radiation that is out
of resonance toward the detector.
Gamma Yield (gammas/Sr)
8.92
6
Energy (MeV)
Energy (MeV)
10-7
8.80
Source emission :
Cotton density:
HMX:
Incident Gamma on the Detector Surface
at Alpha 180
Incident Gamma on the Detector Surface
at Alpha 135
Gamma Yield (/Sr/source)
10-1
Gamma Yield (/Sr/source)
Attenuation (barns/atom)
0
Stand Off Considerations
Distance
(m)
10
50
100
200
300
Nuclear Fluorescence Yield For Various
Explosive Mass M
M15kg
M10kg
M5kg
M05kg
M025kg
M01kg
M005kg
2.0*10-003
Detector:
Air (weight fraction):
Air Density:
Air Attenuation:
N Attenuation
1.5*10-003
270 /s/cm2/mA at distance of 1 m
0.3 g/cm3, (C6H10O5)n, / = 0.0219 cm2/g
1.9 g/cm3, (C4H8N8O8), / = 0.0216 cm2/g
on resonance / = 0.0623 cm2/g
BaF2, 4.89 g/cm3
14N 0.755, 16O 0.232,
0.001225 g/cm3
off resonance / = 0.021 cm2/g,
on resonance / = 0.052 cm2/g, (exp)
Transmission In Air
Off
On
Resonance Resonance
97%
95%
88%
79%
77%
62%
60%
38%
46%
23%
Total
92%
69%
48%
23%
11%
Pixel Geomet.
Factor
Size
1/4r2
(cm)
12
61
122
244
366
7.910-8
3.210-9
7.910-10
2.010-10
8.810-11
1.0*10-003
0
30
60
90
120
150
180
Angle ()
Initial System Modeling
Transmission profile of HMX explosive (5cm radius, 4 cm thick )
embedded in an LD3 container filled up with cotton, detected by
twenty one BaF2 detectors.
Filled Container
Cluster #
1.
Point Source
E=9.17225 MeV
2.
3.
Explosive
4.
d
Summary
• It is critically important for gamma nuclear resonance absorption measurements to modify the
cross section libraries for MCNP to include the nuclear resonance cross sections for the
elements of interest. At present only the nitrogen library has been modified.
• For nuclear fluorescence it is equally important to ascertain that cross sections have been
modified for all nuclear levels of interest and that angular correlation of gamma emission is
included. At present these two factors are not included in the cross sections.
• Use of additional regular detectors to measure nuclear fluorescence simultaneously with the
transmitted radiation will improve the signal.
BaF2 Detectors
r
Photon energy distribution in- and out- of resonance impinging
upon a single detector after traversing 50 m of air, using 10 eV
wide scoring beans the additional attenuation due to resonance
cross section is clearly visible.
• Nuclear fluorescence spectra are preferably collected in a backward configuration reducing the
Compton scattered to the detector and shifting its energy below 500 keV.
• Controlling factor in a stand off configuration is the inverse square distance from the nuclear
fluorescence to the detector. The attenuation in air is manageable even at large distances. The
results will depend on positioning of the detectors.
• Initial simulations of the transmission through an explosive using a monoenergetic gamma
beam and of transmission through air using energy distributed source demonstrate the extra
resonance attenuation.