Detection of Hidden Explosives Using Resonant Gamma
Rays From In-Flight Annihilation of Fast Positrons
Noel A. Guardala+, J.Paul Farrell*, Vadim Dudnikov* and George Merkel§
+
Naval Surface Warfare Center/Carderock Division, Code 644, 9500 MacArthur Blvd, W. Bethesda MD 20817
*
Brookhaven Technology Group, 12-12 Tech Drive, Nesconsett, NY 11793
§
Army Research Lab, 2800 Powder Mill Road, Adelphi, MD
Abstract. Gamma rays with tunable energies can be produced by the in-flight annihilation of fast positrons. The kinetic
energy of the positron beam determines the gamma-ray energy of annihilation photons emitted in a narrow cone in the
forward direction. These photons can be used for nuclear resonant fluorescence determination of explosive materials rich
in 14N selecting gamma rays with energies that match excited states in 14N and then observing the emitted nuclear
fluorescence.
0+ → 0+ transition between the first excited state of
16
O at 6.06 MeV and the ground state of 16O. This
mode of decay, internal pair conversion, is favored in
99.999% of the decays of that first excited state.
Another possible mode of producing e+’s.
INTRODUCTION
Finding and identifying explosive materials which
are buried such as mines or hidden for terrorist
purposes or have been discarded improperly such as
unexploded ordnance (UXO) has become a major
issue recently. Approaches based on elemental
detection and not on object detection are considered
more reliable and accurate. Methods using gamma
rays as probes are desirable due to the penetrating
abilities of both the incoming radiation and the
outgoing radiation (the signal of the element’s
presence).
The positrons are then thermalized, “cooled” to
very low energies and then injected into a 10 MeV
LINAC. After being accelerated to the desired kinetic
energy the positrons are annihilated in-flight via
collisions with a suitable low-Z material. This process
preferentially produces a beam of monoenergetic
gamma rays in the forward direction (the same
direction as the incoming e+ beam) with a very small
angular divergence. This beam of quasimonoenergetic gamma rays is then used to interrogate
for the presence of nitrogen nuclei present in higher
than normal concentrations in a particular area. The
energy of the gamma rays are selected to match
particular excited levels in 14N which then re-emit
characteristic gamma rays. The intensity of these
gamma rays, nuclear fluorescence, is then used to
determine if an anomalous amount of nitrogen is
present in a particular area which is usually an
indication that an explosive material is present in that
area.
Producing monoenergetic gamma rays for use in
nuclear resonant fluorescent methods is a complicated
and demanding task. Moreover the ability to produce
monoenergetic and tunable gamma rays has been an
even more difficult technical accomplishment for
physicists and engineers around the world. The
tunablity, the ability to select and probe with different
gamma ray energies allows for multi-element
capabilities in using the nuclear resonant fluorescent,
NRF, method.
The method we have proposed to produce tunable,
mononergetic beam of gamma rays is one based on the
in-flight anihiliation of fast positrons (1). The
positrons are first produced via the nuclear reaction,
19
F(p,αγ)16O reaction which produced e+e- pairs via a
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
2003 American Institute of Physics 0-7354-0149-7/03/$20.00
905
In addition to this method of nitrogen detection
using NRF which has been demonstrated previously
using the 13C(p,γ)14N reaction (2) other elements of
interest can be detected using the same accelerator
system such as: P, S, C, O, Cl, U and Pu. All of these
are related to either some chemical compound which
can be toxic and/or explosive (either in the
conventional or nuclear sense) and of potential use by
terrorist organizations. Some of these elements are
also pose severe environmental hazards either in their
pure state or incorporated in chemical compounds.
a bit smaller than the yield of monopole e+’s.
Simulations of pair production for the three strong γ
lines produced, e+ transport and thermalization will be
performed using W (110) as the moderator/converter.
Preliminary estimates indicate that about 6 foils of
roughly 1 mm in thickness should give the optimal
yields of e+’s from this production method.
Method involving internal pair conversion
The various proton resonances on F19 which result
in excited states of O16 have been studied extensively
over the past 50 years from the points of view of both
applied and pure nuclear physics. The region of proton
energies we are concerned with in terms of being
applicable with a 2.5 MeV proton beam is 0.340 –2.5
MeV. There are several strong resonances in this
energy window (3).
Discussion of 19F + 1H reaction to produce
O16 excited states
This method involves dealing with a broad
distribution of positrons with energies from near zero
(with a nearly zero probability) up to a maximum at
near 2.5 MeV and then dropping off to near zero again
as the maximum in kinetic energy of 5.04 MeV is
reached. While this type of spectral distribution is a bit
different in structure than what is normally
encountered in typical e+ spectra from radioactive
sources it is considered reasonable that all of the e+’s
can be thermalized using an “electronic cooler”
presently being designed at the Budker Institute of
Nuclear Physics, BINP, located in Novosibirsk,
Russia.
However, the one most significant in terms of
producing monopole e+’s is the broad one which has a
peak at 2.2 MeV proton energy. This resonance is very
wide, ca. 500 keV so in order to get maximum
intensities of monopole e+’s a proton beam of 2.5 MeV
is very desirable.
The other lower-energy resonances contribute
nearly zero intensity in terms of their ability to
populate the 6.05 MeV first excited state of 16O. These
lower - energy resonances can be significantly larger
than the one broad resonance at 2.2 MeV and therefore
they are the primary source of the monoenergetic
gamma rays (3 separate gamma rays) of between 6.13
– 7.12 MeV.
It is thought to be possible to collect and inject with
nearly 100% efficiency all of the e+’s that are produced
from the monopole decay of the first excited state of
O16 that would be produced from the bombardment of
a range-thick suitable F-containing target with protons
of 2.5 MeV energy. Target designs are also presently
being tested at BINP with consideration given to
ability to handle the heat load and accessibility of the
F-containing target to the cooler device so as to
achieve the near 100 % collection value.
The yield of monopole positrons or monoenergetic
γ’s can be calculated using the following:
Method involving external pair
Conversion
Y = n ∫σdx = (n/ρ) x ∫σ(Ep) /S(Ep) dEp x Q/t (1)
Where n is the number of F nuclei in the target per
cm3, σ is the cross section and σ(Ep) is the value of the
cross section at a particular value of the proton energy
inside the target, dx is the penetration distance in the
target of the proton, ρ is the density of the Fcontaining target material and S(Ep) is the proton’ s
stopping power in the target material as a function of
depth and Q is the integrated proton current of 2.5
MeV energy protons and t is the total time of
irradiation.
Another approach to produce positrons is to use the
more abundant amount of gamma rays in the 6-7 MeV
range that are produced form the 19F(p,αγ)16O. These
gamma rays produced from 2.5 MeV protons
impinging on a range-thick F-target are approximately
50 times more abundant than the e+e- pairs produced
via the monopole transition. However, The need to
first convert these γ rays into e+e-‘s pairs and then to
moderate them down to thermal energies (the pairs
should be produced with monoenergetic energies since
the incoming γ rays are monoenergetic as well) leads
to overall losses in intensity such that the final yield of
low-energy e+’s should be no larger and possibly even
When the various parameter are factored in for a
solid SF6 target for a 10 ma proton current running
continuously on the target a value of ca. 3 x 1010 e+’s s-1
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are produced. Using a novel design for an electronic
“cooler” to both thermalize and collect the total
energy spectrum of the monopole e+’s it is believed
that nearly all of the e+’s produced can be injected into
a 10 MeV LINAC and then accelerated to the
appropriate energy to produce a tunable resonant,
quasi-monenergetic γ-ray beam (4).
of magnitude in irradiating a spot with a diameter of ~
20 cm.
The spatial sharpness of this kind of beam is very
desirable in making accurate determination of whether
dangerous or hazardous material in area of roughly
0.15 m2. In order to have a beam of such brightness,
novel conversion techniques similar to the ones
involved in thermalizing the monopole e+’s will be
investigated at BINP in Novosibirsk, Russia.
In-Flight annihilation of MeV-energy e+’s
The resonant, quasi-monenergtic γ rays are
produced by the annihilation of MeV-energy e+’s with
a source of electrons. The less tightly bound these
electrons are the narrower the resultant γ rays will be.
This is a significant concern for NRF or nuclear
resonant absorption, NRA, since the widths of the
most desirable levels to excite in nuclei is typically on
the order of 1-200 eV. It is highly desirable to have the
width of the incoming γ-ray beam to have a
comparable spread in energy as the intrinsic width of
the nuclear state or if possible have an even narrower
spread in energy than the width of the corresponding
nuclear level.
A further possibility to increase the e+ currents that
would be passing through the conversion region is the
use of traps in (5) which e+’s could be stored as they
were produced via the 19F(p,αγ)16O, that is when the
proton beam was on the target. The stored e+’s could
then be released under controlled conditions when it
was necessary to perform the interrogation. When no
interrogating for dangerous or terrorist materials the
traps could be replenished by continous running of the
proton beam.
Nuclear
Resonant
Fluorescence
In past applications, this technique of producing
quasi-monoenergetic γ’s has relied on the use of thin,
low-Z solid converters such as Be or graphite. The
reasons for this approach are a) the low Z minimized
e+ energy loss through process’ such as
bremsstrahlung which would increase the width of the
γ-ray beam and b) the low-Z solids have nearly “free”
i.e. very weakly bound electrons so that the
broadening of the resultant γ-ray beam produced by
annihilating is suppressed relative to annihilation with
more tightly bound electrons due to their broader
momentum distributions. A necessary balance is
sought between the thickness of the target and the
density of “quasi”-free electrons (in cm2) to produce
the most monoenergetic beam and yet to also give the
highest ratio of emitted, high-energy γ rays to incident
high-energy e+’s. However, for a 400 µm graphite
taget the conversion efficiency is only 2 x 10-4 γ rays
per incident fast e+, this for e+’s in the energy range of
5-10 MeV.
Absorption
and
The γ rays produced in the manner described can
be used to detect the various nuclei mentioned in the
Introduction section by either the resonant
fluorescence or absorption method. In the fluorescence
process a second photon either equal in energy to the
exciting γ-ray or of lower energy will be emitted. In
the absorption process the exciting photon is “lost”
and the intensity of γ rays impinging on a certain
object is diminished by this process.
Which method is suitable for a given application
depends on a) the geometry that the object(s) to be
investigated posses and b) the relative intensity and
probability for fluorescence to take place for a given
isolated resonance. As for consideration a) it can be
conceivable that for terrorist attempts the objects may
be placed in a geometry that may allow for
measurements based on a diminished flux of γ rays
that emerge after passing through an object. It may
also be the case that because of the physical inability
or desirability to have a suitable γ-ray detector behind
a suspected object that absorption may not be possible
to measure.
It would be very desirable if the efficiency for this
conversion process was raised two orders of
magnitude or so. Then for an injected e+ current of ~ 3
x 1010 /sec, the γ-ray flux would be around 3 x 108
photons/sec. Given the small angular divergence
associated with in-flight annihilation at e+ energies of
5-10 MeV, i. e. 0.015 mrdn (or about 1deg) this would
correspond to a very dense flux of γ rays even at
distances of 10 m or so. At that distance a flux of 3 x
108 photons/sec would decrease roughly only an order
A look at the equations which govern the
probabilities of both processes shows why absorption
mode is favored in terms of maximum sensitivity
measurements.
The Breit-Wigner formula can be used to determine
the cross section for an isolated resonance:
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σ(E) = πλ2g ΓaΓb/(E -Er) + (Γ/2)2
to detecting at most two different elements using the
same accelerator design. That is in order to have the
system capability to probe for more than selected
elements which have nuclear levels that would be
accessible through the use of a single accelerator so
that a unique proton energy can be used to excite two
different nuclear resonances thereby producing
resonant photons to probe for two different elements.
(2)
where σ(E) is the resonant cross section as a function
of energy in this case excitation γ-ray energy, E is the
energy of the incoming photon, λ is the reduced
wavelength of the incoming photon, g is the nuclear
statistical factor, Γ is the total width of the nuclear
state and Γa,b is the reduced with for either process a,
absorption of the photon and b the emission of the
fluorescent photon (6).
To go beyond this dual element capability would
require a whole new accelerator design which in turn
would have at best dual element capability and so
forth. While the system described here has the
disadvantage of needing two accelerators a tandem 2.5
MeV accelerator to produce a high current of protons
to excite F19(p,αγ) resonances to produce the e+ beam
and then a 10 MeV LINAC to accelerate those e+’s to
the appropriate energy to produce resonant γ rays for
several particular elements; it is believed that several
features of this kind of systems are far more attractive
and versatile than the methods which depend on
isolated nuclear resonances at fixed proton energies to
produce narrow, characteristic γ rays that are useful for
only single element detection.
The reduced wavelength, λ is given by:
λ= hc/E
(3)
where h is Planck’s constant divided by 2π, c is the
speed of light in a vacuum and E is the excitation
photon energy
the multiplicity of the level, g, is given by
g = 2J + 1 /2(2I + 1)
(4)
where J is exit level spin and I is the entrance level
spin
References
The peak cross section is given by,
σmax = 4πλ2gΓaΓb/Γ2
1. Seward, F. D. , Hatcher, C. R. and Fultz, C., Phys.
Rev. 121, 605-609(1961).
(5)
Then the integrated cross section can be expressed
2. Morgado, R. E. et al, SPIE 2092, Substance
Detection Systems, 505-513 (1993).
as:
σint = ∫σdE = π/2σmaxΓ = 2π2λ2gΓaΓb/Γ
(6)
3. Ranken, W. D., Bonner T. W., and McCrary, J. H. ,
Phys. Rev. 109, 1646-1651(1958).
This shows that cross section for resonant
absorption, is proportional to Γγ/Γ while the resonant
cross section for fluorescence is proportional to
(Γγ/Γ)2. So that for all cases the fluorescence cross
section is smaller by a factor of Γγ/Γ than the
absorption cross section. Absorption methods are
therefore the preferred method to use for detection due
to their higher probabilities.
4. Farrell, J. P. et al, SPIE 3730 13th Annual AutoSense Smposyium, 446-453(1999).
5. Greaves, R. G., and Surko C. M., Phys. Rev. Lett.
75, 3846-3849(1995).
6. Malmfors, K. G. and Mossbauer, R., Nuclear
Resononance Flourescence of Gamma-Radiation,
in Alpha, Beta, and Gamma-Ray Spectroscopy, K.
Seigbahn, Editor, Amsterdam, North-Holland, pp.
1281-1291(1961).
Conclusions
A method is described which produces a tunable,
quasi-mononenergetic source of γ rays which can
probe for hidden dangerous or hazardous material
which may be used in future terrorist attacks/actions. It
has the advantage over other methods which although
they produce a more monoenergetic γ-ray beam has
limitations on the number of different γ-ray energies
they can produce. That restriction limits that method
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