Detection of subsurface fissionable nuclear contamination through the application of photonuclear techniques

A photoneutron detection apparatus comprises an X-ray generator and a neutron detector adapted for placement in proximity to subsurface soil. The X-ray generator produces timed pulses of X-ray photons having a range of electronically controlled energies which penetrate the subsurface soil to produce photoneutron emissions that are received by the neutron detector. The neutron detector generates a signal representative of the neutron flux. A signal analysis system time correlates the signal with the timed X-ray pulses to determine the presence of selected contaminants.

STATEMENT OF GOVERNMENT INTEREST 
The invention described herein may be manufactured and used by or for the 
Government of the United States of America for governmental purposes 
without the payment of any royalties thereon or therefor. 
BACKGROUND OF THE INVENTION 
The invention relates to the field of in situ detection of subsurface 
contamination, and more particularly, to a system which detects subsurface 
fissionable nuclear contamination by means of photoneutron emissions and 
detection. 
The purpose of this invention is to provide a cost effective means for in 
situ detection of fissionable materials such as uranium and plutonium 
which may be present as soil contaminants in the vicinity of nuclear 
material processing facilities. Previous methods for detecting such 
contamination require that samples be extracted and sent for laboratory 
analysis. This procedure involves delay and labor intensive handling. A 
far more desirable approach is to detect and quantify fissionable 
contamination in situ. Such an approach allows for the possibility of real 
time three dimensional mapping of the contaminants. 
Passive detection of the natural radioactive emissions from fissionable 
materials is limited by the relatively low level of such emissions and the 
relatively high background level associated with natural terrestrial 
sources. An alternative approach is to selectively increase the radiation 
emission rate of the contaminants by active stimulation. An example of 
active stimulation of radiation emission is photonuclear stimulated 
neutron emission or photoneutron generation. 
Photoneutrons are generated when an energetic photon interacts with a 
nucleus. If the energy of the photon exceeds the threshold binding energy 
of neutrons in the nucleus, a photoneutron can be liberated from the 
nucleus. The energy threshold for this process is dependent on the 
isotopic species of the nucleus with which the photon interacts. By 
observing the threshold at which photoneutron generation occurs it is 
possible to detect and distinguish the presence of specific elements. This 
technique is useful for fissionable contaminants such as uranium and 
plutonium, and also for other selected contaminants such as beryllium. 
Photoneutron stimulation is not commonly used as an analytic technique 
because the photon energy required is very high. The photon energy 
threshold ranges from 1.665 MeV for beryllium to in excess of 7 MeV for 
many common metals and rare earth elements. The threshold energy range for 
fissionable nuclear contaminants is between approximately 3 to 7 MeV. 
Operation of sources which generate such energetic photons involves 
significant safety issues, and for laboratory analysis many alternative 
methods are available, including many techniques of chemical analysis and 
spectroscopy of alpha particle and gamma radiations. 
For in situ analysis of soil contamination the laboratory techniques are 
not generally practical, since laboratory instruments are not designed to 
be operated inside holes drilled in the ground. Therefore, the current 
practice is to remove samples as noted above. 
Therefore, there is a need for an apparatus and method for in situ analysis 
of fissionable nuclear contamination in subsurface soil. 
SUMMARY OF THE INVENTION 
In accordance with this inventive concept, the above noted problems of the 
prior art are overcome and there are provided an apparatus and method 
which employ the process of photoneutron emission to detect fissionable 
nuclear contamination in subsurface soil without removing samples for 
laboratory analysis. 
In accordance with one aspect of the inventive concept, a subsurface 
photoneutron detection apparatus includes X-ray generating means and 
neutron detecting means which are adapted for placement in proximity to 
subsurface soil. The X-ray generating means provides pulses of X-ray 
photons of selected energies which penetrate surrounding subsurface soil 
to produce photoneutron emissions that are received by the neutron 
detecting means. Signals produced by the neutron detecting means are 
conveyed to a signal analysis system which determines the presence of 
selected contaminants in accordance with the relationship between the 
neutron detector signals and the energies of the X-ray photons. 
In accordance with another aspect of the inventive concept, a method for 
detecting subsurface contaminants by photoneutron emission includes 
generating X-ray photons of selected energies at a location proximate to 
subsurface soil. Photoneutrons from contaminants in the subsurface soil 
are detected to produce signals representing neutron flux. The neutron 
flux signals are analyzed in relation to the X-ray photon energies to 
determine the presence of selected contaminants in the subsurface soil. 
OBJECTS OF THE INVENTION 
An object of the invention is to provide an apparatus for in situ detection 
of selected contaminants in subsurface soil. 
Another object of the invention is to provide a method for in situ 
detection of selected contaminants in subsurface soil using the process of 
photoneutron emission.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 there is shown a schematic diagram of a photoneutron 
detection system 10A in accordance with a first embodiment of this 
inventive concept. A cylindrical housing 11 is shown in a cross-sectional 
view. Cylindrical housing 11 is designed so that the photoneutron 
detection system can be disposed in proximity to subsurface soil. For 
example, the housing may be designed or adapted so that it can be deployed 
by cables and other apparatus (not shown) into a bore hole drilled into 
the ground. As another example, the cylindrical housing may be adapted and 
included as part of a system (not shown) which penetrates subsurface soil 
by application of vertical force. Methods for drilling bore holes and 
otherwise penetrating subsurface soil are well known, and such methods are 
generally included within the practice of the present invention. Suitable 
design of housings for apparatus to be deployed in subsurface systems is 
also well known to engineers skilled in the art. 
An electron accelerator 12 provides pulses of high energy electrons 13. 
High energy electrons 13 travel through propagation tube 12A and impact on 
a target collimator 14 to produce X-ray photons 15 by the physical 
phenomenon referred to as Bremsstrahlung. The energy of the electrons is 
converted into X-rays by deceleration in the target, resulting in a 
Bremsstrahlung X-ray energy spectrum with a maximum energy less than the 
energy of the electrons. Power for electron accelerator 12 is provided by 
an X-ray generator power supply 16 and is conveyed to the electron 
accelerator via a supply line 17. Optionally, supply line 17 may include 
water cooling lines (not shown) as may be required by the X-ray generating 
components 12 and 14. Supply line 17 is designed to allow disposition of 
the photoneutron detection apparatus in proximity to subsurface soil by 
various methods as noted hereinabove. This involves providing sufficient 
length and properties of rigidity and/or flexibility as may be required. 
As noted above, design of equipment for introduction into bore holes is 
well understood by engineers skilled in the field. 
Components 12, 14, and 16 are parts of an X-ray generating system. Suitable 
X-ray generating systems that can be adapted as described herein are 
manufactured by Schonberg Radiation Corporation of 3300 Keller Street, 
Suite 101, Santa Clara, Calif. 95054. An example is the Schonberg 
Radiation Corporation portable high energy X-ray system marketed under the 
designation Minac 6. The practice of the present inventive concept may 
also include use of other X-ray generating systems, providing that the 
systems can provide X-ray photons in an approximate energy range of 3 to 7 
MeV, and also providing that the X-ray source can be adapted for 
disposition within a housing suitable for subsurface placement. 
X-ray target collimator 14 is preferably cylindrical in shape, and is 
schematically shown in a cross-sectional view in FIG. 1. X-ray 
target-colimator 14 is designed so that X-ray photons 15 are allowed to 
propagate in all directions perpendicular to the axis of the cylindrical 
housing and with a divergence of about 30 degrees above and below the 
perpendicular plane. X-ray photons propagating in other directions are 
absorbed by target collimator 14. This allows the X-ray photons to 
penetrate soil around cylindrical housing 11 while limiting irradiation of 
other components. A target collimator which provides such a radiation 
pattern is commercially available as part of the Minac 6 X-ray system 
referred to above. 
Also included in cylindrical housing 11 are neutron detector assemblies 18A 
and 18B. The neutron detector assemblies include a plurality of neutron 
detectors 19. Neutron detectors 19 preferably include detectors for both 
"fast neutrons" and for "slow neutrons", where fast neutrons are neutrons 
with energies greater than 0.0253 eV and slow neutrons have energies less 
than or equal to 0.0253 eV. This distinction is made because a detector 
suitable for detection of fast neutrons is generally ineffective for 
detecting slow neutrons. The inventive concept may be practiced using 
either class of detectors, but for best sensitivity use of both types of 
detectors is preferred. A variety of suitable neutron detectors are 
described in "Radiation Detection and Measurement" by Glenn F. Knoll, 
published by John Wiley & Sons, Inc., 1979. 
The signals produced by neutron detectors 19 are conveyed via signal lines 
included in a signal cable 20 to a signal analysis system 21. Signal cable 
20 also contains lines which convey bias voltages to the neutron 
detectors. Optional preamplifiers (not shown) may be included proximate to 
the neutron detectors to amplify and condition the detector signals for 
transmission to the detector signal analysis system. Signal analysis 
system 21 includes bias supplies, amplifiers, and bandpass filters as 
required for operation of the neutron detectors. The design and operation 
of neutron detectors and associated electronic systems is well known to 
persons skilled in the field of radiation detection and electronics. The 
signal analysis system also includes analog to digital converters and 
digital signal processing electronics such as a digital computer system. 
As an example, a personal computer system equipped with D/A boards, IEEE 
488 control boards, programmable amplifiers and bias supplies may be 
included in the signal analysis system. The signal analysis system is 
adapted by well known methods to acquire the neutron detector signal data 
and perform analyses as further described below in order to determine the 
presence of selected contaminants in the subsurface soil irradiated by 
X-ray photons. 
Principle of Operation 
Operation of the invention will be described in reference to the detection 
of beryllium (Be), as an example. Although Be is not a fissionable 
isotope, it is associated with the production of nuclear devices, and the 
principle of operation for this inventive concept is the same for Be as 
for fissionable nuclear isotopes. It will be obvious to persons skilled in 
the art of nuclear science that the teachings herein can be generally 
applied for detection of other isotopic species. 
As an example, the invention may operated according to the following 
procedures: First, the photoneutron detection apparatus is placed in a 
bore hole or otherwise disposed proximate to subsurface soil in a desired 
location. 
Electron accelerator 12 is operated to provide brief pulses of high energy 
electrons. This is a capability of the Schonberg Minac 6 system referred 
to above. As explained below, the energy of the electrons is adjusted so 
that the maximum energy of the Bremsstrahlung generated X-ray photons is 
within a desired range of energies above and below the photoneutron 
threshold for Be of 1.665 MeV. Energy adjustment is accomplished by 
adjustment of X-ray generator power supply 16 in accordance with the 
manufacturer's operating instructions. 
The pulses of X-ray photons cause the emission of pulses of photoneutrons 
from surrounding soil. Photoneutrons return to neutron detectors in a 
delayed pulse following an X-ray pulse. Typically, an X-ray pulse duration 
will be less than five microseconds. The delay in the return of the 
neutrons is due to the propagation velocity of the neutrons in conjunction 
with neutron scattering processes. A neutron pulse will occupy a period 
extending to about 100 milliseconds following the X-ray pulse. The brief 
duration of the X-ray pulses allows discrimination of the X-ray response 
in the neutron detectors, and the subsequent pulses of neutrons are 
discriminated relative to background radiation by virtue of being time 
correlated with the X-ray pulses. 
Signal analysis system 21 is adapted to perform time correlated analysis of 
the neutron detector signals to permit discrimination against the X-ray 
pulses and against continuous background noise. This is accomplished by 
well known box-car integration techniques which can be implemented by 
dedicated electronic components or by software programming of digital 
processors in accordance with procedures well known in the art. 
To selectively detect Be contamination in the subsurface soil, a 
photoneutron response N.sub.1 is first measured for X-ray photon pulses 
with a maximum energy at least 100 keV below the 1.665 MeV photoneutron 
threshold. This gives a response level representing soil components other 
than Be. Then a photoneutron response N.sub.2 for X-ray photon pulses with 
maximum energy at least 100 keV above the Be photoneutron threshold is 
measured. The magnitude of the difference N.sub.2 -N.sub.1 is 
representative of the amount of Be present in the soil. 
Alternative Embodiments 
Referring to FIG. 1, the diameter of cylindrical housing 11 is necessarily 
large enough to include the apparatus contained within. Because 
commercially available X-ray systems occupy a minimum diameter greater 
than about 8 inches, the diameter of housing 11 and any bore hole in which 
the apparatus is placed must also exceed about 8 inches. This limitation 
can be alleviated by miniaturization of commercially available apparatus 
by using shorter wavelength microwaves or by other approaches. 
Another method for reducing the diameter of the portions of the apparatus 
which are inserted below the soil surface is illustrated in reference to 
FIG. 2. A photoneutron detection apparatus 10B shown in FIG. 2 differs 
from photoneutron detection apparatus 10A shown in FIG. 1 in the following 
particulars: 
Referring to FIG. 2, electron accelerator 12 is positioned outside 
cylindrical housing 11 (shown in cross-section) and above the soil 
surface. High energy electrons 13 are propagated through an elongated 
propagation tube 12B (shown in cross-section within housing 1) before 
reaching target 14 (shown in cross-section) which can be placed below the 
soil surface as before. The advantage of this approach is that the 
components inside housing 11 can be made much smaller than presently 
available electron accelerator 12, and hence the diameter of housing 11 
can be made significantly smaller so as to penetrate into a smaller bore 
hole. However, the propagation path for high energy electrons 13 cannot be 
allowed to bend. 
Many modifications and variations of the present invention are possible in 
light of the above teachings. It is therefore to be understood that within 
the scope of the appended claims, the invention may be practiced otherwise 
than as specifically described.