Treaty verification with “passive neutron signatures”

Nuclear Instruments and Methods in Physics Research A 353 (1994) 712-715

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

ELSEVIER

Treaty verification with "passive neutron signatures" R.A. August a

a'*,

G.W. Phillips

a,

S.E. King

a,

J.H. Cutchin

b

U.S. Naval Research Laboratory, Washington, DC, USA n Sachs/FreentanAssociates, Landover, MD, USA

Abstract Cosmic ray interactions with materials result in secondary neutrons which are easily measurable. The magnitude of this neutron flux depends on geomagnetic latitude, altitude, and on the materials that constitute the local environment . In general the neutrons produced by cosmic rays in a given object depend on the atomic number, density, and total mass of the elements which constitute that object . Any massive object will have a unique "neutron signature" depending on its materials and structure. To test if this phenonemenon might be used to determine if a closed canister contains a specific object, such as a missile, we conducted a proof of concept study. The study had two parts, experimental and Monte Carlo simulations . The object was to perform measurements and simulations of a "block missile mockup" . We constructed the mockup at approximately one-sixth the mass of a real missile. A clear neutron signature was observed from this mockup experimentally, and modeling predictions agreed well with the data . Predictions from our results indicate that a full sized missile should produce a very distinct signature . 1. Introduction The Radiation Detection Section of the Naval Research Laboratory (NRL) has completed an initial study of the feasibility of using passive cosmic ray induced "neutron signatures" as a method of treaty verification . The main thrust of this study was to use the "neutron signature" as a unique method for verification of the type of missile or other treaty limited item (TLI) inside a closed container, and during this study we confined our measurements to such an application. However, the general concept need not be confined to this limited role, but can be applied to any situation where a passive characterization of a large object is desired. Cosmic rays consisting primarily of charged particles are constantly bombarding the earth. Neutrons are produced by some of the cosmic rays undergoing reactions as they pass through the atmosphere [1]. We will refer to these secondary neutrons as environmental neutrons . We therefore have two sources of neutrons that must be considered when making a measurement on the earth's surface . The environmental neutrons produced in the atmosphere and those neutrons produced by the interaction of cosmic rays with objects on the earth's surface. To understand how the two types of neutron flux combine to * Corresponding author .

produce a signal in a neutron detector during an inspection of a TLI, consider the following very simple illustration of a larger neutron detection unit sitting on top of a TLI that is not much larger than the detection unit itself. In this case we are making a measurement at only one point directly on top of the TLI. Consider that this detection unit is shielded from neutrons on all sides except the one that faces the TLI. The shielding will be considered to be 100% effective for the purpose of keeping this example simple certainly not the case in reality . In this simple case, the neutron flux that this detection unit sees will be the sum of the neutron fluxes from four different pathways . They are shown graphically in Fig. 1 . 1) Primary cosmic rays (charged particles) interact with the materials in the TLI to produce secondary neutrons . Some of these neutrons are then detected in the detection unit . The magnitude of this neutron production depends on the atomic number, density, and total mass of the elements which constitute the TLI, and on the magnitude of the incident cosmic ray flux which depends on geomagnetic latitude, altitude, and on extraterrestrial factors such as whether the sun is in a solar maximum or minimum. The neutron scattering and absorption cross sections and the geometric arrangement of materials within the TLI are also critical, since the neutrons are susceptible to scattering and absorption before they can be detected . 2) Environmental neutrons scatter within the TLI until they reach the detection unit . The magnitude of the de-

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R.A . August et al. /Nucl. Instr. and Meth. in Phys. Res. A 353 (1994) 712-715 I I I

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makes measurements at a number of points along the

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measurements of the cosmic ray background with a moni-

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length of the missile, while at the same time making tor detector like the one just discussed. The individual

measurements taken along the missile should then be normalized by the monitor counts in order to divide out any effects due to a variation in the incident cosmic ray flux. We then have a set of numbers that is characteristic of the type of missile and independent of variations in the cosmic-ray flux . The collection of all of these points comprises the "neutron signature" of that missile.

Fig. 1. Total neutron flux in the detector : 1) produced in the TLI; 2) scattered in the TLI; 3) scattered outside the TLI; 4) produced outside the TLI.

tected flux depends on the magnitude of the environmental neutron flux, which in turn is dependent on the cosmic ray

flux whose variability was discussed in the preceding paragraph on pathway 1) . This neutron flux is also depen-

dent on the same scattering and absorption considerations discussed for the first pathway.

All missiles of a given type should have the same

neutron signature, since manufacturing irregularities should

not cause two missiles of the same type to be different

enough to have different signatures ; however, this point

cannot be conclusively determined until real missiles are measured . The degree to which different types of missiles can be distinguished is also a point which requires actual missile measurements for an accurate determination ; how-

ever, different types of missiles should have different

signatures, since they have significant differences in their

3) This is the same as pathway 2), except the environ-

gross features . Therefore, if one takes a baseline neutron signature of a specific type of missile, then from that point

the local environment. 4) This is the same as pathway 1), except the cosmic ray interaction now occurs outside the TLI in the local

we are speaking only of missiles here, these methods

mental neutrons are now first scattered outside the TLI by

environment.

Our study results show that neutrons from pathway 4)

only becomes a significant part of the detected flux in

on one should be able to use this method to tell if an unknown missile is of the same type by comparing its neutron signature to the baseline measurement. Although

would apply to any situation involving large, complex objects.

This concept is ideal for treaty verification because: it

cases where objects in the immediate vicinity are very

is completely passive and non-intrusive ; it uses relatively

atomic weights. The neutron flux measured by the detec-

modular, and portable ; it is relatively low technology, and

massive and composed largely of materials with high tion unit will therefore be dominated by the contributions

from pathways 1), 2) and 3), the cosmic ray neutron production within the TLI and the environmental neutron

flux modulated by the TLI and the local environment.

At this point it is important to introduce the concept of a neutron monitor. Consider the same neutron detection unit shown in Fig. 1 - with no TLI present - except now the unit is shielded on all sides by enough polyethylene and paraffin that all neutrons produced outside the monitor

will be blocked from reaching the neutron detection unit for this example we are again assuming that this shielding is

100%

effective. Considering pathways analogous to

those discussed in Fig. 1, we see that pathways 2), 3) and 4) are blocked, since they all involve neutrons produced

outside the monitor. Only the neutrons produced by cosmic

ray interactions within the monitor have a possibility of detection. This neutron monitor is therefore a measure of variability in the incident cosmic ray flux . This is important since when making a measurement like the one in Fig.

1, any variability in the incident cosmic ray flux will result

in neutron flux changes in all four pathways . For simplicity, consider only one type of real TLI, a missile. Assume one takes a neutron detection unit and

inexpensive, "off the shelf" components ; it is rugged,

simple to operate. We project that an optimized detector

system - we based our projections on a scaled up version

of the detectors developed for the INF treaty [2] - would

have missile inspection times on the order of 5-20 min depending on the measurement conditions, the missiles being measured, and assuming that all points were mea-

sured simultaneously with independent detectors. When combined with weighing, measuring, and possibly gravimetry, it would form a system which would be difficult to spoof.

2. Experiments Each detector for these experiments consisted of nine

3He neutron proportional counter tubes that were 1 in . in diameter, 3 ft long, and had a fill pressure of 10 atm. All nine were imbedded in a slab of polyethylene - 3 in . thick by 1 ft wide by 3 ft long - and wired together to act as a single detector . The polyethylene provides neutron energy moderation, which is required since the target neutrons can range up to several MeV, while the tubes are most efficient in the thermal range. We desire neutron detection from VIIIe. TREATY VERIFICATION

714

R.A . August et al.I Nucl. Instr. and Meth . in Phys. Res. A 353 (1994) 71 2-715

only one of the large faces of this slab - a 3 ft 2 cross

135

active face) were shielded against low energy neutrons by a several inch thick case of borated plaster . The entire assembly was sealed in a plywood box that was filled with

1211

o MCNP Calclatms a Experrnental Data

section - so all but one such face (which we called the

moisture absorbing desiccant - such detectors are highly sensitive to moisture . Measurements were made with the active face resting directly on the object being measured. The monitor detector for these experiments was the

same, except that it was buried in a mound of polyethylene and paraffin that was 3 ft high by 4 ft wide by 6 ft long.

This provided a monitor environment that was isolated from, and therefore independent of, the experimental activities and the environmental neutrons .

The object was to measure the neutron signature of a

semi-realistic mockup of a generic missile. The generic missile was based on real missiles [31 using only the bulk features and leaving out details which could confuse inter-

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75 NozzIel 60

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1

2

Fuel 3

I NozzIel 4

5

6

Fuel

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7

PBPS I 8

9

RV 10

11

12

MEASUREMENT POSITION

Fig. 2. Combined experimental results (squares), MCNP simulation results (circles) .

pretation of our results . We felt this would provide the

and five points that straddle between one box and the next .

design will be expected to produce a less detailed neutron signature . The design of the generic missile was then

was centered over the RV . Three passes were made over

hardest proof of concept test, since a less complicated

reduced to a smaller mockup which could be constructed without great expense, while still giving a stringent test . The mockup consisted of six plywood boxes of dimension 6 ft long by 2 ft wide by 2 ft high . The boxes were placed side by side to produce a unit that was 12 ft long by 6 ft wide by 2 ft high. This resulted in six basic sections from

which to mockup the missile. The six sections were a base, motor (fuel section), interstage, second motor (fuel sec-

tion), post-boost propellant stage (PBPS), and reentry vehicle.

Material components were approximated for the con-

Point one was centered over the base, while point eleven

the boxes from point one to point eleven . Each measure-

ment actually consisted of the combination of sixty individual 1 min measurements . A neutron calibration source was used to check detector efficiency if a considerable

time had elapsed between detector uses, or whenever the electronics were modified . A 252Cf neutron source was used to make small efficiency corrections, which never

varied by more than

±3%. The monitor ran at a very

constant rate of 0.36 ± 0.01 counts per second during these

measurements, so it was not necessary to make any correc-

tions due to cosmic-ray flux variations .

There were multiple measurements at each of the eleven

struction of the mockup . Material substitutions were made

points, due to the fact that the entire mockup was mea-

adjusting densities. Thus aluminum was used to substitute

each other within statistical limits, therefore they were

to simplify the mockup using similar atomic weights and

for fiberglass and silica, and polyethylene was specified as

a substitute for acetate and carbon-phenolic . Lead was

used to simulate the high atomic number components of the RV . No fissionable or other radioactive materials were used in the mockup . In practice, the fuel proved to be the limiting factor to achieving the full mockup mass desired. We could only pack the inert components used for the mockup fuel to about two-thirds the density of solid rocket

motor fuel . We therefore constructed the entire mockup at

two-thirds density - this, along with the fact that our overall mass was much less than a real missile, once again

making our test more stringent, since this tends to diminish neutron signature strength . Also, the plywood of which the boxes are constructed served to mockup the acetate material in the side walls of the motor casings. The motor fuel was packed uniformly, while the other components were placed in the positions where they would be found in a real missile [31 - empty cardboard boxes were used as spacers. The mockup had eleven measurement points in total. These were six points directly over the center of each box,

sured several times. The results at each point agreed with combined, and are shown as the square data points in Fig.

2. The error bars shown in the figure represent the combined error of the various measurements . The "combined"

error is the greater of the external error propagated from the errors in the individual measurements and the internal

error calculated as the standard error of the mean of the measurements . This assures that the errors are not under-

stated, and that all comparisons made using these errors

are meaningful . The line connecting the points is simply to guide the eye.

3. Monte Carlo calculations The computer program MCNP was used to make Monte

Carlo simulations of our experiment . This was done for three reasons. The first was to compare the predicted results with our actual data as a way of validating our data .

The second was that the simulation would tell us which of the four pathways shown in Fig. 1 was having the most

R.A . August et al. /Nucl. Instr. and Meth . in Phys. Res . A 353 (1994) 71 2-715 profound effect . The third was that we could use the knowledge gained from the comparison to predict what kind of differences we should see in a full scale missile.

The model calculates the observed signature in two

715

full-blown model was beyond the scope of this project, but some basic calculations were made . Pathway 2) will still remain dominant, but the larger mass now brings the

the mockup and in the immediate environment . The second

contribution of pathway 1) way up . In fact, the results indicate that pathway 1) should comprise 10-20% of the effect on a full scale missile. Also, the full scale missile's

cosmic ray air showers and propagated into the model space. Results of an earlier study [4] - which used the same type of measurements and simulations on very sim-

small mock-up.

parts. The first part consists of those neutrons produced in

part tracks the "environmental neutrons" produced in the

ple large blocks of single materials - provided neutron source terms for the simulations of pathways 1) and 4) for propagation in the first part of the model. The measured ambient background of 1.1 counts per second was used as

the source term for simulations involving pathways 2) and 3) for the second part of the calculation . The neutron

energy spectra were based on one dimensional transport calculations of O'Brien [1] and the experimental measure-

ments of Hess [5]. For the purposes of the simulation, the environmental neutrons were started 1-20 m above the mockup over a wide area . In the simulation both the mockup and the detector unit were modeled as accurately as possible both in terms of component mass and elemen-

tal constituents . The simulation also included the contribu-

tions from the concrete slab and ground on which the experiments were conducted and nearby structures . The results of the MCNP calculations are shown as the circles in Fig. 2. The figure shows that within statistics the simulation reproduces the experimental results accurately .

The MCNP simulations show that the dominate effect comes from pathway 2), the scattering and absorption of environmental neutrons within the mockup . This is espe-

cially obvious over the fuel sections, where Fig. 2 shows a

larger mass, greater mass density, and greater overall complexity of mass distribution, should result in a more detailed neutron signature than the one achieved with our

4. Conclusions Fig. 2 shows clear variations in the neutron signal at different points over the mockup . This shows that measurable neutron signatures do exist. The agreement of the

simulations with the experiments both confirms and explains this conclusion . The simulations also showed that neutron scattering and absorption is the dominant effect on the scale of the mockup, but that on a full scale missile we can expect a very significant additional contribution from neutron production within

the missile . We expect the overall neutron signature of a real missile to be far more complex than the one obtained on our simple mockup .

Acknowledgement This work is an extension of an NRL program origi-

nally funded by the Defense Nuclear Agency (DNA).

clear suppression of neutrons over these sections . This

References

is thus an efficient neutron absorber . The greatest contribu-

[1] K. O'Brien, H.A. Sandmeier, G.E . Hansen and J.E . Campbell, J. Geophys. Res. 83 (1978) 114. [2] K.W . Marlow et al ., Sandia National Laboratory, private communication . [3] W.W . Craig, DIA Report #DST-1000B-365-89 (1989) (S/NF/WN). [4] R.A . August, G.W . Phillips, S.E . King and J.H. Cutchin, Neutron Signatures Proof of Concept, Phase I Final Report to DNA (1992) . [51 W.N . Hess, H.W. Patterson, R. Wallace and E.L. Chump, Phys . Rev. 116 (1959) 445.

makes sense since the mock fuel is very hydrogenous, and tion from another pathway comes over the RV . Here

pathway 1), neutron production within the mockup, plays a more significant role (10% contribution) because of the high mass and atomic number of the RV mockup, but the experimental data is still not significantly above the ambient environmental flux. Overall, the conclusion has to be that pathway 2) is heavily dominant on our small mock-up. The final phase of our simulations was to use the

MCNP results to make predictions for a full size missile. A

VIIIe. TREATY VERIFICATION