Personnel Plutonium Monitor

Abstract

A system for monitoring a zone for the presence of a source of plutonium in which the updated background level of gamma rays is compared with the readings obtained when a possible carrier of a gamma ray source enters the zone. Counting is initiated, and an alarm is sounded when the counting after a predetermined period of time accumulates to a value in excess of the background total plus a significant statistical deviation as a factor of the background total.

Description

SOURCE OF THE INVENTION

The invention described herein was made in the course of, or under a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION

Based upon economic as well as strategic considerations, the control and security of nuclear materials is important to all parties responsible for handling and using them. One of the materials subject to this degree of concern is plutonium which, aside from its significance from the point of view of national security, is employed in appropriate form for the manufacture of fuels for certain types of nuclear reactors.

In facilities which handle or process this material the plutonium is present largely in the metallic, oxide, fluoride, or nitrate form with a few percent abundance of Pu.sup.240 in Pu.sup.239.

The particular area of concern with respect to these isotopes found to be the most difficult security problem is that of the unauthorized removal of plutonium in relatively small amounts, i.e.,down to 0.1 gram of Pu.sup.239/240, carried by plant personnel. The extent of the monitoring problem is indicated by the traffic encountered in one typical installation, that of as many as 700 persons a day with a peak rate of 60 persons per minute passing through standard 3-foot-wide doors and available for observation for about 1 to 1.5 seconds. Physical examination of all those persons on a regular basis is time consuming and inefficient while the difficulty of monitoring by conventional radiation detection instruments is indicated by the fact that these materials produce passive radiation with the bulk of the energy being alpha emissions which can be effectively shielded or blocked by the thinnest and lightest of material. Gamma radiations, which are more penetrating, peak out at levels which are about 10 percent of the alpha energies. Conventional gamma detection instruments are sufficiently sensitive to sense small amounts of plutonium. However, conventional techniques are based on count rate measurements which require either a decrease in effective sensitivity or an increase in the false alarm rate.

BRIEF SUMMARY OF THE INVENTION

The present invention overcomes the major deficiencies of presently available passive radiation detection systems for the monitoring of personnel to detect the presence of plutonium in gram quantities. It provides a unique arrangement capable of detecting gamma radiation at low energy levels and at counting rates heretofore considered to be incapable of being attained while maintaining a low false alarm rate.

In accordance with the principles of this invention, a preferred embodiment of the invention employs a small cluster of spaced gamma ray detectors, a summing circuit to combine the sensed information, and provision to accumulate or integrate the information received during the period the subject is present in the zone of detection. During the time when no subject is present within this zone, the invention maintains an up-to-date reading of background gamma information for the same energy levels. When the subject enters the zone of detection, background information is frozen and maintained until counting of the subject is completed. Background information is then compared with the information taken from the subject in accordance with a predetermined relationship which can be altered somewhat by the operator to accommodate local conditions and instant requirements. Based upon this comparison the presence of plutonium may be indicated and an alarm sounded if desired.

In addition to obtaining high plutonium sensitivity, a system embodying this invention has the additional advantages of simplicity of operation making it feasible for use by people of limited skills, minimum interference with plant operation and personnel traffic, and low capital investment. Good system reliability and the elimination of costly maintenance normally associated with one-of-a kind components are obtained as a result of utilizing as the components of the system readily available commercial equipment.

It is thus a principal object of this invention to provide a personnel monitoring system for plutonium sensitive to gram amounts of the material.

Other objects and advantages of this invention will hereinafter become evident from the following description of a preferred embodiment of this invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an isometric view of a doorway arrangement for the detectors utilized in a preferred embodiment of this invention; and

FIG. 2 is a block diagram of a system embodying the principles of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The plutonium to be monitored in accordance with this invention appears as largely Pu.sup.239 with a few percent of Pu.sup.240 present in metallic, oxide, fluride or nitrate form. As shown in the table, measurement of passive gamma radiation would be most effective at 375 and 414 KeV for Pu.sup.239. Experiments and studies have shown that at these energies the gamma radiation will penetrate moderate shielding. By moderate shielding is meant shielding equivalent to lead thickness not in excess of 1-2 cm. It is assumed that for the detection of plutonium carried in shielding of greater equivalent thickness normal visual surveillance procedures provide adequate security. ##SPC1##

For a description of the overall system of a preferred embodiment utilized for monitoring a doorway reference is made to FIGS. 1 and 2. Through doorway opening 10 pass the personnel or subjects undergoing monitoring. As will be described later, initiation of counting of subject is begun upon depression of floor mat switch 12 the length of which would be such as to provide the normal time span pointed out below for a subject to cover in walking its entire length.

In the particular embodiment, four detectors 16a, 16b, 16c, and 16d are mounted flush in the side walls of opening 10 so that only the entry to each detector is exposed. These detectors would be encased in shielding entry to each detector as to have a 2.pi. directional response as indicated by broken lines 18a, 18b, 18c, and 18d effectively enhancing the source-to-background ratio. The use of four detectors reduces average detector-source separation thereby increasing sensitivity of the system. Subject 22 is carrying a plutonium sample 24.

Detectors 16 a - d are conventional gamma counters and may be NaI scintillation devices coupled to photomultipliers to view and respond to the photon energy spectra from the Pu.sup.239/240.

As briefly pointed out above, the system functions to count integrally and to compare with a continuously updated background level. The comparison is automatically made at a preselected number of standard deviation intervals. Mathematically, the system solves the relation

(1) S.sub.c + B.sub.c > B'.sub.c + N .sqroot.B'.sub.c

where

S.sub.c = source counts

B.sub.c = background counts

B'.sub.c = stored background counts

.sqroot.B'.sub.c = standard deviation of background counts

N = integral multipler, for selecting the number of standard deviation intervals considered to be significant.

In other words, the above mathematical statement becomes true and plutonium is presumed to be present on the subject when the cumulative counts taken from the subject during the prescribed interval exceeds the sum of the stored background counts for a like interval and the preselected number of standard deviations of the stored background counts.

Referring to FIG. 2 for the system for obtaining relation (1) and carrying out the principles of this invention, it will be seen that the outputs of detectors 16a - 16d are summed in amplifier 26 and amplified in amplifier 28, single channel analyzer (SCA) 32 selecting and passing on pulses representing the particular channel selected. Pulses from analyzer 32 are fed to a monostable multivibrator 34 whose amplitude is fixed and whose width is adjustable depending on the setting of range switch 36. The output of multivibrator 34 is passed first through a low-pass filter 38 whose 10-second time constant is long compared to the second or so that the subject is in the counting area but short compared to any changes in average background count rate. This produces an analog voltage that is proportional to the average background rate. A digital voltmeter (DVM)42 reads this output voltage and displays the results.

When a subject enters the area or zone of counting, switch 12 is stepped on and actuated, causing a flip-flop 44 to place a hold on DVM 42 at its last reading which is for the purpose of subsequent computation the average stored background B'.sub.c. Digital-to-analog converter (D/A) 46 continuously produces a negative analog voltage proportional to the stored voltage so that when switch 12 is depressed the negative voltage then produced by D/A 46 is representative of the average background gamma condition.

As the same time that switch 12 sets flip-flop 44, it also initiates 8-second timer 48, reset timer 52, and 20-second timer 54, the purposes of all of these to be explained below.

A data integrator 56, which is reset to zero by reset timer 52 when mat switch 12 is closed, receives positive pulses through resistor R.sub.1 from multivibrator 34 representing, during the counting period just begun, background plus possible signal counts (S.sub.c + B.sub.c) from the subject. These positive pulses cause integrator 56 to produce a "stair-step" count in the negative direction (due to a polarity reversal). At the same time as the preceding occurs, the stored background data from D/A 46 combined with the output of a sigma circuit 57 to be described, in the form of a steady DC negative voltage by way of leads 58 (including balance rheostat 62) and 59, causes integrator voltage to rise as a linear ramp in the position direction. Those two-inputs cause the output voltage from data integrator 56 to average zero. Since the background pulse rate is random the output voltage will look like noise with an average DC value of zero. This will cause red and green output lights 64 and 66, respectfully, to flash alternately.

It will be seen that stored background current is fed by lead 59 to integrator 56 as already noted from sigma circuit 57 consisting of a passive diode squareroot circuit 68 and a two part attenuator circuit 72. The latter is provided with a range switch S.sub.1 to set sigma proportional to the range being used and a switch S.sub.2 to select the number (N) of sigmas to be added.

Date integrator 56 serves the dual purpose of accepting the difference between data pulses (S.sub.c + B.sub.c) from multivibrator 34 and the stored and modified background counts (B.sub.c + N.sqroot.B'.sub.c) from D/A 46 and sigma circuit 57, and integrating the results. The time of integration is not critical, but longer times gives more accurate results.

The output of integrater 56 is fed to a comparator 74 which continuously determines the polarity of the result. When the subject leaves the counting area (releasing floor mat switch 12) the comparator output is strobed. If the output is negative (as noted, there is a polarity reversal in integrator 56) when mat switch 12 is released the signal is passed through and "AND" circuit 76 and alarm 78 is sounded.

Twenty-second timer 54 is designed to indicate sticking of mat switch 12. If mat switch 12, upon being closed by a subject, does not reopen within 20 seconds, then alarm 78 is sounded indicating the possibility of a failure, or the placement of a heavy object on the match switch 12.

The system just described operates in the following manner. When no subject is within the monitoring area, DVM 42 continuously displays and updates the average background count rate.

When a subject steps on mat switch 12, flip-flop 44 "holds" the output of DVM 42, and data integrator 56 is reset to zero. Output counts from analyzer 32, combined with the stored average background count fed through D/A 46, squared in circuit 68, multiplied by N in attenuator circuit 72 and added to the output of D/A 46 is fed into integrator 56, causing the latter to integrate the difference. Thus integrator 56 subtracts the stored and modified background counts B'.sub.c + N.sqroot.B'.sub.c from the data pulses S.sub.c + B.sub.c, and integrates the difference. Switch S.sub.2 in attenuator circuit 72 may be used to alter the multiple N to obtain the degree of selectivity desired.

Comparator 74 receives the output of integrator 56 and determines continuously the polarity of the result. When the subject leaves the counting area as indicated in "AND" gate from switch 12, the output is passed to alarm 78 which is designed to go off when the output is negative.

Background updating occurs at all times that there is no subject on floor mat switch 12. This automatic feature is important for any sudden changes in background due to movement of active materials in the neighborhood. The pulses are averaged by low-pass filter 38. The averaging period has no maximum value but would have a minimum time, such as 5 seconds. If mat switch 12 is open for less than this period, DVM 42 will not be strobed and the previous value will be used.

Rheostat 62 is used as a balance control which equalizes the electrical response of the low-pass filter 38, DVM 42, and D/A 46. Consider a steady train of pulses at the input multivibrator 34. Current from the multivibrator flows through 38, 42, 46, 62, 58 and then to the integrator 56. Current also flows from the multivibrator 34 through resistor R.sub.1 directly to the integrator 56. Balance rheostat 62 is used to insure that these currents are equal in magnitude although opposite in sign.

The output on connector 82 is used to monitor the short term behavior of the integrator. Such things as analytically monitoring the count rate during a counting period or adjustment of rheostat 62 can be accomplished.

It will be seen that the system as just described, which utilizes commercially available components, functions with little or no attention as there are no critical adjustments to be made during its normal operation. The system is readily adaptable to regular field use as it fits in with normal plant routines. The system will detect unshielded plutonium samples in sizes from very large down to less than 1 gram. It is unobtrusive and will provide a positive warning signal for use by plant personnel in controlling unauthorized plutonium removal.

In the event that an organized theft operation is suspected or to be protected against, in which larger quantities of plutonium in well gamma shielded containers would be carried out, a neutron monitoring system can be integrated with the described system. Detectors sensitive to neutron radiation would be added along with an electronic system similar to the gamma detection system described. In the neutron system the minimum source size detectable might be as low as 17 grams. Any neutron shield capable of effectively attenuating the spontaneous fission and (.alpha.,n) neutrons from the Pu.sup.239/240 sample will be large and easily detected by visual inspection.

It is thus seen there has been provided a unique system for the personnel monitoring on a large scale of clandestine diversion of small amounts of plutonium. The system as described which is sensitive to gram quantities or less of plutonium makes it unprofitable and risky for anyone to expend the labor and time required for the unauthorized removal of an appreciable amount of this material.

Claims

What is claimed is:

1. A gamma ray source detection system for use in a monitoring zone comprising:

a. means for measuring and maintaining continuously the updated background count rate of gamma rays in said monitoring zone;

b. means responsive to the movement of a possible source of gamma rays into said monitoring zone for holding the then background count rate;

c. means for producing a first signal corresponding to the sum of the held background count rate and a preselected factor of said held background count rate;

d. means for taking a second signal of polarity opposite to that of said first signal from said measuring means corresponding to the count rate of gamma rays in said zone;

e. means also responsive to the aforesaid movement for cumulatively counting the sum of said first and second signals; and

f. means responsive to the movement of said possible source of gamma rays out of said monitoring zone for indicating that said possible source is an actual gamma source when said cumulatively counting means produces an output corresponding to the same polarity as said second signal.

2. The detection system of claim 1 in which said indicating means delivers an alarm signal warning of the presence of a gamma source when said source leaves said zone.

3. The detection system of claim 1 in which the preselected factor is a multiple of the square root of said held background count rate.

4. The detection system of claim 3 in which the means responsive to the movement of a possible source of gamma rays includes a switch initiated by said movement, the release of said switch when said possible source leaves said zone causing termination of the aforesaid cumulative counting and indicating of the presence of the gamma ray source.

5. The detection system of claim 4 having means to change said multiple.