ACTIVE AND PASSIVE NEUTRON EXAMINATION AND ASSAY SYSTEM

Abstract: The assay of a drum is performed by treating the drum as a collection of virtual subvolumes, each separately treated in the system response. Then, neutron measurements are made so as to include azimuthal orientation and time relative to a generator pulse in the data stream. An external matrix probe provides a detailed measure of neutron transmission through a drum, so that scans of mock-up drums can be compared to the scan of a target drum, so that appropriate system response functions can be determined.

Description: ACTIVE AND PASSIVE NEUTRON EXAMINATION AND ASSAY SYSTEM This invention was made with Government support under contract DE-AC05-840R21400 awarded by the U.S. Department of Energy to Martin Marietta Energy Systems, Inc. and the Government has certain rights in this invention. FIELD OF THE INVENTION The present invention relates generally to the field of measuring and testing and, more specifically, to systems and methodology for assaying drums containing transuranic materials. Active and passive measurements are made of a drum or other container suspected of containing transuranic waste. BACKGROUND OF THE INVENTION A variety of devices and systems for assaying waste materials have been developed to determine or approximate the quantity and/or type of nuclear species contained therein. One such system is described in U.S. Patent No. 4,620,100 to Schoenig et al. The system described therein determines 23S U and 238 U content in a box of radioactive waste material 5 by using a passive arrangement which detects spontaneous gamma ray emission from the material. An active component of the system includes a neutron source producing neutrons which cause the 235 U to fission in proportion to 10 its mass. U.S. Patent No. 5,002,720 to Berggren describes a thermal neutron flux detector which uses a plastic film electret containing a fissionable material, such as 235 U. Ion fission 15 fragments which result from interaction with thermal neutrons alter the electric potential of the electret, thereby providing a measure of thermal neutron activity. Another example of a detector system 20 employing both a scintillator and a neutron emitting source is found in U.S. Patent No. 4,879,550 to Bernard et al. U.S. Patent No. 4,724,118 to Grenier describes a device for detecting fissionable 25 material which uses 3 He detectors disposed in polyethylene panels. The 3 He detectors are surrounded with cadmium and boron carbide. A neutron generator is disposed in an area juxtaposed to a sample. Fissionable products in the sample produce fission neutrons when exposed to the thermal neutrons from the neutron source. These fission neutrons are then detected by the 3 He detectors. U.S. Patent No. 4,483,816 to Caldwell et al. describes a detecting system that uses 3 He neutron detectors as a passive component of the system and a neutron generator as the active component. The passive detectors detect spontaneous neutron emission from 240 Pu, 244 Cm, 2S2 Cf, and spontaneous alpha emitters such as 241 Am. The active detector, which includes a pulsed neutron source, measures total fast neutron flux emerging as a result of fissioning of any fissile isotopes present in the sample. In general, present neutron-based systems for assaying drums containing plutonium or enriched uranium evolved from smaller, simpler assay systems which were used to assay small and uniform samples such as fuel pellets or small storage cans. These systems are poorly adapted to the current problem of assaying waste contained in 55-gallon drums, particularly when the characteristics of the waste are poorly documented or when the waste is moderately dense (> 0.5 g/cm 3 ) . Some of these problems can be attributable 5 to certain drawbacks endemic to the prior art systems. For example, the necessary corrections for the neutron moderating and absorbing effects of the waste matrix on both the signal neutrons and on the interrogating neutron flux were done 10 in a single, gross-volume correction. In addition, little attempt has been provided by the prior art to sufficiently account for inhomogeneous distributions of transuranic (TRU) material within the waste matrix. 15 Another problem associated with the prior art has been that determining the absolute efficiency of the systems based upon standards traceable to national standards has been problematic. Moreover, there exists little 20 basis for understanding the causes of, and solutions to, uncertainties in the assay result. These inadequacies result in large, indeterminate uncertainties in the assay results which adversely impact the ability to transport, 25 dispose, store, or treat transuranic waste. SUMMARY OF THE INVENTION An object of the present invention is to provide a method and apparatus for assaying drums containing transuranic materials, wherein inhomogeneous distributions of transuranic materials within a matrix material are accounted for. Another object of the present invention is to provide a system which can make more accurate and reliable assays of drums containing transuranic materials, for which the necessary matrix corrections are determined by totally independent means. Another object of the present invention is to provide a neutron examination system which provides a more accurate and reliable determination of the uncertainty in the assay of the transuranic content in a drum. These and other objects of the invention are met by providing a method of assaying a drum suspected of containing transuranic material within a matrix material which includes the steps of dividing a subject drum into a plurality of rotational and vertical segments, taking a plurality of neutron measurements at positions corresponding to the rotational segments, and correlating the neutron measurements to the type and location of fissioning materials contained within the test drum. 5 An apparatus for carrying out the inventive method includes means for taking a plurality of neutron measurements at a plurality of rotational positions of the drum, and means for correlating the neutron measurements to a type 10 and location of fissioning materials contained within the drum. In another aspect of the invention, the amount of attenuation of the neutron measurements is determined by measuring the 15 transmission characteristics of neutrons in different matrix materials relative to that of the drum being measured. The present invention further provides matrix dependent functions to account for both 20 detection and thermal flux efficiency differences throughout the matrix. The detection efficiency functions and the thermal flux functions are determined by the system by matrix characterization methods totally 25 independent of outside information. Another aspect of the invention is a method for providing an absolute calibration for the passive measurement of spontaneously fissioning transuranic material, without requiring a spontaneous fission source which has been absolutely calibrated. Other objects, advantages and salient features of the invention will become apparent from the following detailed description, which taken in conjunction with the annexed drawings, discloses preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of an active and passive neutron examination and assay (APNEA) unit according to the present invention; Figure 2 is a horizontal cross-sectional view of the APNEA unit of Figure 1, showing details of the test chamber and detectors; Figure 3 is a perspective view showing only the 3 He sensor packs used in the APNEA unit of Figure 1; Figure 4(a) is a schematic top plan view of the virtual volumes of a drum using the assaying method and apparatus of the present invention; Figure 4 (b) is a top view of a mock drum having five pipes disposed therein; Figure 5(a) is a perspective view of the virtual volumes of Figure 4(a), with azimuthal 5 sections 1 and 8 removed for illustration; Figure 5(b) is a side elevational view of the mock drum of Figure 4(b); Figure 6 is a perspective view of an external matrix probe (EMP) unit which forms a 10 part of the APNEA system of the present invention; Figure 7 is a top plan view of the EMP unit of Figure 6; Figure 8a is a graph showing the damping 15 factor attributable to the present invention; Figure 8b is a graph showing the dR correction addition attributable to the present invention; Figure 9 is a system schematic showing the 20 basic electronics for effecting the present invention; Figure 10 is a histogram of the various transmission measures of a set of drums filled with soil; Figure 11 is an expanded, graphic illustration of the N2 and B2 peaks of Figure 10; Figure 12 is a graph showing the yield in a detector (N2) for a 5 Cf source placed at a height (h) of 18 inches and moved in radius out from the drum center; Figure 13 is a graph showing the total response to a source placed at various (r,h) positions in four different matrices; Figure 14 is a graph showing the same total response for the S47 matrix as Figure 13, along with the adjusted results; Figure 15 is a graph showing the correlation between Y(0,18) and N2, where Y(0,18) is the efficiency of the total APNEA unit to a source placed at r=0, h=18, and N2 is an EMP-like transmission measurement; Figure 16 is a graph showing the thermal flux distribution within a drum for six (6) different matrices, summed from 300 μs to 2 ms; Figure 17 is a graph showing the exponential die-away time (delta-t) for the same six matrices as in Figure 16; Figure 18 is a graph showing a comparison of useable flux available at three different times, F 0 (t=0) , F 300 (t=300μs) , and F 700 (t=700μs) , for the MTD matrix; and Figure 19 is a graph showing a comparison of useable flux available at three different 5 times, F 0 (t=0) , F 300 (t=300μs) , and F 700 (t=700μs) , for the SOIL matrix. DETAILED DESCRIPTION OF THE INVENTION According to the present invention, the assaying of a drum is performed by treating the 10 drum as a collection of virtual subvolumes, each subvolume being of the order of 2% of the total volume. This is made possible by utilizing specialized data acquisition techniques such as either rapid, repetitive sealer readout or list 15 mode readout to include in the data stream the azimuthal position of the drum and, for the active mode, a detailed time history of the pulsed system. One aspect of the invention is a system 20 which includes two major components: an external matrix probe (EMP) unit and an active and passive neutron examination assay (APNEA) unit. The EMP unit provides transmission data indicative of the absorption characteristics of 25 a matrix material. The APNEA Unit Referring to Figure 1, the APNEA unit 10 includes a housing 12 having a front opening, a door 14 slidably movable between open and closed positions with respect to the opening, and a loading mechanism 16 for loading and removing containers, such as a drum 18, into a test chamber 20 disposed within the housing 12. The housing 12 has a frame made of a material which preferably experiences minimal neutron activation. One such material is aluminum. The test chamber 20 is dimensioned and sized to accommodate a single 55-gallon drum, but can also accommodate an 85-gallon overpack drum. A cabinet 22 contains various components of the APNEA electronics, to be described more fully below. The cabinet 22 can contain the power supply for the various sensors as well as the requisite amplifiers and discriminators. The various signals are processed and fed from the cabinet 22 to a computer (not shown) . The computer can be located remotely from the APNEA housing 12 and electronics cabinet 22, if desirable. As shown in Figure 2, the test chamber 20 is defined by four polyethylene sidewalls 24, 26, 28, and 30. The floor and ceiling (not shown in Figure 2) of the chamber 20 are also 5 made of polyethylene. As seen in Figures 2 and 3, all sides of the test chamber 20, as well as the top and bottom, are lined with shielded 3 He detectors which detect signal neutrons. Behind all 10 detector packs is a four inch thick wall 31 of carbon. The detectors are 3 He gas proportional counters or "tubes" which detect thermal neutrons. The 3 He gas detectors also provide excellent discrimination between neutrons and 15 gamma-rays. The tubes are implemented in packages which are arranged either horizontally or vertically and then arranged into "packs" consisting of one or more packages. The door assembly includes 20 four packages 32, 34, 36, and 38, each containing four 3 He tubes 40 arranged horizontally. Packages 32 and 34 are combined to form one pack (and thus one "count" when a neutron is detected) , while packages 36 and 38 25 are combined to form a second pack. The wall opposite the door similarly has four packages 42, 44, 46, and 48, each containing four tubes 50 arranged horizontally. Packages 42 and 44 are combined to form a third pack, and packages 46 and 48 are combined to form a fourth pack. The different packs thus provide a signal from different sides and vertical positions around the chamber 20. The ceiling of the chamber 20 is provided with three sensor packages 52, 54, and 56, each containing four horizontally oriented 3 He tubes 58. Packages 52 and 56 are combined to form a fifth pack, while package 54 constitutes a sixth pack. The floor of the chamber 20 is provided with two sensor packages 60 and 62, each containing nine 3 He tubes 64. Packages 60 and 62 constitute the seventh and eighth packs. Two 3 He detector packages 66 and 68 are disposed in one of the sidewalls of the chamber 20, with four tubes 70 in each package arranged vertically. These packages constitute the ninth and tenth packs. The opposite wall has three packages 72, 74 and 76, packages 72 and 76 containing four vertically oriented tubes 78 and package 74 containing three. Packages 72 and 76 are combined to form an eleventh pack, while package 74 forms a twelfth pack. All totalled, the illustrated embodiment of the APNEA unit 10 includes eighty-one tubes 5 arranged into twelve packs. A neutron detected by any tube within a pack constitutes a single "count" for that particular pack. The horizontally oriented tubes give vertical sensitivity (in that they are located at 10 different vertical positions) , while the vertically oriented tubes give azimuthal sensitivity. The top and bottom packs are split to increase the azimuthal sensitivity. The detector packs and their relative 15 chamber geometry are designed to provide a high- efficiency neutron detection system. It is estimated that a neutron in the chamber has less than a 5% chance of leaving the chamber without traversing a detector pack. The overall neutron 20 detection efficiency of the system is also estimated to be about 16%. Part of this high efficiency is a result of the four inch carbon wall 31 which reflects escaping signal neutrons back into the detector packs. The exact number 25 of "packs" and tubes is not, however, critical to the operation of the APNEA, although in general greater resolution can be achieved with larger numbers of tubes and packs. To a certain extent, the number of packs and tubes is limited by the hardware and by the software's ability to process the incoming data. As seen in Figures 1 and 2 , unshielded 3 He detectors 80, 82, and 84, which are smaller than the detector tubes in the sidewalls, are disposed inside the chamber 20 at vertically spaced positions along one wall. These detectors provide a measure of flux activity within the chamber, and thus constitute chamber flux monitors. Referring to Figure 2, two drum flux monitors 86 and 87 are disposed within the chamber 20. Each monitor is a vertical, position sensitive 3 He tube with a cadmium jacket and directional baffle which allows it to see thermal neutrons emanating only from the drum surface. The baffle additionally divides the vertical view into many segments. Monitor 87 is in the back corner nearest a neutron generator 88 (to be described below) , and monitor 86 is in the farthest corner. This positioning makes the system more sensitive to the fact that there is generally more thermal flux internal to the drum in the side nearest the generator 88. The pulsed neutron generator 88 introduces an interrogating flux through a gap in one wall 5 of the chamber 20. The neutrons from the generator 88 pass through two inches of lead and several inches of polyethylene before entering the chamber 20 in order to enhance the quantity of usable thermal flux deposited in the drum. 10 The generator 88 is mounted so as to have a vertically adjustable disposition used to enhance sensitivity in the top and bottom sections of the drum. A particularly suitably generator is a 14- 15 MeV neutron generator known as the "Zetatron", made by Martin Marietta Specialty Components. The neutron generator 88 is housed in a moderating assembly 90, which includes a two inch lead girdle, four inch thick carbon back 20 and sides, and several inches of polyethylene over the entrance to the chamber. The generator 88 and assembly 90 are positioned between detector packs to permit it to be as close to the chamber 20 as possible. 25 As seen in Figure 2, an additional shielded detector pack 92, which functions as a pulse monitor, with a relatively longer time constant, is placed behind the neutron generator 88 away from the chamber 20. This detector pack 92 provides detailed monitoring of the primary output of the neutron generator 88. As seen in Figure l, the drum 18 is placed on a turntable 94. The angular orientation of the drum 18 is determined by an optical sensor 96 which cooperates with a reflector 98 placed on the seam of the drum 18. This enables determination of the absolute orientation of the drum with respect to the chamber 20. Other types of position sensors could be employed. Neutrons are normally tagged in time by being associated with a pulse from a neutron generator. The neutron generator 88 produces a lOμs wide pulse at a repetition rate of 50Hz or 100Hz. Since the detector recovery time of the APNEA is of the order of lOOμs, the lOμs width of the generator pulse plays no significant role. After the pulse, the detector packs detect fission neutrons induced by the thermal flux encountering fissile materials. All of the flux monitor detectors measure thermal flux which dies away even more slowly. The pulse tagging aspect of the neutron generator 88 is essential to the differential die-away technique which is the basis of the active assay. The data acquisition system for the APNEA 5 unit provides passive data which include the relative and absolute drum position of the revolving drum and which include independent data for each detector pack of both singles and correlation events. The active data include a 10 detailed time history of the system response after the generator pulse for all detector packs, for all monitor detectors, and for the pulse monitor detector. The drum rotation information is also included. 15 By including drum azimuthal position in the data stream, the drum can be treated as a plurality of sub-volumes, as shown in Figures 4(a) and 5(a). The sub-volumes are shown as eight different azimuthal sections (numbered in 20 Figure 4(a) as sections 1-8) and a circular core section, and a plurality of height sections (numbered in Figure 5(a) according to the number of inches from the bottom, i.e., 1, 6, 12, 18, 24, 27, and 30) . The EMP Unit The purpose of the external matrix probe (EMP) is to provide an external (non¬ destructive) and independent determination of the neutron absorption characteristics of a drum. This is significant since information from x-ray scans or generator manifests are inadequate for specifying whether signal neutrons emitted from fission or from (α,n) reactions in a drum will be detected in the APNEA unit. Hydrogen content is the main factor in establishing the neutron attenuation in a drum, and detailed information about the density of hydrogen in a drum is normally not known. A second purpose of the EMP unit is to measure the vertical variation in hydrogen density. Since many drums are not packed so as to have a homogeneous distribution of material, this is an important variation to measure. In particular, compacted material is often composed into large chunks and stacked into drums. The result is very dense materials with many surrounding voids and with many individual variations. The EMP unit provides information to allow the analysis of the assay data to account for these variations. As seen in Figures 6 and 7, the EMP unit 100 includes a d+d neutron generator 102 as the probe source of tagged neutrons. It may alternatively be equipped with a time tagged 2S2 Cf 5 source. The strength of the source for typical drums should be chosen to maximize statistics within an acceptable scan time. A very strong source may be necessary if the drum in question is emitting a large number of neutrons. 10 Twelve neutron detectors 104 are mounted on a vertically translatable frame. The detectors 104 have their ends pointed toward the drum, so as to give the best geometric definition and efficiency for detecting neutrons which survive 15 traversing the drum. Extraneous materials around the EMP unit 100 are kept to a minimum so that neutrons from the source 102 will not scatter off them and thence into the detector assembly. 20 Two additional detectors 106 and 108 are mounted to the sides of the source 102 and serve to measure the scatter of neutrons back from the drum matrix material. These detectors give a rough measure of the scattering density of the 25 drum. An optical sensor 110 indicates when a reflector 112 mounted on the drum passes by a fixed position. Typically, the reflector 112 is a reflective tape mounted vertically on the seam of the drum. The reflector 112 measurement gives a fixed fiducial mark on the drum and is used in the final analysis to align the assay data with the physical drum. There are two mechanical motions associated with the EMP unit 100. The drum platform rotates at a typical speed of three rpm, and the source or generator 102 and detector assemblies 104 move slowly upwardly at the speed of, for example, 0.5 inches per revolution of the drum. Typically, a scan of a drum, from bottom to top, takes about twenty minutes. The optical detector 110 is fixed elevationally and does not move with the scan. A rotational shaft encoder is coupled with the rotation mechanism so that measurements of the rotation aspect of the drum can be included in the data acquisition. A height transducer (not shown) allows the vertical position of the scanning assembly to be recorded. To conduct an EMP examination, the source and detector assemblies are lowered out of the way so that the drum may be placed on the platform. The optical assembly stays in place but is far enough removed that small misplacing of a drum during loading will not damage it. 5 A critical aspect of the EMP is that the source of neutrons for transmission measurement must be time tagged. The reason is that the unit 100 cannot be easily shielded from all sources of outside neutrons, since it is built 10 so as to have little in the way of scattering surfaces near it. More importantly, a drum is normally a source of neutron background to a transmission measurement that is difficult to deal with. If the neutron source is pulsed or 15 otherwise time tagged, then transmission measurements can be related in time to the source neutrons. The transmission detectors in array 104 are small and have an exponential response time less than 100 μs. This means that 20 background neutrons, either from the environment or from the drum itself, can be largely excluded from the results, and this is critical since the transmission through a dense matrix (such as concrete) is only a few percent of that for an 25 empty drum. The data acquisition system for the EMP unit is similar to that for the active mode of the APNEA unit. The data stream includes, for all detectors, a detailed time history of the system response with respect to the time-tagged interrogation pulse. In addition, the elevation of the source and detectors is included in the stream along with the drum rotation information. Formation of Mock-drums Since the present system is used to assay drums containing a variety of matrices, it is necessary to characterize the system by measuring its response to drums with a full range of matrices. Referring to Figures 4(b) and 5(b), a mock-drum 114 is provided with five internal vertical pipes 116, 118, 120, 122, and 124, disposed at radii of 0, 4, 6, 8, and 10 inches, respectively. Each pipe is made of thin stainless steel or aluminum and has an inner dimension of about 1.25 inches, which is sufficiently large to accommodate a one inch diameter 3 He tube. A reflector 115 is placed on a seam of the drum 114 to give absolute orientation of the drum during rotation Calibration devices are positioned in each of these pipes at elevations with respect to the bottom of the drum of h=l, 6, 12, 18, 24, 27, and 30 inches. Most of these elevations can be 5 seen in Figure 5(a) . This leads to a possible total of 35 different (r,h) combinations. If target drums have a liner or some special internal structure, such as a bird cage, then the mock-drum, in addition to having an 10 appropriate matrix, should include that liner or structure. Mock-drums are intended to contain a matrix that is similar to the target matrix. If possible, the actual matrix is used, e.g., soil 15 or concrete. In general, a mock matrix is constructed that uses sufficient polyethylene to approximate the hydrogen content of the target matrix, plus sufficient steel or other heavy material to match the weight of the target 20 matrix, and finally, enough vermiculite to fill the remaining volume. Mock matrices should bracket the hydrogen content of the target matrix. Target matrices which are known (or 25 suspected) to contain significant amounts of thermal neutron absorbers, such as boron, cadmium, or Raschig Rings, require that the mock-matrix contain similar amounts of absorbers, especially for the active response measurements. Figure 10 is a graphic display of EMP-type measurements which were made in the APNEA unit on a series of drums filled with soil. The "N2" peak is an EMP-like measurement of the transmission of neutrons from a 2i2 Cf source horizontally through a drum. The "B2" and "T2" peaks correspond to neutrons passing, at an angle, through the drums to a bottom or top detector, respectively. The N2 detector is 74, the B2 detector is pack 60, and the T2 detectors are from the pack 54. The "S+S" peak corresponds to neutrons detected behind the source in packs 66 and 68 and is located beyond 100% because of the scattering of neutrons from the surface layers of a drum. The "SUM" peak is the sum response of all detectors except the S+S detectors. It has been demonstrated (to be shown below) that either the N2 or B2 measurement is a good indication of the absorption of neutrons within a drum. The SUM peak is a poorer indication because it includes scattered neutrons and neutrons which did not pass through the drum. The T2 peak demonstrates that care must be taken when determining the transmission characteristics of a drum because the drums may not be filled to the same height. 5 In Figure 11, the N2 and B2 peaks from Figure 10 are expanded to show in more detail critical aspects of the necessary transmission measurement. The secondary N2 peak at 10% arises from the different filling heights of the 10 drums — the B2 bottom measurement is insensitive to this problem. The EMP unit has been specially designed to deal with vertical inhomogeneities in a matrix, as shown in Figures 10 and 11, and it measures 15 both the transmission and the scattering properties of the matrix. While all matrices are treated as if they were azimuthally uniform, a matrix is allowed to change significantly in the vertical dimension. 20 The simplest example of this is a target drum which is only partially filled, but could include cases of obvious segregation, as when glass is introduced into a drum already partially filled with concrete. The EMP 25 measurement should detect such vertical variations, independent of external information PCMJS96/00045 as might come from x-ray analysis or generator manifests. The fitting algorithms incorporate the corresponding mock-drum characterization information directly without any modification. The mock-drum undergoes a standard EMP measurement. Nothing is placed in any of the internal pipes. The object of the EMP measurement is to obtain the detailed transmission characterization of the mock-drum. These are the results which are used to pick which mock-drum best approximates a target drum's matrix with respect to system detection efficiency. A second set of measurements of the mock- drum is made in the APNEA unit 10 using the passive mode. First a passive measurement of the mock-drum is conducted. Then a 52 Cf source or an (α,n) source is positioned at one of the (r,h) points and a passive measurement is performed. These passive measurements are repeated with the sources at all of the (r,h) point possibilities. These measurements give the information necessary to generate the efficiency response function, E(d,V c ), giving the efficiency for detectors (d) of the APNEA unit for detecting fission neutrons originating from a (r,h) point within the mock-drum. The (α,n) source gives the values to be used in fitting the singles data, which are normally dominated by lower energyneutrons from (α,n) reactions. 5 The 252 Cf source gives the values to be used in fitting the correlation data and the active data, both of whose neutrons are associated with fission. Finally, the measurement set includes the background contribution to the correlation 10 data arising from cosmic rays interacting with the combination of the APNEA unit and the mock- drum. Figure 12 is a graph showing the yield in a detector (N2) for a 2i2 Cf source placed at a 15 height (h) of 18 inches and moved in radius out from the drum center, for three different matrixes: "MT" is an empty chamber, "S47" is a drum filled with 650 pounds of steel and 47 pounds of polyethylene, and "S140" is a drum 20 filled with 550 pounds of steel and 140 pounds of polyethylene. Positive radius (r) values are closer to the detector, and negative values are farther away. The "N2" detector corresponds to the vertical wall detector pack 74 shown in 25 Figure 2. As seen in Figure 12, the MT matrix results map out the geometric falloff of yield as the source is moved further from the detector. Both steel matrices give a higher yield at r=10 because of scattering of neutrons from the matrix. At the center of the drum (r=0) , 38% and 75% of yield has been lost for the S47 and S140 matrices, respectively. It is this spacial variation which enables the present invention to determine accurately the position of neutron- emitting material within a drum. The total response to a source placed at various (r,h) positions in four different matrices is shown in Figure 13. The abscissa is proportional to height within the drum. Within each group of measurements at the indicated height are measurements at r=0, 4, 6, 8, and 10, shown going from left to right. The top triangles are the MT (empty chamber) measurements, the circles are the "S" matrix measurements (a drum containing 700 pounds of steel) , the squares are the S47 measurements, and the bottom triangles are the S140 measurements. As seen in Figure 13, the total response is nearly flat for the MT and S matrices. The slightly higher efficiency of the bottom of the unit is evident in the measurements at h=l. However, as soon as significant hydrogen is introduced into the matrix, the response is very 5 position dependent. This provides a clear indication of the strong affect that a matrix can have on the response of a device such as the present invention. Figure 14 shows the same total response (Raw) for the S47 matrix along 10 with the adjusted results (Connected) . When the efficiency measurements are compared to the corresponding EMP measurements, it is found that there is a strong and smooth correlation between them. Figure 15 shows the 15 correlation between Y(0,18) and N2, where Y(0,18) is the efficiency of the total APNEA unit to a source placed at r=0, h=18, and N2 is the corresponding EMP-like transmission measurement. This is plotted from left to right 20 for five significant matrices in Figure 15 respectively: "CONC" (a drum filled with 1,000 pounds of concrete), S140, "SOIL" (a drum filled with 650 pounds of soil), S47, and "RR" (a drum filled with 300 pounds of Raschig rings) . This 25 comparison demonstrates that, pursuant to the present invention, the EMP measurement is an independent measure of the detection efficiency characteristics of a matrix. The third set of measurements is performed in the APNEA unit using the active mode. A 3 He tube or other thermal neutron detector is introduced into the mock-drum at as many (r,h) points as possible (the active center of the tube should be at the (r,h) point) . The response of this thermal flux monitor tube is included along with the normal active data. This set of measurements gives the thermal flux distribution, F(V c ,t), for all (r,h) points within the mock-drum as a function of time (t) . At the same time, the response of the various flux and drum flux monitors is measured for this mock-drum matrix. Because the active measurements are made with multiple time-gates, the fast response of the shielded detectors to the generator pulse is measured. The functions which are derived from these measurements are F(V c ,t), flux-monitor(t) , drum-flux monitor(t), and fast-response(d,t) . All of these functions are normalized with respect to the pulse monitor which is recorded for each active measurement. Figure 16 shows the thermal flux distribution within a drum for six (6) different mε -rices: S47, S140, SOIL, CONC, MT, and MTD (an empty drum with an inner plastic liner) . The flux is measured at h=18, at various values of r. Positive values of r are closer to the 5 neutron generator. Figure 17 shows the exponential die-away time (delta-t) for the same seven matrices. As seen in Figures 16 and 17, the MT and MTD flux and delta-t values are nearly flat as 10 there is no significant matrix to influence the die-away of flux. Thus, these values are essentially a measure of the response of the APNEA chamber itself. The four dense matrices generate flux proportional to their hydrogen 15 content and have a die-away time which varies inversely with the hydrogen content. Because the die-away times for these matrices are relatively short, there is a clear advantage arising from the fact that the APNEA has pushed 20 the analysis times in to 300 μs. Previous devices have begun their analysis at 700 μs and have given up at least a factor of two in flux in so doing. Figures 18 and 19 compare the useable flux 25 for systems which begin analyzing at t=0, t=300, and t=700μs, respectively. When the matrix is void or benign, as in Figure 18, the advantage to the 300 μs analysis is about a factor of two. When a dense matrix, such as SOIL (Figure 19) , is encountered, the advantage grows to about a factor of four. As is clear from Figure 17, the relative gain is biggest in the center of the drum where the die-away time is shortest. Virtual Sub-volumes Figures 4(a) and 5(a) schematically illustrate the virtual volumes associated with the (r,h,0) values. Each of these volumes is defined by being close to a particular calibration position. When an arbitrary position is closer to another calibration position, it is effectively in that virtual volume. In the illustrated embodiment, there are 54 virtual volumes, and these are referred to as V c or V b , if they are treated as being fixed in the chamber or the drum, respectively. For each V b there is a corresponding V c , though this correspondence changes as the drum rotates. The number 54 comes from the fact that there are eight azimuthal sections, a center or "core" section, and six vertical divisions. These 54 sub-volumes are distinct but not equal in volume. If the radial values chosen are r=0 and 8 5 inches, then a given vertical segment is divided into 9 pieces, a core cylinder and 8 equal wedge-shaped segments. The core cylinder and the wedge-shaped segments should be roughly equal in area as shown in Figure 4a. The 10 typical choice of heights, illustrated in Figure 5(a), means that many of the vertical slices have a 6 inch thickness, but some are much thinner. In particular, the virtual thicknesses for the listed h-values are 3.5, 5.5, 6, 6, 6, 15 and less than 6 inches, respectively. However, the essential outcome is that the drum is analyzed as a collection of sub-volumes, each accounting for about 2% of the total volume. Data Analysis 20 The analysis of data generated by the various detectors involves solving the linear equations listed below: (1) Y,(d,0) = E,. m (d,V c ) - R, (V b ) (Singles) (2) Y lf (d,0) = E, f>m (d,V c ) -R jf (V b ) (Correlated) (3) Y f (d,0,t) = E, m (d,V c ) -R f (V b ) • F B (V e . 1 ) (Fissile) (4) Y cf (d,0,t) = E., m (d,V c ) .R,(V b ) .F π ,(V c ,t) (Active Correlated) where: Y is the measured (detected) yield of neutrons s refers to "singles" neutrons (gross neutron output) sf refers to spontaneous fission (with correlated neutrons) f refers to fission induced with thermal neutrons cf refers to correlated neutrons from induced fission d refers to one of 12 detector packs θ refers to the angular orientation of a drum within the chamber V c is a volume element fixed within the chamber V b is a volume element fixed within the drum t is the time after a neutron generator pulse m refers to the matrix in the drum E is the efficiency for detector "d" to detect a neutron originating in chamber volume V c R is the quantity of assay material within a volume element, V b F is the thermal-neutron flux associated with the neutron generator pulse which impinges on chamber volume V c at time t after the pulse. In the foregoing, it is noted that a sum over the elements V b is assumed. Equation (1) above is the equation to be solved for the "passive singles" analysis. The 5 methodology for solving this equation is essentially the same as that for solving the passive-correlation and the active assays. It is assumed, since there are more data points (96 for the passive) than there are unknowns (54 R 10 values) , that an appropriate solution can be obtained by minimizing the equation for X 2 : X 2 = SUM[(y - E-R) -ω-(Y - E-R)]/df ω (d , θ ) = 1.0/ΔY(d,0) 2 where 15 ω is the uncertainty weight assigned to Y ΔY is the uncertainty in Y, and df is the number of degrees of freedom. In this form, X 2 should have an expectation value of 1.0, if the functional form 20 E(d,V c ) -R(V b ) is a good representation of Y(d,0). Since approximations have been made in obtaining this form, higher values of X 2 will normally be observed when the statistics are good. When the minimization of X 2 is performed, 25 unphysical solutions are obtained wherein some of the R(V b ) elements are found to be negative. As a consequence, a method for minimizing X 2 while constraining the values of R(V b ) to be non- negative was developed. The equation is solved in an iterative fashion using an adaptation of Newton's method for finding zeros of a function. An initial guess is made, then that guess is improved by changing each element of R by an amount proportional to dR: dR(V b ) = X^'/X 2 " where X 2 ' is the partial derivative of X 2 with respect to R(V b ) and X 2 " is the second partial derivative with respect to R(V b ). The basic constraint is that no element of R can become negative. The various R values are strongly correlated, meaning that there is only a small difference in general between having source strength in a certain volume or in the one next to it. Consequently, the iterative procedure will tend to oscillate as it seeks to improve X 2 . For this reason, a dynamic damping function is introduced which reduces the size of dR values to prevent the fitting procedure from oscillating out of control. At the same time this damping function is constantly modified so as to push the speed with which the fitting procedure converges. Figure 8a, a graph of the damping factor associated with the fitting procedure for a drum 5 with a super-compressed matrix, shows how the value of the damping settles into a value near 0.2. The damped incremental change to R is shown in Figure 8b and reduces with time but continues to oscillate. On the other hand, the 10 value of R, after the first few iterations, converges rather smoothly to its asymptotic value. A feature of systems which have cylindrical symmetry is that it is difficult to tell the 15 difference between source strength in the core and an equivalent source strength spread uniformly throughout the annulus. It is the nature of the data associated with the APNEA unit that irregularities in the characterization 20 functions or statistical fluctuations in the assay data will tend to suggest that a core source is in fact partially distributed out into the annulus. Less troublesome is the case when the source is distributed evenly throughout the 25 annulus. This appears to the analysis similar to a core source. As a result, it is important that the appropriate starting condition be found for the iterative fitting algorithm. If a drum is known to have a homogeneous distribution of source material, then the starting condition should reflect that. If, however, a drum has a somewhat non-uniform matrix, as is the case for much compressed waste, then a better starting point is with the source concentrated in the core. The fitting algorithm will move strength as necessary out into the annulus. The fitting algorithm will accurately place a point source regardless of the starting configuration. It is only for distributions which could be azimuthally uniform that special care must be taken. The top and bottom detector packs reduce the cylindrical ambiguity making the fitting more precise near the top or bottom of the drum. The fitting result gives the distribution of source strength throughout the virtual volume set. Since the azimuthal variable (0) is measured relative to the seam on the drum, the absolute (r,h,0) location of a "hot" volume is identified. Thus, if a given volume is seen to possess most of the total source, then the corresponding physical volume could be removed as part of a remediating program. Equation (2) above is for treating correlated neutrons. It has the same functional form as that for the singles determination. In this case, Y refers to the number of correlated 5 neutrons seen (i.e., the number of coincidences seen) , and E $r (d,V c ) can be derived from E sf (d,V c ) : E lf (d,V c ) = E lf (d,v e ) -E sf (d,V c ) The main difficulty with fitting the correlation data is that they are statistically 10 weak and that there are often large corrections to the data arising from accidentals. The accidentals are a problem because the response time of the detector packs is relatively slow and the singles rate may be quite large due not 15 to spontaneous fission but to (α,n) reactions taking place within the matrix. The third equation above is for the active analysis, and is much the same as for the passive analyses. In this equation, "Y" is the 20 detector yield with the fast response removed, "F" is the thermal singles flux function for the matrix, and "E" is the fission efficiency function used to generate the correlation efficiency function. 25 The fourth equation above is for the active analysis of correlated neutrons. Y cf is the yield of correlated neutrons corrected for accidentals. R f and E, f are the same quantities as in equations (3) and (2) , respectively. The relevant aspect of the active analysis for equations (3) and (4) is that the data are dominated by the influence of F(V c ,t), which is strongly directional whenever the matrix is reasonably dense. Then the thermal flux tends to drop off within the drum as one moves further from the neutron generator as can be seen in Figure 16. For very dense drums, the thermal flux in the center of the drum is considerably less than that in the forward part of the annulus of the drum, and the part of the annulus nearest the generator encounters much more flux than does the part farthest from the generator. Although it is clear that active neutron assay systems, generally, will have a reduced sensitivity to fissile material in the center of a drum, the APNEA system, by explicitly including the time dependence of the flux in its fitting procedure, has significantly enhanced the sensitivity of the assay to the faster decaying drum core, as seen in Figure 17. This enhanced sensitivity comes in large measure from the utilization of both time and subvolumes in the fitting procedure. However, a very big gain comes from utilizing the time dependence in the yield measurement to allow the fast component in the yield (arising from the 5 direct response to the generator pulse) to be removed from the yield. This means that the analysis can be moved to within 300 μs after the pulse, in from 700 μs. At 300 μs, not only is there more flux to work with, but there is 10 relatively much more core flux to work with. With additional electronics, the beginning analysis time should be pushed into under 200 μs. The active analysis is a clear beneficiary 15 of the virtual volume approach as not only is the local efficiency treated explicitly but the local flux and its time variation are treated explicitly. The result of this approach is that inhomogeneous distributions of material 20 throughout dense matrices are treated directly and not by "averages," and assumptions about the distribution of material do not have to be relied upon. One assumption that can be made, in most 25 cases, is that R sf should have the same shape as R f . ; In fact, it is probable that R s will also have the same shape because the (a,n) activity is usually related to the fissile source material. The APNEA analysis uses these comparisons as a consistency check on its derivation of the three source strengths. System Schematic Figure 9 is a system schematic showing the basic electronics for effecting the present invention. Each detector tube, such as tube 76, is coupled to a preamplifier 126, linear amplifier 128, and discriminator 130. The signal emanating from the discriminator represents one "count," and is in digital logic form. The basic electronics for preamplification, amplification, and discrimination are conventional. In the list data acquisition mode, the separate digital logic signals (neutrons) are fed to multi-bit time tagging (MBTT) modules 132 where the neutrons are tagged with a 24 bit time with a micro-second time base and placed in an internal first-in-first-out (FIFO) buffer. A fast, programmable, auxiliary CAMAC crate controller 134 gathers the resultant neutron words from all MBTT modules 132 and places them in a FIFO 136 which is accessed directly by a host computer 138 through a PC interface 140. The auxiliary CAMAC crate controller ("Event Handler") 134 is commercially available 5 from EVENT HANDLERS, Inc. of Oak Ridge, Tennessee USA. It consists of a module capable of plugging into the CAMAC crate. The module includes printed circuit boards and microprocessors programmed to perform and manage 10 data acquisition, e.g., set gates and read sealer modules. The host computer, which may be an IBM PC or PC clone, accesses the CAMAC crate and the final FIFO through the commercial PC-CAMAC 15 interface 140 which allows the host computer to be located up to 500 feet from the APNEA or EMP units. Drum rotation data and height data are input to a register 142 from a rotation sensor 20 144 and a height sensor 146, respectively. A "neutron word" from the MBTT module consists of six bytes; one byte identifies the module, two bytes identify which of sixteen inputs has been accessed, and three bytes give 25 the time (in micro-seconds) when the event took place. The auxiliary controller 134 also includes in the data stream to the FIFO 136 additional information related to the rotation of the drum, the reflector sensor for the absolute drum position, and, for the EMP unit, the height of the scanning assembly. The MBTT modules 132 are also used to tag the occurrence of generator pulses and reflector sensors. The list mode acquisition can readily handle neutron rates above 100 kHz. The various components of the system electronics are assembled in modular form in a CAMAC crate 141 which typically includes a power supply for operating the electronic components. In the sealer data acquisition mode, the neutron signals are fed into sealers 148 and into logic circuitry which provides the auto¬ correlation time gate for correlation data acquisition. The auxiliary controller 134 controls the sealers 148 and the logic circuits to provide a data stream which includes the drum rotation and reflector information. The data stream for the passive mode is a list of all sealers taken every 1/32 of a drum rotation. The sealers are split into two identical groups. The first group is left free running to record the singles data, and the second group is gated by the auto-correlation gate and represents the correlation data. Note that each detector pack and each monitor detector has its own, separate sealer channel. Thus, both 5 singles and correlations are recorded separately for each detector. In the active mode of sealer data acquisition, the two sealer banks are gated by the auxiliary controller 134 and not by the 10 auto-correlation gate. The auxiliary controller 134 arranges that the two banks are alternately gated on and off at times of 300, 350, 400, 500, 600, 700, 800, 1,000, 1,200, 1,400, 1,600, 1,800, 2,000, 2,500, 3,000, 3,500, 4,000, 4,500, 15 5,000, 5,500, 6,000, 8,000, 10,000, 12,000, 14,000, and 16,000 micro-seconds relative to the neutron generator pulse. During the time that a bank of sealers was gated off, the auxiliary controller 134 reads the sealers, zeros them, 20 and transfers the result to the FIFO 136. The sealer readout includes all detector packs and monitor detectors, a 1 MHz clock, a rotation counter, a reflector counter, a pulse counter, and logic information. This is done 25 for every generator pulse until the end of the pulsing cycle (typically 2,000 pulses), at which time the auxiliary controller 134 automatically changes the acquisition mode over to passive, and a three-rotation passive scan of the drum is performed. This follow-up passive scan looks for any delayed neutron events and provides a background measure for the active analysis. An advantage of the list mode or the segmented sealer acquisition system is that they give much useful information on the stability of the system. Since the detectors are handled separately, it is a simple matter to check the integrity of the system and of its many components. Noise bursts in a pack or the entire system can be identified and worked around. Variations in the generator output are monitored and accounted for. In many cases, if a component falters or fails, this condition can be identified and corrected for in the assay analysis. When working close in time to the generator pulse, the system is capable of sensing when the detectors have sufficiently recovered from the pulse to give reliable results. When the digital logic signal is delivered to a sealer 148, the logic circuitry provides the necessary gating for the correlations analysis. The logic circuitry includes the auxiliary controller 134 which collects sealer data and puts it into a queue for the computer 138. One of the important functions of the 5 auxiliary controller 134 is to generate and control time gates. While the drum is rotating the auxiliary controller 134 monitors a clock, and performs the necessary acquisition step as the drum rotates through each of its eight 10 angular sections. The computer 138 is equipped with a software program which incorporates the various algorithms noted above. For the operations required for the present invention a 15 conventional 486 speed personal computer with nominal memory would suffice. A turntable motor can be switched on manually, as can the neutron generator. The computer 138 receives input signals from the 20 position sensor 110, a drum height detector 126, and a generator pulse controller which is built into the Zetatron neutron generator system. The last item provides information on the timing of each pulse of the generator. 25 The Event Handler 134 is a fast, programmable auxiliary crate controller which a allows microsecond response to incoming data. It acts as a data acquisition preprocessor, and cooperates with the FIFO buffer 136 and the commercially available PC-CAMAC interface 140 to enable very rapid front-end data pre-processing. As a result, all data, both active and passive, are divided into rotational segments (at least eight segments per rotation for the APNEA system) , and neutron time correlations in the passive mode are recorded per individual detector pack. Also, all measurements are aligned with respect to a fixed marker located on a drum. Moreover, in the active mode, multiple, variable-width time gates with respect to each neutron generator pulse are used to acquire neutron time spectra. If multi-bit time tagging modules are used in the CAMAC-based electronics, list mode data acquisition can be performed. The auxiliary controller 134 and FIFO allow this approach to be made with no dead-time loss up to fairly high count rates (< 150,000 neutrons per second). In this mode of acquisition, six bytes of information are transmitted for each recorded neutron event. The six bytes identify which detector fired and the time (in microseconds) at which it fired. Time Tagging An alternative way to tag neutrons is to 5 tag the fission of a spontaneously fissioning nucleus, such as S2 Cf or 20 Pu. If 2i2 Cf is placed in an ion chamber, then the fission fragments can be detected in the chamber and the fission is tagged. Alternatively, gamma-ray detectors 10 could be used to detect the gamma-ray burst associated with the fission. In either case, since the 252 Cf fission is tagged, then the resultant 3.72 neutrons associated with the fission are also tagged. 15 Tagged fission neutrons are extremely valuable as they can be used to map out detector time response and to determine the detection efficiency of neutron detectors. Once the detection efficiency is known, then the absolute 20 strength of the fission source can be determined, without the use of a calibrated standard. Pursuant to the present invention, tagged fission neutrons are used to map out the 25 response of the APNEA detector system. Because the passive mode of assay requires that time correlations between correlated neutrons be measured, the time response functions of the APNEA system are used to determine what the correlation efficiency is, depending on whether auto-correlation, full correlation, or shift register techniques are utilized. It should be noted that this correlation efficiency is different from and independent of the detection efficiency — it is chiefly a function of the time window for correlating. The absolute passive calibration is calculated directly from the correlation efficiency and the absolute detection efficiency, and no calibrated standard source is required. The absolute active calibration still requires a calibrated source to normalize the thermal flux function, though the detection efficiency part of the absolute calibration is accurately determined by this neutron tagging technique. One of the principal advantages to the present invention is its ability to utilize a virtual volume analysis which independently treats virtual volumes as small as 2% of the total volume. This is made possible first by including drum rotation (relative and absolute) information into the data stream and by segmenting the data stream into bunches which can be reformed into angular data segment 5 groups. Another advantage offered by the present invention is that the data, both sides and correlated, for each detector are kept separate and that the detector packs are oriented so as 10 to enhance the spacial resolving power of the system. A third advantage resulting from the present invention is the inclusion of fast time dependence in the active data. This allows the 15 analysis to be pushed closer in time to the pulse, and, as a consequence, to significantly improve the sensitivity of the system to material in the drum center. In order to achieve the aforementioned 20 advantages, it is necessary that mock drum characterization generate the necessary system response functions to be used in the fitting. The EMP unit measurements then link a drum to the appropriate efficiency functions, and the 25 monitor detectors and fast pulse analysis link in the appropriate flux function. The overall result is that the APNEA system of the present invention produces assays that are largely independent of the spacial distribution of material and that are largely independent of outside guesses and assumptions concerning the matrix. A further benefit of independently determining the appropriate mock- matrix function is that the uncertainty in this determination can be directly related to the uncertainty in the final assay value. This particular source of uncertainty has not been rigorously treated by other systems and is often the largest piece of the final uncertainty. Thus, the present invention is likely to be not only more accurate, but more sensitive, more robust, and ultimately, more credible. While advantageous embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A method of assaying a drum suspected of containing a fissile material and/or spontaneously fissioning material within a 5 matrix material, the method comprising the steps of: placing the drum in a test chamber; rotating the drum at a predetermined rate and through a predetermined number of rotations; 10 taking a plurality of neutron measurements of plural azimuthal positions of the drum while rotating the drum; and correlating the plurality of neutron measurements to a level and location of 15 fissioning materials contained within the drum. 2. A method according to claim 1, wherein the step of taking a plurality of neutron measurements includes positioning a plurality of detectors outside all sides, top and bottom of 5 the test chamber, positioning at least one drum sensor and at least one chamber sensor inside the test chamber to measure interrogating thermal neutron flux, and exposing the test drum to an interrogating flux from a neutron 10 generator disposed outside the test chamber. 3. A method according to claim 1, further comprising providing a reference point on the test drum, and detecting the reference point while rotating the test drum. 4. A method according to claim 2, wherein the step of taking a plurality of neutron measurements further includes positioning a sensor outside the test chamber adjacent the neutron generator to monitor output of the neutron generator. 5. A method according to claim 1, further comprising determining neutron transmission through the test drum. 6. A method according to claim 5, wherein the step of determining neutron transmission comprises placing the test drum in an external matrix probe unit, and exposing the test drum to an interrogating neutron source. 7. A method according to claim 1, wherein the correlating step comprises solving at least one of the following: Y s (d,0) = E.. m (d,V c ) R 5 (V b ) 5 Y sf (d,0) = E sf . m (d,V c ) R sf (V b ) Y f (d,0,t) = E f,m (d,V c ) R f (V b ) -F m (V, t ) Y cf (d,0,t) = E., m (d,V c ) .R f (V b ) -F nι (V c ,t) where: Y is the measured (detected) yield of neutrons 10 s refers to "singles" neutrons (gross neutron output) sf refers to spontaneous fission (with correlated neutrons) f refers to fission induced with thermal 15 neutrons cf refers to correlated neutrons from induced fission d refers to one of a plurality of detector packs 20 0 refers to the angular orientation of a drum within the chamber V c is a volume element fixed within the chamber V b is a volume element fixed within the drum 25 t is the time after a neutron generator pulse m refers to the matrix in the drum E is the efficiency for detector "d" to detect a neutron originating in chamber volume V c 30 R is the quantity of assay material within a volume element, V b , and F is the thermal neutron pulse flux associated with the neutron generator which impinges on chamber volume V c at time t after the pulse, wherein all equations assume a sum over the elements V b . 8. A method according to claim l, further comprising constructing a plurality of mock drums with different matrices, transmitting a source of neutrons through each mock drum, and measuring transmission through and scattering of neutrons. 9. A method according to claim 8, further comprising transmitting a source of neutrons through each drum to be assayed, and comparing the transmission and scattering characteristics of each assayed drum to those of the mock drums to select a mock-drum system efficiency response function for use in the correlation step. 10. A method according to claim 2, further comprising determining the transmission of generator neutrons through the drum matrix by measuring the strength of the fast response, 5 determining the time and strength response of the various flux monitors, and comparing these parameters to those of mock-drums to select an appropriate system response for the thermal flux function for use in the correlation step. 11. An apparatus for conducting assays of a test drum suspected of containing a fissile or fissioning material within a matrix material, the apparatus comprising: 5 means for taking a plurality of neutron measurements at a plurality of rotational positions of the test drum; and means for correlating the neutron measurements to a type and location of 10 fissioning materials contained within the test drum. 12. An apparatus for assaying a drum suspected of containing a fissile material and/or spontaneously fissioning material within a matrix material, the apparatus comprising: means for separating the drum into a collection of virtual sub-volumes; and means for providing separate, detailed volume maps of at least one of fissile material, spontaneously fissioning material, and of uncorrelated singles neutron activity within each virtual sub-volume. 13. An apparatus according to claim 12, further comprising means for applying a separate system response value to each sub-volume. 14. An apparatus according to claim 12, further comprising a test chamber having sidewalls, a ceiling and a floor, means for rotating the drum within the test chamber, and means for sensing the angular orientation of the drum. 15. An apparatus according to claim 14, further comprising a plurality of neutron detectors disposed outside the test chamber on all sidewalls, the ceiling and the floor. 16. An apparatus according to claim 15, further comprising at least one chamber flux detector disposed within the chamber. 17. An apparatus according to claim 16, further comprising at least one drum flux detector disposed within the chamber. 18. An apparatus according to claim 17, further comprising pulsed neutron generator means for transmitting an interrogating thermal flux into the chamber. 19. An apparatus according to claim 18, further comprising pulse monitor means for monitoring a primary output of the pulsed neutron generator means. 20. An apparatus according to claim 19, further comprising data acquisition means, coupled to the position sensing means, the pulse monitor means, the drum flux detector, the chamber flux detector, and the plurality of neutron detectors, for taking a plurality of neutron measurements of plural azimuthal positions of the drum while rotating the drum. 21. An apparatus according to claim 16, wherein the at least one chamber flux detector includes three chamber flux detectors disposed horizontally sidewalls of the chamber. 22. An apparatus according to claim 17, wherein the at least one drum flux detector is a 3 He tube having a cadmium jacket and a directional baffle. 23. An apparatus according to claim 18, wherein the pulsed neutron generator means is vertically movably mounted outside one of the sidewalls of the chamber. 24. An apparatus according to claim 23, wherein the pulsed neutron generator means is a 14 MeV neutron generator disposed in a moderating assembly. 25. An apparatus according to claim 12, further comprising matrix probe means for making a transmission and scattering measurement of the drum. 26. An apparatus according to claim 25, wherein the matrix probe means comprises an external matrix probe which includes a time-tagged source of neutrons with energies near the fission energies to be encountered, and at least one sensor for detecting neutrons transmitted through the drum. 27. An apparatus according to claim 26, wherein the time-tagged source is a 232 Cf source in an ion chamber. 28. An apparatus according to claim 26, wherein the time-tagged source is a d+d generator. 29. An apparatus according to claim 26, further comprising means for rotating the drum while making the transmission and scattering measurements, and means for moving the time- tagged source and sensor vertically during drum rotation. 30. An apparatus according to claim 26, wherein the matrix probe means comprises a 25 Cf source disposed within the chamber in juxtaposition to the drum and opposite a vertically oriented one of the plurality of neutron detectors. 31. An apparatus according to claim 25, further comprising mock-drum matrix probe means for making a transmission and scattering measurement of a plurality of mock-matrices expected to be encountered during drum assaying. 32. An apparatus according to claim 31, wherein the mock-drum matrix probe means comprises a mock-drum having a plurality of vertically oriented tubes located at different radial locations, and being adapted to contain different matrix materials and filler materials selected to approximate hydrogen content and density of matrices expected to be encountered during drum assaying. 33. An apparatus according to claim 32, wherein the mock-drum further includes means for giving absolute orientation of the mock-drum during rotation. 34. An apparatus according to claim 33, further comprising means for positioning a neutron emitting source at plural radial and height positions within the mock-drum. 35. An apparatus according to claim 34, further comprising means for positioning an (α,n) source at plural radial and height positions within the mock-drum. 36. An apparatus according to claim 32, further comprising means for positioning a thermal flux detector at plural radial and height positions within the mock-drum. 37. An apparatus for providing an external, non-destructive, and independent determination of the neutron absorption characteristics of a drum, comprising: a neutron source disposed on one side of the drum, and being operable to transmit neutrons horizontally through the drum; a neutron detector disposed on another side of the drum opposite the neutron source; means for rotating the drum during operation of the neutron generator; and means for translating the neutron source vertically upwardly during drum rotation. 38. An apparatus according to claim 37, wherein the neutron source is a 252 Cf source. 39. An apparatus according to claim 37, wherein the neutron source is a d + d source. 40. An apparatus according to claim 37, further comprising means for indicating the absolute orientation of the drum during rotation.

Inventors: COFFEY DON E, HENSLEY DAVID C, PIERCE LARRY A, SCHULTZ FREDERICK J, COFFEY, DON, E., HENSLEY, DAVID, C., PIERCE, LARRY, A., SCHULTZ, FREDERICK, J.
Applicants: LOCKHEED MARTIN ENERGY SYS INC, LOCKHEED MARTIN ENERGY SYSTEMS, INC.
Application year: 1996
Main classification: G01T3
IPC codes: G01N23