WO1996022550A2 - Systeme actif et passif d'examen et de controle de neutrons - Google Patents

Systeme actif et passif d'examen et de controle de neutrons Download PDF

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Publication number
WO1996022550A2
WO1996022550A2 PCT/US1996/000045 US9600045W WO9622550A2 WO 1996022550 A2 WO1996022550 A2 WO 1996022550A2 US 9600045 W US9600045 W US 9600045W WO 9622550 A2 WO9622550 A2 WO 9622550A2
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WO
WIPO (PCT)
Prior art keywords
drum
neutron
chamber
source
neutrons
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Application number
PCT/US1996/000045
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English (en)
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WO1996022550A3 (fr
Inventor
David C. Hensley
Frederick J. Schultz
Larry A. Pierce
Don E. Coffey
Original Assignee
Lockheed Martin Energy Systems, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Application filed by Lockheed Martin Energy Systems, Inc. filed Critical Lockheed Martin Energy Systems, Inc.
Priority to AU46932/96A priority Critical patent/AU4693296A/en
Priority to EP96902584A priority patent/EP0803072A4/fr
Publication of WO1996022550A2 publication Critical patent/WO1996022550A2/fr
Publication of WO1996022550A3 publication Critical patent/WO1996022550A3/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
    • G01N23/02—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material
    • G01N23/025—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material using neutrons

Definitions

  • 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.
  • 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
  • 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.
  • TRU transuranic
  • 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.
  • 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
  • the amount of attenuation of the neutron measurements is determined by measuring the
  • the present invention further provides matrix dependent functions to account for both
  • the detection efficiency functions and the thermal flux functions are determined by the system by matrix characterization methods totally
  • 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.
  • Figure 1 is a perspective view of an active and passive neutron examination and assay
  • 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);
  • FIG. 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
  • Figure 16 is a graph showing the thermal flux distribution within a drum for six (6) different matrices, summed from 300 ⁇ s 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;
  • the assaying of a drum is performed by treating the
  • One aspect of the invention is a system
  • EMP external matrix probe
  • APIA active and passive neutron examination assay
  • 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.
  • 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.
  • 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
  • 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
  • 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
  • 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.
  • 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
  • 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.
  • 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.
  • 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)
  • 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.
  • 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-
  • the neutron generator 88 is housed in a moderating assembly 90, which includes a two inch lead girdle, four inch thick carbon back
  • the generator 88 and assembly 90 are positioned between detector packs to permit it to be as close to the chamber 20 as possible.
  • 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.
  • 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 lO ⁇ s wide pulse at a repetition rate of 50Hz or 100Hz. Since the detector recovery time of the APNEA is of the order of lOO ⁇ s, the lO ⁇ s 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.
  • 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
  • the drum 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
  • 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 ( ⁇ ,n) reactions in a drum will be detected in the
  • the EMP unit 100 includes a d+d neutron generator 102 as the probe source of tagged neutrons.
  • 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.
  • the 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
  • 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.
  • 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
  • An optical sensor 110 indicates when a reflector 112 mounted on the drum passes by a fixed position.
  • 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.
  • 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.
  • a scan of a drum 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.
  • the source and detector assemblies are lowered out of the way so that the drum may be placed on the platform.
  • 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
  • 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
  • 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 ⁇ s. This means that
  • 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.
  • the elevation of the source and detectors is included in the stream along with the drum rotation information.
  • 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
  • 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
  • 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
  • Mock matrices should bracket the hydrogen content of the target matrix.
  • FIG 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
  • 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
  • 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 fitting algorithms incorporate the corresponding mock-drum characterization information directly without any modification.
  • the mock-drum undergoes a standard EMP measurement. None 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.
  • 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.
  • a 52 Cf source or an ( ⁇ ,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 ( ⁇ ,n) source gives the values to be used in fitting the singles data, which are normally dominated by lower energyneutrons from ( ⁇ ,n) reactions.
  • 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.
  • the measurement set includes the background contribution to the correlation
  • Figure 12 is a graph showing the yield in a detector (N2) for a 2i2 Cf source placed at a
  • the "N2" detector corresponds to the vertical wall detector pack 74 shown in
  • the total response to a source placed at various (r,h) positions in four different matrices is shown in Figure 13.
  • 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.
  • FIG. 14 shows the same total response (Raw) for the S47 matrix along
  • 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) .
  • the response of the various flux and drum flux monitors is measured for this mock-drum matrix.
  • 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 m ⁇ -rices: S47, S140, SOIL, CONC, MT, and MTD (an empty drum with an inner plastic liner) .
  • Figure 17 shows the exponential die-away time (delta-t) for the same seven matrices.
  • 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.
  • V c 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.
  • 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 drum is analyzed as a collection of sub-volumes, each accounting for about 2% of the total volume.
  • 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 ⁇ 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.
  • 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 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.
  • X 2 " is the second partial derivative with respect to R(V b ).
  • 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.
  • Equation (2) above is for treating correlated neutrons. It has the same functional form as that for the singles determination.
  • the main difficulty with fitting the correlation data is that they are statistically
  • F is the thermal singles flux function for the matrix
  • E is the fission efficiency function used to generate the correlation efficiency function
  • 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.
  • 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 ⁇ s after the pulse, in from 700 ⁇ s. At 300 ⁇ s, not only is there more flux to work with, but there is
  • the active analysis is a clear beneficiary
  • 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.
  • the separate digital logic signals 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.
  • MBTT multi-bit time tagging
  • 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,
  • 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.
  • 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.
  • the two sealer banks are gated by the auxiliary controller 134 and not by the
  • 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,
  • the auxiliary controller 134 reads the sealers, zeros them,
  • 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
  • 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.
  • the logic circuitry 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
  • 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
  • a turntable motor can be switched on manually, as can the neutron generator.
  • the computer 138 receives input signals from the
  • 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.
  • 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.
  • multiple, variable-width time gates with respect to each neutron generator pulse are used to acquire neutron time spectra.
  • 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).
  • 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
  • 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
  • tagged fission neutrons are used to map out the
  • 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.
  • 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
  • a third advantage resulting from the present invention is the inclusion of fast time dependence in the active data. This allows the
  • 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
  • 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.

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  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
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Abstract

On contrôle un fût (18) en considérant ce fût comme un ensemble de sous-volumes virtuels dont chacun est traité séparément dans la réaction du système. Ensuite, on effectue des mesures de neutrons de façon à inclure l'orientation azimutale et le temps par rapport à un générateur d'impulsions dans le flot de données. Une sonde extérieure (100) à matrice fournit une mesure détaillée de la transmission de neutrons au travers d'un fût, de telle façon que des scannages de fûts factices (114) peuvent être comparés au scannage d'un fût (18) à contrôler. On peut ainsi déterminer des fonctions appropriées de réaction du système.
PCT/US1996/000045 1995-01-12 1996-01-11 Systeme actif et passif d'examen et de controle de neutrons WO1996022550A2 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
AU46932/96A AU4693296A (en) 1995-01-12 1996-01-11 Active and passive neutron examination and assay system
EP96902584A EP0803072A4 (fr) 1995-01-12 1996-01-11 Systeme actif et passif d'examen et de controle de neutrons

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US37169095A 1995-01-12 1995-01-12
US08/371,690 1995-01-12

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WO1996022550A2 true WO1996022550A2 (fr) 1996-07-25
WO1996022550A3 WO1996022550A3 (fr) 1996-09-26

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CN107202807A (zh) * 2017-04-10 2017-09-26 中国矿业大学(北京) 一种基于中子照相实验台的加载装置
US10114130B2 (en) 2016-11-29 2018-10-30 Battelle Energy Alliance, Llc Detectors for use with particle generators and related assemblies, systems and methods

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US10114130B2 (en) 2016-11-29 2018-10-30 Battelle Energy Alliance, Llc Detectors for use with particle generators and related assemblies, systems and methods
CN107202807A (zh) * 2017-04-10 2017-09-26 中国矿业大学(北京) 一种基于中子照相实验台的加载装置
CN107202807B (zh) * 2017-04-10 2023-07-18 中国矿业大学(北京) 一种基于中子照相实验台的加载装置

Also Published As

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AU4693296A (en) 1996-08-07
WO1996022550A3 (fr) 1996-09-26
EP0803072A4 (fr) 1999-05-26
EP0803072A2 (fr) 1997-10-29

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