Abstract:
A method for monitoring a container or the contents in a volume ( 39, 47 ), including allowing at least one of a beta, gamma, neutron, and proton radiation emerging from said container or volume ( 39, 47 ), and/or secondary particles or radiation brought forth by said radiation, to pass through a measuring volume ( 12, 12 ′) of at least one radiation detector ( 10, 10′, 10 ″), said measuring volume ( 12, 12 ′) containing a noble gas and/or a noble gas isotope, or a mixture of noble gases and/or noble gas isotopes and detecting the photons generated within said measuring volume ( 12, 12 ′) by an interaction ( 18 , W 1 , . . . , W 4 ) of the radiation with the noble gas or noble gases and/or their isotopes of the measuring volume ( 12, 12 ′). The output of said photon detecting means ( 15, 16, 53 ) is then used to derive information about the container or the contents in said volume ( 39, 47 ), whereby this information is used to discriminate protons, neutrons, beta and gamma rays respectively.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method for monitoring an unknown container or the contents in a volume, a respective monitoring system, and a radiation detector for being used in such monitoring system. 
     BACKGROUND OF THE INVENTION 
     It is a challenge to assess the contents of container in a non-intrusive way. The desire to do so is often fuelled by the threats posed for example by radioactive or explosive contents. 
     For such purposes, passive and active assessment systems exist. Passive systems can be in the form of radiation portal monitors (RPMs). Such systems are composed of radiation detectors that detect radiation emitted by radioactive substances within the bulk in question. The prime drawbacks of conventional technologies used in RPMs are the following:
         1. Plastic Scintillator technology (PVT), as disclosed in the U.S. Pat. No. 5,679,956: Has very poor energy resolution, preventing isotope identification. This leads to frequent innocent alarms due to naturally occurring radioactive material (NORM) or technically enhanced NORM (TENORM) material.   2. There are attempts to surmount these insufficiencies by using scintillating crystals (such as Nal(Tl)). The price of crystals prevents production of large devices, limiting their scalability.   3. Commonly used scintillation materials (such as the examples above) are not capable of distinguishing neutron from gamma ray signals.       

     Efforts have been made to perform such a task by using passive radiation detectors (see for example WO-A1-2005/009886 or U.S. Pat. No. 5,679,956 or U.S. Pat. No. 6,768,421 or US-A1-2005/0029460). The shortcomings of commonly employed solutions are various: Detectors with sufficient energy resolution to give clues to the source identity are too costly to use for a detector with a large detection volume. Further, to detect neutron-emitting substances, separate neutron detectors need to be installed, as most radiation detectors are not capable of discerning between neutron and gamma radiation (see the US-A1-2005/0029460). 
     A problem with passive monitoring is nuisance alarms caused by naturally occurring radioactive materials (NORM) and technically enhanced naturally occurring radioactive materials (TENORM). To avoid such incidents, it is desirable to have a detector that can measure the spectrum of the detected radiation in order to identify the source isotope. A further way to reduce nuisance alarms is the use of a detector capable of discerning strong point sources from conglomerations of weak sources in a large partial volume. A powerful distinction criterion between benign and harmful radioactive sources is the uniformity of emission. While benign sources often are large volumes of low activity material, dangerous sources often are much more point like. 
     Active systems consist of a radiation source of some and a detector of some form. The bulk in question is illuminated by the radiation source referred to as the interrogating radiation. The radiation of the radiation source is commonly in the form of neutrons and/or gamma rays. The detector can measure the effects of the materials within the bulk in question on the interrogating radiation. Such effects may be attenuation, scattering, or neutron resonance. The detector may also measure radiation emitted upon stimulation by the interrogating beam, due to physical processes such as stimulated fission or photo fission. 
     The invention is a response to the demand for new technologies for the assessment of materials in a bulk volume. In particular, for homeland security purposes, systems capable of quantifying the threat posed by containers, trucks, trains, or other freight forms are seeked. Explosives or radioactive substances for use in dirty bombs or nuclear weapons may pose these threats. 
     The challenge can be stated as the following: Conceive a detector whose scalability to large dimensions is feasible from a technical and from a financial point of view, which is capable of detecting and discerning neutrons from other forms of radiation, as well as giving the most precise possible information regarding the energy of the incident radiation. The detector ideally provides means for identifying point sources. 
     On the other hand, radiation detectors using noble gas have been used for radiation detection from small scales up to very large scales. For example, the ICARUS collaboration has deployed neutrino detectors with hundreds of tons of detector volume. In most noble gas based detectors, the ionization charge brought forth by energy radiation interactions is measured. 
     It has already been suggested that different particles, i.e. alpha particles, electrons, and fission fragments, lead to different scintillation pulse shapes in liquid argon and liquid xenon (see for example Hitachi et al. Phys. Rev. B, 27(9), p. 5279-5285 (1983)). This effect is assumed to be brought forth by the fact that different particles interact differently with the target material, transferring their energy either to target nuclei or target electrons, or to a combination of the two. The effect is also assumed to depend on the density of energy deposition. 
     Recently, it has been proposed (Boulay et al., Direct WIMP Detection Using Scintillation Time Discrimination in Liquid Argon, arXiv: astro-ph/0411358v1 (15 Nov. 2004)) to use this fact to detect dark matter in the form of WIMPs (Weakly Interacting Massive Particles). The proposed detector is based solely on the detection of liquid neon or argon scintillation light to discern between WIMPs and the internal background caused mainly by beta radioactivity proceeding from detector components, in particular radioactive impurities in the noble gas. The method of discrimination relies on the different scintillation light pulse shape emitted by beta as opposed to assumed WIMPS interactions. 
     Passive monitoring procedures are commonly used to detect illicit radioactive sources in containers. For this purpose, plastic scintillators are often employed, detecting the gamma ray count rate. Efforts have been made to construct and deploy detectors relying on scintillating crystals in order to measure the radiation spectrum and identify the source isotope. 
     Active interrogation techniques have been proposed, where the working principle includes a radiation source and a detector. Material assessment is performed, relying on physical effects such as induced fission, photo fission, nuclear fluorescence, and beam attenuation. 
     Liquid noble gas ionization drift chambers have been proposed for active interrogation with cosmic muons. 
     Imaging techniques using gamma rays (Compton imaging) or neutrons are described in scientific publications. For neutrons, a good description of this technique is given in 2005  IEEE Nuclear Science Symposium Conference Record, “Demonstration of a directional Fast Neutron Detector ” by P. E. Vanier. 
     DESCRIPTION OF THE INVENTION 
     It is therefore an objective of the invention, to provide a method for monitoring the unknown contents in a volume, a respective monitoring system, and a radiation detector for being used in such a monitoring system, which avoid the disadvantages of the prior art methods, systems and detectors, have an improved resolution, and are simpler to install and operate. 
     This objective is achieved by the solutions according to claims  1 ,  14  and  23 . 
     The invention proposed here relies on a pulse shape discrimination method discriminating between fast neutrons and gamma rays penetrating into the noble gas based detector from the outside. This allows the deployment of the apparatus for the assessment of materials as described below. 
     Noble gas in general is a price worthy scintillator of high light yield. Noble gases, permitting discrimination between neutron and gamma ray events on grounds of their different pulse shapes, allow the design of scalable, multifunctional detectors: In a single unit, such an apparatus is capable of detecting neutrons and other radiation, and performing spectroscopy of the measured radiation. This allows assessment of the radiation source if the device is used as part of a passive screening system. 
     In an embodiment as part of an active screening unit, the same qualities allow assessment of the material via methods described below. 
     According to an embodiment of the invention the processing step comprises the steps of comparing the derived spectrum with known spectra stored in a database; and issuing an alarm, when the derived spectrum is characteristic for a dangerous material contained in said volume, whereby the comparison of the derived spectrum with the spectra of the database gives a threat-likelihood; and the alarm is issued, when the threat-likelihood is above an adjustable threshold. 
     According to another embodiment of the invention the wavelength of the photons generated by the interaction is shifted by means of a wavelength-shifting material before the photons are detected by means of said photondetecting means. 
     In another embodiment of the invention the measuring volume extends along an axis and the photon detecting means are positioned at opposite ends of the measuring volume with respect to said axis, and photons, which do not directly hit the photon detecting means, are reflected towards the photon detecting means by means of a reflecting wall surrounding said measuring volume, whereby all photons are detected within a first given time period by means of said photondetecting means, when two or more photon detecting means coincidentally detect a photon within a second given time period, the first given time period having a typical length of a few microseconds, and the second given time period having a typical length of a few nanoseconds. 
     According to another embodiment of the invention a plurality of radiation detectors is arranged in a detector cluster; the gamma radiation and/or fast neutrons undergo multiple scattering within the measuring volume of various radiation detectors of said detector cluster; and the photons detected by means of the photon detecting means of the various radiation detectors of said detector cluster are used to derive information on the incident particles. In addition, the radiation may pass through a coded aperture before entering the radiation detectors and a decoding algorithm is used to derive directional information on the trajectories of the incident particles. 
     According to another embodiment of the invention the volume with the unknown contents is moved along at least one row of radiation detectors while monitoring the unknown contents in said volume. 
     According to another embodiment of the invention an interrogation beam from a radiation source emitting photons or neutrons is sent through the volume to be screened, whereby the interrogation beam splits up into a remnant interrogation beam and a scattered interrogation beam; and the remnant interrogation beam and/or the scattered interrogation beam and/or radiation generated by stimulated emission in the unknown contents is measured by means of the at least one radiation detector. Either a pulsed interrogation beam or a continuous interrogation beam may be used. 
     According to an embodiment of the monitoring system of the invention the at least one radiation detector comprises an elongated measuring volume extending along an axis; there are two photon detecting means provided at opposite ends of the measuring volume with respect to the axis; and the signal processing means comprises a coincidence unit connected to the two photondetecting means. A plurality of radiation detectors are arranged next to each other in a detector arrangement with their axes being in parallel, wherein the radiation detectors are arranged in at least one detector cluster. The radiation detectors of each of the detector clusters may share a common volume of noble gas or of a mixture of noble gases. 
     According to another embodiment the radiation detectors are arranged in at least one detector row, especially in parallel detector rows. On the other hand, the radiation detectors may be arranged in orthogonal detector arrangements. 
     In another embodiment a radiation source is provided for sending an interrogation beam through said volume with said unknown contents. 
     According to an embodiment of the inventive radiation detector the measuring volume is bounded by a container; and the photon detecting means are optically coupled to the measuring volume by means of light guides, which are coated at their measuring volume sides with a wavelength shifting coating. 
     The inner walls of the container may be covered with a photon reflecting coating, especially of PTFE or MgF 2  coated Al. 
     On the other hand, the inner walls of the container may be covered with a wavelength shifting coating, especially of tetraphenyl butadiene (TPB) and/or P-Terphenyl or a mixture containing one or more of these substances. 
     According to another embodiment of the invention the noble gas or noble gas mixture or mixture of noble gases and/or noble gas isotopes in the measuring volume is in the liquid state or in the pressurized gas phase. 
     According to another embodiment of the invention the noble gas or noble gas mixture or mixture of noble gases and/or noble gas isotopes in the measuring volume is contained in an inflatable container. The respective detector may therefore be an airborne device. 
     According to another embodiment of the invention the noble gas or noble gas mixture or mixture of noble gases and/or noble gas isotopes in the measuring volume is in contact with an in-situ purification means, which is especially based on an impurity-gettering spark gap. 
     According to another embodiment of the invention the noble gas or noble gas mixture or mixture of noble gases and/or noble gas isotopes used in the measuring volume is purified in-situ. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments, which are illustrated in the attached drawings, in which: 
         FIG. 1  shows a single radiation detector according to an embodiment of the invention with incoming radiation depositing energy in the detector by interaction with a noble gas in the measuring volume; 
         FIG. 2  shows a cutout of the detector according to  FIG. 1 ; 
         FIG. 3  shows the simplified circuit of a measuring system with one radiation detector according to an embodiment of the invention; 
         FIG. 4  shows various steps within a method for operating the measuring system according to  FIG. 3 ; 
         FIG. 5  shows an embodiment of the invention, wherein several radiation detectors are grouped to a detector cluster, sharing the noble gas of a common noble gas vessel; wherein the cluster may be used as an imaging system; 
         FIG. 6  shows how, according to another embodiment of the invention, several radiation detectors can arranged in detector rows to be operated together in a passive monitoring system; 
         FIG. 7  shows an embodiment of the invention using the radiation detectors in an active interrogation system; 
         FIG. 8  shows a detector arrangement with crossed rows of radiation detectors and a coded aperture used with this arrangement for imaging in accordance with the invention; 
         FIG. 9  shows a single radiation detector according to another embodiment of the invention with incoming radiation depositing energy in the detector by interaction with a noble gas contained in an inflatable container; and 
         FIG. 10  shows a single radiation detector according to another embodiment of the invention, wherein the photon detecting means, especially solid state devices like avalanche photodiodes, are located at any position inside the noble gas volume. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a single radiation detector according to an embodiment of the invention. The radiation detector  10  comprises a cylindrical, elongated container  11 , which extends along a longitudinal axis  21  and contains a measuring volume  12  of noble gas, especially argon, neon or helium, or a mixture of any of these gases or their isotopes, e.g. helium-3. The noble gas or noble gas mixture in the measuring volume  12  is in the liquid state or in the pressurized gas phase. An incident ray  17 , typically a neutron or a gamma ray, deposits energy in the radiation detector  10 . An energy-dependent part of the deposited energy is converted by an interaction  18  with the noble gas into scintillation photons  20 . Some of these scintillation photons  20  arrive at two photon counters  15 ,  16  or other comparable photon detecting means arranged at opposite ends of the measuring volume  12 . The photon counters  15 ,  16  are optically coupled to the measuring volume  12  via light guides  13 ,  14  ( FIG. 1 ,  2 ). The scintillation photons  20  may possibly be shifted in wavelength and reflected at the wall of the container  11 . The photon counter closer to the point of the interaction  18  typically detects more photons. The elongated geometry of the measuring volume  12  is advantageously for material analysis. 
     An example of how the classification of radiation types detected by the invention may be achieved can be found in “2006 IEEE Nuclear Science Symposium Conference Record, “Noble Gas Scintillation-Based Radiation Portal Monitors and Active Interrogation Systems” by R. Chandrasekharan et al. (2006 IEEE Nuclear Science Symposium Conference, Oct. 29-Nov. 4, 2006, San Diego, Calif.). 
     As is shown in  FIG. 2  in greater detail, the measuring volume  12  with the noble gas is confined in an elongated container or vessel  11  terminated by view ports in the form of light guides  13 ,  14  made of an optically transparent material such as PMMA or quartz glass. Each of these light guides  13 ,  14  is optically coupled to a photon counting device or photon counter  15 ,  16 , for instance a photomultiplier tube (PMT). The short wavelength (VUV or below) noble gas scintillation light, to which most materials are opaque, can be reflected off the container walls, for example by a reflector made of PTFE or MgF 2  coated Al, until it reaches one of the light guides  13 ,  14 . Else, or additionally, the container walls may be coated with a wavelength shifting (wls) coating  22  such as Tetraphenyl Butadiene (TPB) or P-Terphenyl, or a mixture containing one or both components, with the purpose to down-convert the scintillation photons  20  to a more practical wavelength, for instance 420 nm. Such down-converted photons can easily be reflected until they reach the terminating light guide  13  or  14 . The face of the light guide  13 ,  14  exposed to the measuring or detection volume  12  can also be coated with a wavelength shifting (wls) coating  23 , that is transparent to its own emission wavelength. This may be achieved by dip coating with a polystyrene-TPB-chloroform solution. The light guides  13 ,  14  conducts photons to the respective photon counter  15 ,  16 , either by total internal reflection or due to a reflective coating or wrapping not explicitly shown in  FIG. 2 . 
     As has been said already, possible noble gases to be employed may be pure argon, neon, helium, or mixtures of pure noble gases. The gas choice may be optimized for optimal performance in the specific embodiment. Light gases may be better suited for neutron detection for kinematics reasons. On the other hand, heavier gases have higher density, resulting in a higher light yield, and longer wavelength emission. Mixing noble gases (doping) leads to a wavelength conversion. As the purity of the noble gases is essential for the detection process used, it is advantageous to have an in-situ gas purification. Accordingly, as shown in  FIG. 2 , a noble gas purification means  52  may be arranged at or in the detector  10  in contact with the noble gas. The noble gas purification means  52  may contain a spark gap with electrodes made of a gettering metal, as has been described in an article of S. G. Pokachalov et al., “Spark discharge method of liquid rare-gas purification”, Nucl. Instr. And Meth. A327 (1993) 159-162. 
     Furthermore,  FIG. 10  shows a single radiation detector according to another embodiment of the invention. The photons are detected by photon detectors  53 , that can be enclosed by the container  11  to be inside the measuring volume  12 , eliminating the need for light guides. The photon detectors  53  may be solid state devices (e.g. avalanche photodiodes) sensitive to the noble gas scintillation light and/or a shifted wavelength of this light. The photon detectors are connected to the detector electronics by some form of feed-through  54 . In this embodiment, high detection yields can be achieved by reducing eliminating losses arising from light guides, as well as through convenient photon detector placement and area coverage. 
     The gas may be in the liquid phase or in a pressurized gas phase, such as to optimize the gas density for ideal performance in the desired application. With reducing density, the detector becomes less sensitive to gamma rays. This effect may be desired when neutron detection has high priority. 
     Furthermore, as shown in  FIG. 9 , the radiation detector  10 ″ may be of an inflatable design, whereby the container  11 ′ is made of rubber or any other light weight, gas tight, possibly flexible and/or elastic, material. The noble gas is filled into the container  11 ′ by means of a valve  51 . The photon counters or photon detecting means  15 ,  16  are used as terminations of the container  11 ′. The shape of the container  11 ′ may be cylindrical or spherical or of another suitable form. When helium is used as a gas, the detector  10 ″ may rise into the air like a balloon or zeppelin to be used as an airborne detector. Such an inflatable detector  10 ″ can be made small when not used, can have a big measuring volume  12 ′ to detect radiation even at a longer distance to the object to be inspected, and can be easily manufactured and installed. It may be used not only for monitoring a container or the contents in a volume, as explained in this application, but for any other noble-gas-based detecting applications. 
       FIG. 3  illustrates a measuring system  24  according to an embodiment of the invention with a simplified circuit layout for triggering and data analysis. The operating steps of the system are illustrated in  FIG. 4 . 
     An occupation sensor  26  arms the data acquisition chain, when a volume  47  in question, for example a truck or container, is in place. A coincidence unit  27  gives a trigger if more than one of the photon counters  15 ,  16  detects a photon within a short time window, typically of the order of a few nanoseconds. Given a trigger, the event, that means the full information on detected photons in all photon counters  15 ,  16  within a designated time window, is recorded. This time window extends from typically tens of nanoseconds before the trigger time to typically a few microseconds after the trigger time. By comparing the number of photons detected at each end of the measuring volume  12 , the z-coordinate (vertical coordinate in a system like that of  FIG. 6  or  7 ) of the interaction can be measured. The x and y coordinates of the interaction are given by the coordinates of the detector in which the interaction took place. The signals are at the same time summed up in a summation unit  28  and subtracted in a subtraction unit  29 . A processing unit  30  interacts with a buffer storage  31  and a database  32 . By fitting the shape of the sum of the signals, the type of particle (neutron or gamma), as well as the amount of deposited energy can be determined. The information from these analyses is stored in the temporary buffer storage  31 . Out of this information, spectra are constructed, increasing in precision with time as many events are acquired. These spectra are compared with data from the database  32 , giving a threat-likelihood. If the threat-likelihood is above an adjustable threshold, an alarm is issued at an output unit  33 . When the measurement is completed, the occupation sensor  26  turns off, and the buffered information from buffer storage  31  is stored in the database  32 , clearing the buffer storage  32  for a new measurement. 
       FIG. 5  shows an embodiment of the invention, wherein several identical radiation detectors  10  are arranged in and operated together as a detector cluster  34 . This detector cluster  34  may be useful for increasing the sensitive or measuring volume of the system and/or allowing directional or imaging applications. An incident gamma ray  17  or neutron can undergo multiple scattering. If the interaction points W 1 , . . . , W 4  of such a process are located within the cluster&#39;s detection volume, information on the trajectory of the incident particles can be extracted. This is done by measuring the coordinates of the interactions W 1 , . . . , W 4 , given in part by the z coordinate and the x-y coordinates of the radiation detectors  10 , the time and the deposited energy of each interaction W 1 , . . . , W 4 . This technique has been shown to be feasible with neutrons (neutron imaging) as well as gamma rays (Compton imaging). Directional information of this sort allows identification of point sources. The directional information may be derived in a different way by a so-called coded aperture imaging method (see for example E. E. Fenimore et al., Applied Optics, Vol. 17, no. 3 (1978)). As is shown in  FIG. 8  in a simplified scheme, the radiation may pass through a coded aperture  50  before entering a detector arrangement  49  with a cluster of radiation detectors  10 ,  10 ′. A decoding algorithm is then used to derive directional information on the trajectories of the incident particles from the photons counted by the photon counters of the various radiation detectors  10 ,  10 ′. 
     The geometric cross section of each radiation detector  10  may be optimized for dense packing, for example by using hexagonal measuring volumes. The individual measuring volumes of each detector do not necessarily need to be identical with the noble gas container. The volumes may be defined by reflective/wavelength-shifting foil and the noble gas contained by larger vessel  35  surrounding all radiation detectors  10  ( FIG. 5 ). 
       FIG. 6  shows a possible embodiment of the invention with a monitoring system  36 , wherein a plurality of radiation detectors  10  is arranged in detectors rows  37 ,  38  to form a passive detection system, said detectors rows  37 ,  38  extending on opposite sides along the volume in question, which may be a truck, a train, or a freight container  39  on a conveyor belt moving in a direction  40 . The container  39  in question is passed alongside the detector. Sufficiently penetrating radiation from possible radioactive sources within the container  39  passes through the systems fiducial measuring volume. A certain fraction of this radiation will deposit sufficient energy within the detector volume to allow detection. Although the monitoring system  36  of  FIG. 6  is a stationary one, it may sometimes be advantageous to miniaturize it to get a hand-held system, or to make it a vehicle-based system, which can be moved to different locations, when needed. 
     Such a distributed arrangement may be advantageous for discerning strong point sources from weak sources distributed over larger volume. Due to the 1/r 2  standoff intrinsic to radiation, the interaction rate is higher in the measuring volumes of those detectors, which are at closer proximity to the source. 
       FIG. 7  shows another embodiment of the invention with a possible arrangement for an active interrogation system  48 . A radiation source  43  directs an interrogation beam  44  to the volume  47  in question. A first detector arrangement  42  positioned behind the volume  47  in question receives the remnant interrogation beam  45  coming out of the volume  47  in line with the interrogation beam  44 . A second detector arrangement  41  positioned at one side of the volume  47  receives the scattered interrogation beam  46 . The detector arrangements  41  and  42  each comprise plural rows of detectors arranged one behind the other in closest packing; only the first row is shown in  FIG. 7 . The active interrogation system  48  is used as a detector for the remnant and/or scattered interrogation beam  45 ,  46  and/or stimulated emission radiation from the material in question. The interrogation beam  44  may be emitted by any photon or neutron source as the radiation source  43 . The interrogation beam  44  may be continuous or pulsed. 
     Although the detector arrangements described so far comprise only a group of radiation detectors oriented in parallel, other detector arrangements may be advantageous, where various radiation detectors are oriented in different directions.  FIG. 8  shows a detector arrangement  49 , wherein rows of radiation detector  10  and  10 ′ are arranged in a crossed configuration. 
     The method and system according to the invention may furthermore be used not for the monitoring of the contents of a container but for identifying a container itself, i.e. for radiation imaging of casks (containers) for spent nuclear fuel (see for example K. P. Ziock et al., Radiation Imaging of Dry-Storage Casks for Spent Nuclear Fuel, IEEE Nuclear Science Symposium Conference Record, N30-1, p. 1163-1167 (2005)). Such a container is characterized by a radiation “fingerprint”, which may be used to monitor the location and/or transport of the casks. 
     LIST OF REFERENCE NUMERALS 
     
         
           10 , 10 ′, 10 ″ radiation detector 
           11 , 11 ′ container 
           12 , 12 ′ measuring volume 
           13 , 14  light guide 
           15 , 16  photon counter (e.g. PMT) 
           17  incident ray (gamma and/or neutron) 
           18  interaction 
           19  emerging ray 
           20  scintillation photon 
           21  axis 
           22 , 23  wls coating 
           24  measuring system 
           25  power supply 
           26  occupation sensor 
           27  coincidence unit 
           28  summation unit 
           29  subtraction unit 
           30  processing unit 
           31  buffer storage 
           32  database 
           33  output unit (alarm) 
           34  detector cluster 
           35  vessel 
           36  monitoring system 
           37 , 38  detector row 
           39  container 
           40  direction 
           41 , 42  detector arrangement 
           43  radiation source 
           44  interrogation beam 
           45  remnant interrogation beam 
           46  scattered interrogation beam 
           47  volume in question 
           48  active interrogation system 
           49  detector arrangement 
           50  coded aperture 
           51  valve 
           52  noble gas purification means 
           53  photon detector (e.g. avalanche photodiode) 
           54  feed-through 
         W 1 , . . . , W 4  interaction