in the present invention , it is recognised that the use of high energy x - ray inspection devices is becoming increasingly common for cargo screening , and that the x - rays produced by these devices interfere with the standard passive radiation detection systems so reducing the sensitivity of the passive radiation detection system to hidden special nuclear materials and other gamma - ray emitting sources . the independent claims define aspects of the invention for which protection is sought . the dependent claims define preferable inventive features . any of the features of the dependent claims may be used in combination with the features of other claims , even if they are not explicitly dependent upon them — this will be clear to a person skilled in this field . in the first aspect of this invention , a set of radiation detectors 18 for passive radiation monitoring are co - located with a set of imaging detectors 20 for high - energy x - ray inspection . an example embodiment is shown in fig2 . the x - ray source 22 will typically be an x - ray linear accelerator in which the output beam comprises a series of intense pulses of x - rays as described in fig3 . the period between pulses is typically 10 ms with a pulse width of typically 5 μs . therefore , for approximately 99 . 95 % of the time , the x - ray beam is not emitting . other pulse arrangements will be apparent to the skilled person . in the time between x - ray pulses , the passive radiation detectors 18 are able to accumulate the gamma - ray signal as planned . however , every 10 ms or so , the operation of the gamma - ray detectors will be significantly perturbed the presence of the x - ray beam , and this will destroy the validity of the gamma - ray signals that were collected in the previous time period . in order to address this issue , in the present invention a circuit is described that effectively renders the gamma - ray detectors 18 inactive during the x - ray pulse period , but enables the detectors 18 at all other times . typically , a gamma ray detector that is designed for monitoring of low activity gamma ray sources will be as large as possible in order to subtend the largest possible solid angle back to the source . therefore , it is uncommon to use a semiconductor detector for this purpose . further , the gamma radiation is often at high energy and so the use of a large volume gas ionisation based detection system is generally considered to be impractical . generally , such detectors use a scintillation detector that may be a high density in - organic material such as csi , nai or bgo or a low density in - organic plastic scintillator . in the case of a passive radiation monitor , it is conventional to employ a large plastic scintillator since this is generally a less expensive solution than using an in - organic detector material . in either case , the scintillator converts the gamma - ray energy to an optical light pulse which is transported through the bulk of the scintillator material to one or more optical detectors . these detectors could be made from a semiconductor material such as silicon . however , the area that would need to be covered by detector material makes such an approach expensive , and the large area drives up capacitance which in turn makes forming a low noise readout circuit extremely difficult . a more practical solution is to utilise one or more photomultiplier tubes . a photomultiplier tube comprises a vacuum envelope within which is a photocathode , a number of dynodes and an anode . light passes into the photocathode from the scintillator and results in the generation of low energy photoelectrons . these photoelectrons are drifted in an electric field towards a first dynode . as they pass through the field , the photoelectrons pick up energy such that when they strike the first dynode , each photoelectron is capable of generating multiple secondary electrons ( typically 3 to 5 ). these secondary electrons accelerate in an electric field towards a second dynode where further secondary electrons are generated . finally a large cloud of electrons reach an anode where a large current pulse is generated per incident optical photon . a typical gain is 10 6 electrons per photon . a photomultiplier tube is typically wired up using a resistor chain substantially as shown in fig4 . often , a photomultiplier tube will have 10 dynodes and will be operated with a positive high voltage of around 1000v . using ohms law , it can be seen that the voltage between dynodes is then around 83v . to improve system linearity and uniformity of signal collection from the photocathode of the photomultiplier tube , the accelerating voltage between the cathode and first dynode is often increased to around 165v . at high dynode numbers , it is often advantageous to place capacitors in parallel with the resistor chain in order to support the high current pulses in this region of the dynode chain to ensure stable operating voltage which in turn ensures good linearity of the detection system . this is shown with dynode dn in fig4 . when subjected to a large signal pulse due to exposure to radiation from an x - ray linear accelerator , voltage drops start to occur across the dynode chain due to the exceptionally high current flows that result from such a bright signal and this causes the voltage across the photomultiplier tube to collapse . this sends a large current pulse through the coupling capacitor , c l , into the preamplifier which in turn saturates the subsequent readout electronics . it can take tens of milliseconds for the detection system to return back to a usable operating state . in the context of a combined x - ray and gamma - ray detection system , this is an unacceptable operating condition . in the present invention , an alternative circuit topology 50 is described and is shown in fig5 . here , the first dynode , d 1 , is connected to ground and a standard resistor ladder is used from this point forwards . the cathode is now connected to a switching potential which may be set to a negative value with respect to ground potential or to a small positive potential with respect to ground potential . since there is a physically large separation between the cathode and first dynode and the dielectric constant of vacuum is essentially zero , the capacitive coupling between the cathode and dynode is negligible . this means that the potential that is applied to the cathode can be varied with very high slew rate with no significant measureable effect at the first dynode or consequently at any further point in the signal chain . this provides a very clean way to disable the electronics during the high intensity burst of x - ray signal from the x - ray linear accelerator source . referring to fig5 and 6 , it is seen that if the cathode potential normally rests at a negative potential ( typically − 165v ) with respect to ground then the photomultiplier tube operates normally and behaves as a passive radiation monitor . during an x - ray output phase , the cathode potential is switched to ground potential or to a small potential that is positive with respect to ground ( as shown graphically in fig6 ) and the operation of the photomultiplier tube is totally inhibited . as soon as the x - ray pulse has passed , the cathode potential can be switched back to a negative level and the passive radiation monitor will continue to operate . of course , the + hv value should be decreased by the cathode normal operating potential in order to achieve the required signal gain in the dynode chain ( i . e . + hv should be reduced from 1000v to 835v using the figures from the above examples ). by establishing a small positive potential between the cathode and the first dynode ( for example 10 v ), any energetic photoelectrons that are produced during the x - ray burst will be actively returned to the cathode during the burst itself so that the vacuum gap between the cathode and first dynode is free of any drifting electrons which could otherwise contaminate the signals to be recorded immediately after the cathode potential is returned to its active value . a suitable circuit for driving the cathode switching potential is shown in fig7 . this “ push - pull ” circuit can achieve switching times from − von ( for example − 165v ) to + voff ( for example + 10v ) potential in times of the order of 100 ns . it is convenient to opto - couple the switching signal into the buffer in order to level shift the signal from typical digital control circuitry referenced to ground potential to this circuit which is referenced to − von potential . an example data acquisition system for use with the passive radiation detector is shown in fig8 and 13 . in this example , the passive detection system comprises an array of detectors — each passive detector comprises a photomultiplier tube 130 . each tube 130 is arranged in a known spatial position . an active scanning system arranged to scan an object ( in this example a vehicle ) comprises a further linear detector array 132 for detecting radiation emitted by a radiation source of the scanning system . the radiation from the active radiation source irradiates the object and the active system &# 39 ; s linear detector 132 detects radiation that passes through or is scattered by the object . the position of the scanning system detector 132 relative to each detector 130 is known . as the vehicle moves past the detectors 130 , 132 , information from the detectors 130 , 132 is sent to a controller 134 . the speed of the vehicle can also be taken into account when producing the spatially correlated information . the controller correlates spatially the information from the two sets of detectors 130 , 132 . this is possible for the passive detector since it is in the form of an array of separate detectors 130 — each detector measures absolute levels of radiation , in order to detect radiation emitted from any radiation - emitting material in the object — this detection is controlled by the controller 134 such that it is carried out whilst the active radiation source is not irradiating the object . in the prior art , passive detectors are not provided in such arrays , instead there is only a large passive detector ( as described with reference to fig1 ) and spatially correlated information is not provided in the same way . the spatially correlated information can be used to determine where within an actively scanned region a passively detected radiation - emitting material is located . here , each photomultiplier tube ( pmt ) 130 is coupled to its own shaper circuit , discriminator and digital data buffer . therefore the overlaid image shown in fig1 can be obtained in some embodiments ( as described in further detail below ). as shown in fig9 , the shaper circuit formats the data from the pmt into a pulse suitable for subsequent discrimination . the first stage in the shaper is a gain stage with a time constant set that is typically several milliseconds . the second stage is rc ( low pass ) filtering followed by cr ( high pass ) filtering to leave a positive going pulse with duration of typically a few microseconds . the selection of filtering times in the rc - cr shaping stage will define the noise performance of the system but is typically set for both parts at a few microseconds . other settings will be apparent to the skilled person . the discriminator circuit may have different design depending on specific application . when using plastic scintillators in which there is no true photopeak ( due to the low atomic number of the scintillator material ) it is typically sufficient to introduce a single discriminator window using a circuit of the type shown in simplified form in fig1 . here , the output from the shaper stage passes into two analogue comparator circuits , the first checks the height of the pulse with respect to a lower level discriminator ( lld ) setting and the other checks the height of the pulse with respect to an upper level discriminator ( uld ) setting . pulses which are greater in magnitude than the lld but lower than the uld are accepted and generate a digital output pulse at the output of the logic circuit . in an alternative embodiment which is better suited for use with in - organic scintillators where the magnitude of the optical signals generated better reflects the shape of the input gamma - ray energy distribution , the intensity of the light flash is measured using an analogue - to - digital converter 110 and the values of successive values can be histogrammed to form an energy dependent analysis of the material under inspection . such a circuit is shown in simplified form in fig1 . here the logic circuit is used to determine whether the signal is above the noise floor . if it is , an adc conversion is initiated and the resulting digital value is then histogrammed to a digital memory 112 . at the final stage of data acquisition , the digital data from each photomultiplier detection chain is multiplexed down to a single data stream which is advantageously in the ethernet format . this data is passed to a computer for final analysis . in the event that a discriminator of the form shown in fig1 is used , the analysis will normally consist of a dynamic background subtraction followed by comparison of the residual signal against threshold . if an above threshold signal is observed , an alarm will typically be raised . in the event that a discriminator of the form shown in fig1 is used , the analysis will normally proceed as noted above . however , if an above threshold signal is detected , the shape of the energy distribution will typically be analysed against known spectral lines in order to provide a preliminary assessment of the type of radioactive material that has been detected , and both the type and activity of the radioactive source will be passed on to the operator . in a further aspect of this invention , data 122 from the passive radiation monitoring system is overlayed with image data 124 from the x - ray imaging system . an example data overlay is shown in fig1 . here , the total count per x - ray pulse is displayed in graphical form in a window at the base of the screen . to improve noise performance , the operator is provided with means to change the integration window for the gamma - ray data so that the signal distribution appears “ smoother ”. in the event that gamma - ray spectral data has been collected , the operator is provided with a pop - up window to view both the energy distribution and fitting data to a suitable gamma - ray library . this graphical view should normally utilise the same integration window that is used for display of the primary count data . as the operator moves their cursor about the image , the graph is advantageously updated in real - time to reflect the gamma - ray spectrum that was collected from the equivalent region of the object under inspection . it is advantageous to combine spectral data collection with the use of data deconvolution algorithm . such an algorithm is provided with the energy response function of the detector itself so that this response can be de - convolved from the measured gamma - ray spectral data . further , the energy response function for a number of known gamma - ray sources is provided to the de - convolution algorithm such that signals that fall into the compton continuum can be restored to their full energy peak . this provides a considerably sharper spectrum which can be used to good advantage when fitting to data from a standard gamma - decay library . in another aspect of the invention , fig1 shows an alternative system set - up to that shown in fig5 . typically x - rays having a peak energy value of 900 kev or higher are used for x - ray scanning in this invention . there are three primary parts to this front - end electronics configuration circuit : 1 . power supply unit , psu . this takes + 12v input and converts this to an adjustable voltage of up to − 1 kv for driving the photomultiplier tube . the psu also generates a cathode potential which may be switched to a negative or positive potential with respect to the − hv output in order to inhibit the photomultiplier tube , pmt operation during x - ray pulses . 2 . analogue to digital converter , adc . this directly digitises the output of the pmt . this should be a high speed device that is capable of recording pulse heights during the x - ray pulse as well as pulses due to passive gamma interactions . 3 . field - programmable gate array , fpga . the fpga is configured as a digital pulse processor . this provides time and magnitude information for every above - threshold radiation interaction . the fpga drives the event based data stream out to the rest of the data acquisition system . the detectors are not temperature stabilised in this embodiment since it is expensive to undertake this and also leads to unreliability of systems in practical customer installations . the present system should therefore be capable of calibration by a relatively untrained operator . one way to do this is to provide a reference radioactive source for every detector and to use this for spectrum stabilisation . however , this may be difficult to achieve and leads to transportation issues . as an alternative , a light source with line of sight to the scintillator can be pulsed and this used as a way of confirming the calibration of the pmt and electronics system . this is simple to achieve and by splitting the light source so that it irradiates a photodiode as well as the scintillator , it is possible to provide a reasonably accurate calibration of the system .