Abstract:
The apparatus and method provide a technique for significantly reducing capacitance effects in detector electrodes arising due to movement of the instrument relative to the item/location being monitored in ion detection based techniques. The capacitance variations are rendered less significant by placing an electrically conducting element between the detector electrodes and the monitored location/item. Improved sensitivity and reduced noise signals arise as a result. The technique also provides apparatus and method suitable for monitoring elongate items which are unsuited to complete enclosure in one go within a chamber. The items are monitored part by part as the pass through the instrument, so increasing the range of items or locations which can be successfully monitored.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/930,899, filed Aug. 15, 2001 now abandoned, which is a continuation of patent application Ser. No. 09/307,228, filed May 7, 1999, abandoned, which claims priority to Great Britain Application Nos. 9,809,748.8, filed May 8, 1998 and 9,809,749.6, filed May 8, 1998 which applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     This invention is concerned with improvements in and relating to ion detectors and particularly, but not exclusively to detectors in which the detector moves relative to the ion source. Relative movement may arise due to movement of the detector and/or due to movement of the ion source, for instance due to passage of the item carrying the source through the detector. 
     2. The Relevant Technology 
     The monitoring of alpha emissions from an item or location is of particular significance during decommissioning, material accounting and a variety of other applications. 
     The long range detection of alpha emissions, indirectly, through the monitoring of air ions generated by the passage of alpha particles is known. The item believed to be carrying the alpha sources is placed in a container, the container completely enclosing the item so as to exclude ambient ions. The ions generated by alpha particles are attracted to electrode(s) in the detector system and a current arises as a result. 
     The very small size of this current makes it prone to interference from noise currents arising due to other variables in the system. Movement of the detector electrode relative to the source of the alpha particles causes significant changes in the system capacitance and significant noise currents as a result. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention aims to provide a detector system which is far less prone to noise interference, even where the detector is actively moved relative to the source, for instance when mounted on a vehicular survey system or, for instance, when an elongate item is moved through the detector during monitoring. 
     According to a first aspect of the invention we provide an instrument for detecting ions originating from a monitored location, the instrument having a body portion and one or more electrodes at an electrical potential relative to the monitored location, and one or more electrically conducting element(s) provided with one or more apertures, the element(s) being provided between the electrode(s) and the monitored location and further comprising means to monitor ions discharged on the electrode(s). 
     The element(s) are preferably provided with a large number of apertures, for instance greater than 80% of their effective area as apertures. The element may be in the form of a grid. A single metal element is preferred. 
     The element(s) may be planar. A single continuous element or a plurality of elements may be provided. 
     The element may be at a different potential to the location, including item, being monitored. The element may be earthed. The element may have an applied potential or an electrostatic potential. 
     The element may be provided across the full extent of the electrode(s), or at least a substantial part thereof. Preferably the configuration of the element mirrors that of the electrode(s), for instance a planar electrode may be provided together with a planar element or grid. 
     The element may be provided at a significant separation from the location/item to be monitor, for instance greater than 5 cm, greater than 10 cm and even greater than 20 cm. 
     Equally the element may be provided in proximity to the location/item to be monitored, for instance less than 5 cm and more preferably less than 1 cm from the closest surface of the item. 
     The instrument may be provided with a plurality of detection electrodes. The electrode(s) may be provided close to or away from the element. A continuous detector electrode may be used, such as a plate. A discontinuous detector may be used, such as an apertured plate or grid. 
     Preferably an electrical potential is applied to the electrode(s). An electrical potential may be provided electrostatically. The potential is preferably higher than that applied to the element. The potential of the element may be lower than that of the electrode(s), but higher than that of the location/item. The location/item and element may both be grounded. 
     The ions may be attracted to the electrode(s) electrostatically by the electric field. Alternatively or additionally air flow within the instrument from the location were ions are generated into proximity with the electrode(s) may be promoted. A fan may be used to promote the movement of ions to the electrode(s). 
     The electrode(s) may be provided within a continuous enclosure with a closeable opening for introducing the item to be monitored. A support may be provided within the enclosure for the item so as to maximize the surface area in contact with air. Preferably means to promote air flow around the item are provided. Preferably the air flow circulates within the sealed enclosure. 
     The electrode(s) may be provided within an enclosure which is open to its surroundings on at least a portion of one side. The enclosure may be in the form of a hood with an open side. Such an enclosure is suited to monitoring large areas or surfaces, such as floors and walls. The perimeter of the opening may be provided with a laterally extending flange. The lateral extent of the flange is preferably greater than the gap between the flange and surface. An extent at least 5 times and more preferably 10 times is provided. Preferably the flange is provided around the opening. Preferably a planar flange, opposing the surface to be monitored is provided. 
     The location to be monitored may be a surface, such as a wall, ceiling or floor, including these of a building, room or vessel; or a surface of a piece of equipment, such as a glove box, tank or vessel. Monitoring of open ground, rubble and soil is possible. The location to be monitored may be material passing the detector on a conveyor system. 
     The item to be monitored may be a piece of equipment, or a part thereof, such as a pump, pipe, beam, glove box, tool, filter, cable, rod or the like. 
     The item/location may be placed within the instrument and/or placed against and/or in proximity to the instrument. 
     Preferably the item/location is electrically grounded. 
     The item/location may be moved relative to the instrument, for instance a beam on a roller bed or soil on a conveyor belt and/or the instrument may be moved relative to the location/item, for instance a vehicularized instrument moved across a stretch of ground. 
     The means for monitoring ion discharge preferably comprises current monitoring, and more preferably current measuring means. The means for monitoring ion discharge may comprise means for monitoring the remaining electrostatic potential, such as surface charge monitors. 
     The current monitoring means is preferably an electrometer, most preferably the electrometer is provided as a floating input electrometer. The electrometer may be provided as a ground referenced electrometer. 
     An additional detector may be provided in the instrument to detect background ion levels. The additional detector may comprise an electrode and an apertured element. Preferably the additional detector electrode is separated from the electrode by a guard plate. 
     Preferably an equivalent potential is applied to the electrode and the background electrode. Preferably an equivalent potential is present for the element and the background element. 
     The ions may originate from the location directly or indirectly. Preferably the ions are generated by alpha particles, most preferably alpha particles emitted from material on or at the location being monitored. 
     According to a second aspect of the invention we provide a method of detecting ions originating from a monitored location, the method comprising providing one or more electrode(s) at an electrical potential relative to the location so as to attract at least some of the ions to the electrode, and additionally providing one or more electrically conducting element(s) with one or more apertures therein to allow the passage of ions, between the electrode(s) and the monitored location, the discharge of ions on the electrode(s) being monitored. 
     Other features, options, possibilities and details provided in the first aspect of the invention and/or elsewhere in this document are included in the possibilities for the second aspect of the invention. 
     According to a third aspect of the invention we provide an instrument for monitoring alpha emitting sources on an item, the instrument comprising a detecting chamber defining a detecting volume, the detecting chamber being provided with an inlet through which the item can be introduced and an outlet through which the item leaves the detecting chamber, the detecting chamber being provided with one or more electrodes for collecting ions produced in the detecting volume by the portion of the item in the detecting volume, the instrument further being provided with means to monitor ions discharged on the electrode(s). 
     In this way the monitoring of items considerably longer than the detecting volume, and indeed the instrument, is possible. 
     The detection chamber may be provided with one or more electrodes opposing the portion of the item within the detection volume. A single electrode surrounding the portion of the item is preferred. 
     The electrode(s) are preferably configured to the cross-sectional profile of the item being monitored. A cylindrical electrode may be provided, most preferably with its axis aligned with the axis of the item and/or instrument. 
     An applied, preferably externally generated, potential may be employed or an electrostatic potential may be employed. 
     Preferably the voltage gradient between different portions of the electrode(s) and the portion of the item is substantially constant for different portions of the electrode and of the item. 
     The detection chamber may be provided with one or more pairs of opposing detection electrodes. Preferably the electrodes of a pair are provided such that the item passes between them. A potential difference between the opposing electrodes of a pair may be provided in use. Preferably a potential difference between the item and at least one of the electrodes is provided in use. An applied or electrostatic potential may be employed. 
     The chamber is preferably provided with an inlet and outlet on a common axis. The chamber may be cylindrical. The inlet and/or outlet may be provided in the end wall(s) of a right cylinder. 
     The inlet and/or outlet may lead to a further chamber externally provided to the detecting chamber. The further chamber(s) may be provided with an opening to the surrounding environment. Preferably the further chamber aperture is axially aligned with the aperture (inlet/outlet) into the detecting chamber. Most preferably the aperture of the inlet further chamber, the inlet to the chamber, the outlet from the chamber and the aperture in the outlet further chamber are all axially aligned. 
     The detecting chamber may have an inlet closed to the passage of ions from outside the chamber to inside the chamber and/or an outlet closed to the passage of ions from outside the chamber to inside the chamber. 
     The inlet and/or outlet further chambers may have an internal configuration approximately conforming to the external configuration of the item. A limited clearance, most preferably over a significant length may be provided between the internal surface of the further chamber(s) and the external surface of the item. The clearance may be less than 5 mm and more preferably less than 2 mm. The significant length may be greater than 10 cm and more preferably greater than 25 cm. The length may be at least 5 times, more preferably at least 10 times and ideally at least 20 times the minimum clearance presented. 
     The detection chamber may be provided with alternative means for excluding ambient ions. The atmospheric pressure in the detection chamber may be higher than the ambient atmospheric pressure. 
     The means for monitoring ions discharged on the electrode(s) may comprise electrostatic charge monitoring means. Preferably the means for monitoring ions discharged on the electrode(s) comprise current indicating means and more preferably current measuring means. A ground referenced electrode may be provided. 
     Preferably one or more of the electrodes is connected to an electrometer. 
     The item may be a continuous item of more than 5 cm, preferably of more than 10 m, more preferably more than 20 m and potentially 50 m or more in length. 
     The item may be a discrete item such as a pipe, beam (such as an I beam), pole, fuel element, cladding, cable, wire, rail or other elongate or large item or a surface, such as a material traveling on a conveyor. 
     The instrument may be provided with associate means for supporting the item and/or moving the item through the instrument. Such means may be provided on both sides of the instrument. 
     The instrument may be provided with means for supporting it on the item to be monitored. The support means may enable the instrument to be moved along and/or over the item. 
     According to a fourth aspect of the invention we provide a method of monitoring alpha emitting sources on an item, the method comprising introducing the item through an inlet connected to a detecting chamber in an instrument and removing the item through an outlet in the instrument, the detecting chamber defining a detecting volume and being provided with one or more electrodes for discharging ions produced in the detecting volume by the portion of the item in the detecting volume, the method including monitoring ions discharged on the electrode(s). 
     The item may be introduced by moving the item into the detector and/or by moving the detector along the item. 
     The fourth aspect of the invention includes the features, options and possibilities set out elsewhere in this application, including the steps necessary to implement them. 
     It is particularly preferred that the third or fourth aspects of the invention include the various options, features and possibilities set out above for the electrically conducting elements provided with one or more apertures which are provided between the electrode or electrodes and the monitoring location. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 
     FIG. 1 illustrates schematically a prior art alpha detection instrument; 
     FIG. 2 illustrates an instrument according to a first embodiment of the present invention; 
     FIG. 3 illustrates a second embodiment of the invention; 
     FIG. 4 illustrates a third embodiment of an instrument according to the present invention; 
     FIG. 5 illustrates the signal expressed in equivalent alpha response against time for a prior art instrument moved relative to the location being monitored; 
     FIG. 6 indicates the results on an equivalent basis to those of FIG. 5 but for an instrument according to the present invention; 
     FIG. 7 illustrates the results on an equivalent basis to FIGS. 5 and 6 but for an instrument according to the present invention at a higher rate of movement; 
     FIG. 8 illustrates an instrument according to a further embodiment of the present invention; 
     FIG. 9 illustrates a cross-section through FIG. 8 along line AA; and 
     FIG. 10 illustrates an instrument according to a still further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The monitoring of alpha and/or beta and/or gamma emissions from a location or item is of particular significance during decommissioning, material accounting and a variety of other applications. An accurate calculation of the level of contamination present enables the correct decision to be taken in classifying items/locations in terms of grade or the most suitable decontamination process. 
     Alpha determination for locations, such as floors, are known in terms of the type of instrument schematically illustrated in FIG.  1 . 
     The instrument uses the principal that when direct alpha particle detection is not practicable, detection of alpha particles emitted into air from the location can successfully be indirectly monitored. Whilst alpha particles only travel a few centimeters in air and, as a consequence, a detector unit at any distance from the item cannot detect alpha particles directly, during the course of their travel through the air the alpha particles cause ionisation of a significant number of air molecules. These ionized molecules remain in that state for a substantial period of time and this is sufficient to enable them to be attracted/drawn from in proximity with the location to a detector array. 
     Thus in the instrument of FIG. 1 a hood style container  2  is placed over a floor location  4  to be monitored with the edge  3  of the container in contact with the floor. The container is open to the floor  4  and contains a detector array  6  which employs an electric potential V P , from source  7 , so as to attract the charged ions generated by alpha emission from the floor  4  to the plate of the detector array  6 . The current resulting from these ions is detected by an electrometer  8  so as to give a measure of the ion level presence and hence the alpha contamination presence. 
     To obtain an accurate reading it is necessary to place such an instrument at the desired location and then wait for a significant period of time for vibrations arising from the movement to settle down. While such systems function, therefore, they and a variety of other instruments based around this principal suffer problems where the instrument is moved relative to the location being monitored. The instrument and/or the location may actually be moved, but the problem is the same in each case. 
     Whenever a location, in a grounded state, moves relative to the detector array, at an applied potential, capacitance changes occur which effect the instrument&#39;s detection capabilities through the generation of high noise levels. The problem is particularly acute for detectors of the FIG. 1 type; those involving systems in which ions are attracted towards an electrode by the application of an applied potential of opposing polarity. The capacitance changes result in a noise current, whose value is determined by the expression:            I   ~               Q          t         =     V             C          t                       (for  constant     V   )                              
     Since the capacitance of the detector is directly related to the distance from the electrode in the detector to the grounded object, variations in the distance cause a directly proportional noise current. 
     The problem is addressed in the present invention by providing an electrically conducting grid between the electrode of the detector and the item/location. This has the effect that the relevant capacitance for noise purposes becomes that of the electrode and grid system. The spacing of these two components is far more consistent than between the electrode and the object and the variation in capacitance and hence the noise current is greatly reduced as a result. 
     The use of a grid, however, as opposed to a solid plate, ensures that the ion collection, necessary to make the alpha particle level determination, is not significantly hindered. 
     In the embodiment of the invention illustrated in FIG. 2 the instrument  100  is held in close proximity to the location  102  to be monitored. So as to exclude extraneous ions present in the surrounding air as far as possible a limited gap due to skirt  103  is provided. 
     Movement of the item  102  past the instrument  100  or movement of the instrument  100  along the item  102  would cause capacitance problems with the prior art, but in this embodiment a grid  104  is placed inside the volume defined by the instrument  100 . The grid  104  is placed between the item  102  and the detector plate  108 . The grid  104  is electrically connected to the detector plate  108 . 
     The detector plate  108  is electrically connected to provide an applied potential V P +V G  due to source  114  and source  116  through a floating input electrometer  112 . The grid  104  is also electrically connected to provide a lower applied potential V G , due to source  116  alone, than that for the detector plate  108 . The item  102  is grounded. 
     Alpha emitting sources present on the item  102  emit alpha particles which in turn stop within the volume  105  below the grid  104  and generate ions as they pass. The applied potential electrostatically attracts the ions through the grid  104 , into volume  106  and hence to the detector plate  108  where they give rise to a current. The current is in turn detected by the electrometer  112  and the value is used to calculate the level of alpha contamination present. 
     The grid  104  in this embodiment is positioned at a distance from the item  102  greater than the penetration distance in air of alpha particles, generally greater than 5 cm. In this way no alpha particles, only ions, reach the grid  104 . 
     In the second embodiment of the invention, illustrated in FIG. 3 a similar arrangement of instrument  100 , item  102 , volume  106 , detector plate  108  and applied potential V P  from source  114  for the detector plate, together with an electrometer  112  are employed. In this embodiment, however, the grid  104  is positioned in close proximity to the surface of the item  102  under consideration. Additionally the grid  104  is itself grounded. A system provided in this way allows a ground reference electrometer to be used. 
     Alpha emission sources on the item  102  emit alpha particles which pass through the grounded grid  104  and enter the volume  106  between the grid  104  and detector plate  108  where they stop and cause the ionisation which is to be detected. 
     The item  102  could be a wall or floor of a building or construction or a piece of land or soil. The item  102  under consideration in such systems could equally well be a pipe, I-beam, pump or other piece of equipment, as described in more detail below. 
     The third embodiment of the invention illustrated in FIG. 4, as is possible for all of the embodiments described above, has been modified to provide a background canceling instrument. 
     The instrument  302  consists of an enclosure  305  defining a volume  307  which is in proximity with the surface  303  to be monitored. Limited clearance or other means are employed to exclude as far as possible extraneous ions present in the outside air from volume  307 . 
     The volume  307  contains a front detection plate  308  and a front grid  304 . The plate  308  is at a potential V P +V G  due to sources  334 ,  336  and the grid  304  is at a lower potential, potential V G  due to source  336  alone. 
     The back volume  325  of the instrument is similarly provided with a back detection plate  330  and a back grid  332 . Equivalent voltages to V P +V G  and V G  are applied to the back plate  33  and the back grid  332  respectively. A common potential source  336  is used in this regard for the grids  304 ,  332 , together with a common potential source  334  for the detector plates  308 ,  330  and the ground plate  338 . 
     Between the two detection plates  308 ,  330  a guard plate  338 , to exclude location ions from the background count, is provided mounted on a mechanical support  339 . 
     The plates  308 ,  338 ,  330  and grids  304 ,  332 , are spaced by insulators  370 . 
     Each detector plate  308 ,  330  is connected to an electrometer  312 ,  340  respectively, and a current reading for each detector is obtained as a result. The background, electrometer  340 , can be subtracted from the item+background value, electrometer  312 , to give an absolute item value using software. 
     The embodiments of the invention described above address alpha particle determinations but it is perfectly possible to incorporate gamma and/or beta detectors in such an instrument alternatively or additionally. The gamma detectors may be of the thick plastic scintillator type, sodium iodide type or semi-conductor type. Beta detection can be undertaken directly or alternatively by calculation from the gamma emissions recorded. 
     To demonstrate the effectiveness of the technique presented by the present invention a plate detector in a hood container was positioned at a spacing of 55 mm from a surface, without a grid, and one corner of the surface was moved at a rate of 0.5 mms −1 . 
     The results obtained for the current signal, and shown in FIG. 5, give a calculated limit of detection of 210 Bq due to the noise present in the signal and generated by the movement. 
     The apparatus was then provided with an earthed grid of 1 cm squares, between the detector plate and the source. A test with a 55 mm spacing from the surface and a higher rate of movement, 1 mms −1 , gave the results of FIG.  6 . The substantial removal of noise gave a limit of detection of 18 Bq. 
     A further test with a far higher level of movement, 20 mms −1 , was performed (again using a gridded detector) and produced the signal results set out in FIG.  7 . Even for this level of movement the limit of detection, 149 Bq, was still significantly lower than for the ungridded detector even at low movement. 
     As previously discussed, long range alpha detection techniques based on detecting ions have previously been concerned with enclosures for areas or items to be monitored. This renders them suitable for relatively small or flat items, but prevents the technique being suitable for analyzing longer items which cannot practically be enclosed within the detection instrument in the manner of FIG. 1, for instance. 
     To successfully monitor long items, including I-section beams and other elongate items, a detector of the type embodying the invention is preferred, as illustrated in FIG.  8 . Such instruments preferably include the grid between the item being monitored and detector to reduce variation in capacitance and hence reduce noise in the detector signal. 
     The instrument  700  according to the first embodiment of the invention comprises an elongate central portion  702  of cylindrical cross-section. The cylinder  702  is provided with end faces  704  which define apertures  706  lying on the axis of the cylinder  702 . The apertures  706  allow the passage of an elongate item to be measured through the cylinder  702 . 
     The cylinder  702  is itself provided with a cylindrical electrode  708  spaced, and thereby electrically insulated, from the cylinder wall  702 , see FIG.  9 . The electrode has an known electric potential applied to and as a consequence ions generated in the cylinder  702  are drawn away from the item to the electrode  708 . The ions generated by alpha emission from the article and attracted to the electrode  708  give rise to a current which can be measured using an electrometer, not shown. 
     This monitoring technique is effective even during the continuous movement of the item through the instrument. 
     Capacitance of variations due to the relative movement of the detector electrode and item can be mitigated to a very large degree by including a grid, not shown, between the electrode  708  and the item, shown in cross hatching. The grid may have a corresponding profile to the electrode  708  and/or to the item. 
     The above embodiment employs a cylindrical electrode to monitor, preferably, cylindrical items, but the electrode array can be configured to the particular type of item under consideration. Thus an I beam cross-section could be monitored using electrodes spaced from the end surfaces and closer together electrodes space from the intervening web portion. Uniform spacing from the item is preferred. 
     As ambient air includes extraneous ions of its own it is desirable to exclude these from detection at the electrode  708  to give a truer measurement of the alpha particle generated ionisation. The detection currents employed are around 10 −12 A and as a consequence easily distorted by extraneous ions. 
     In some prior art systems featuring closed containers this was readily achievable by a filter which closed off the air flow route into the container. This option, however, is not viable where the elongate item exceeds the length of the measuring chamber  702  thus rendering filters over the air inlet impractical. 
     To physically counteract this problem the present invention provides for a further body portion  710  on each end of the cylinder  702 . These body portions  710  extend from the end faces  704  of the cylinder  702  for a substantial length to ends  712  of their own. The aperture  713  through the body portion  710 , which allows access for the elongate item to be measured, takes a form closely configured to that of the item itself, for instance a pipe (the item) passing down a slightly larger pipe (the inlet). This structure gives minimal air flow and hence minimal flow of ions into the detection chamber. 
     In an alternative form, not shown, the ambient ions can be excluded by a slight positive pressure within the detecting chamber and/or further chambers, which promote air flow out of the instrument. Other forms, not shown, exclude ambient ions using an extended restricted clearance between the item/location and the inlet and outlet passage, for instance flexible air excluders, such as brushes, may be provided. 
     In a still further alternative form the “background” ions present in the air around the unit can be determined in advance of the item&#39;s passage through the instrument and/or afterwards to provide a base count which can be deducted to give a measurement of the ions arising from alpha particle emission. Such background determinations could be made with a clean, uncontaminated item corresponding to those to be analyzed. 
     In the above mentioned embodiments continuous movement of the item/location through the instrument is preferred. 
     Whilst the system has been discussed in this embodiment in relation to a fixed instrument through which the elongate item is moved, for instance on a conveying roller bed, the instrument is equally applicable to the embodiment of FIG. 10 in which the instrument itself moves. 
     In this embodiment an elongate item  200 , for instance a railway rail, is being analyzed by an instrument  202  formed of a first measuring cylinder portion  204  and air excluding body portions  206 . As with the first embodiment of the invention the measuring cylinder  204  is provided with a cylindrical electrode  208  to detect ions generated by alpha particle emission whilst within the cylinder  204 . 
     The entire instrument is supported on the item to be monitored  200  by wheels  212  which can be driven to advance the unit along the item  200 . Signals from the instrument  202  relating to its position and the level of alpha detection are sent to a remote monitoring unit, not shown. 
     Once again, this embodiment of the invention may also include a grid between the electrode  208  and the item  200  so as to minimize capacitance variation effects on the detecting currents. 
     The embodiments of the invention described above address alpha particle determinations but it is perfectly possible to incorporate gamma and/or beta detectors in such an instrument alternatively or additionally. Beta detection can be undertaken directly or alternatively by calculation from the gamma emissions recorded. 
     This type of monitoring renders full analysis of long items (10 meters+) possible, whilst avoiding the cost and practical difficulties of enclosing large items, and facilitating continuous fed of the item through the monitor. The instrument is suitable for items such as cables, wires, beams, pipes, rails, indeed any item or location having a conductivity capable of carrying the currents involved. 
     The instrument also provides spatial information as to the location and/or spread of any alpha emitting sources present.