Ion monitoring

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.

What is claimed is:  
     
         1 . An instrument for detecting ions originating from a location/item, the instrument comprising: 
 a body portion at least partially bounding a chamber, the chamber being configured to at least partially receive or cover the location/item;    at least one electrode at least partially disposed within the chamber, the at least one electrode being adapted to receive an electrical potential relative to the location/item,    at least one electrically conducting element provided with one or more apertures, the at least one electrically conducting element being disposed between the at least one electrode and the location/item; and    means for monitoring ions discharged on the at least one electrode.    
     
     
         2 . An instrument according to  claim 1  in which the at least one electrically conducting element is provided with greater than 80% of its effective area as apertures.  
     
     
         3 . An instrument according to  claim 1  in which the at least one electrically conducting element is in the form of a grid.  
     
     
         4 . An instrument according to  claim 1  in which the at least one electrically conducting element is at a potential different to the location/item.  
     
     
         5 . An instrument according to  claim 1  in which the at least one electrically conducting element is earthed.  
     
     
         6 . An instrument according to  claim 1  in which the configuration of the at least one electrically conducting element mirrors that of the at least one electrode.  
     
     
         7 . An instrument according to  claim 1  wherein the body portion completely encloses the chamber and is provided with a closeable opening for introducing the location/item to be monitored.  
     
     
         8 . An instrument according to  claim 1  wherein the body portion comprises a hood so that the chamber has an opening exposed to the environment.  
     
     
         9 . An instrument according to  claim 8  in which the opening has a perimeter that is provided with a laterally extending flange.  
     
     
         10 . An instrument according to  claim 1  further comprising an additional detector to detect background ion levels, the additional detector comprising a second electrode and a second apertured element, the second electrode being separated from the at least one electrode by a guard plate.  
     
     
         11 . A method of detecting ions originating from a location/item, the method comprising: 
 positioning an ion detecting instrument adjacent to a location/item, the location/item emitting radiation that produces ions in the surrounding environment, the ion detecting instrument comprising: 
 at least one electrode; and  
 at least one electrically conducting element having at least one aperture extending therethrough, the at least one electrically conducting element being disposed between the at least one electrode and the location/item;  
   applying an electrical potential to the at least one electrode relative to the location/item so that at least some of the ions produced by the radiation travel to the at least one electrode by passing through the at least one aperture of the electrically conducting element; and    monitoring the discharge of ions on the at least one electrode.    
     
     
         12 . A method according to  claim 11  further comprising moving the location/item relative to the instrument.  
     
     
         13 . A method according to  claim 11  further comprising moving the instrument relative to the location/item.  
     
     
         14 . An instrument for monitoring alpha emitting sources on an item, the instrument comprising: 
 a housing bounding a detecting chamber, the housing comprising an inlet through which an item can be introduced into the detecting chamber and an outlet through which the item can be removed from the detecting chamber;    at least one electrode disposed within the detecting chamber, the at least one electrode being configured to collect ions produced in the detecting chamber by the item; and    means for monitoring ions discharged on the at least one electrode.    
     
     
         15 . An instrument according to  claim 14  in which a single electrode disposed within the detection chamber surrounds at least a portion of the item.  
     
     
         16 . An instrument according to  claim 14  in which the at least one electrode is configured to the cross-sectional profile of the item being monitored and the axis is aligned with the axis of the item and/or instrument.  
     
     
         17 . An instrument according to  claim 14  in which the inlet and/or outlet leads to a further chamber externally provided to the detecting chamber, the further chamber(s) being provided with an opening to the surrounding environment.  
     
     
         18 . An instrument according to  claim 17  in which the inlet and/or outlet further chambers have an internal configuration approximately conforming to the external configuration of the item to define a limited clearance between the internal surface of the further chamber(s) and the external surface of the item.  
     
     
         19 . An instrument according to  claim 14  in which detection chamber is higher than the ambient atmospheric pressure.  
     
     
         20 . 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).  
     
     
         21 . A method according to  claim 20  in which the item is introduced by moving the item into the detector.  
     
     
         22 . A method according to  claim 20  in which the item is introduced by moving the detector along the item.  
     
     
         23 . An instrument for detecting ions originating from a monitored location, the instrument comprising: 
 a body portion and one or more electrodes at an electrical potential relative to the monitored location;    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;    means for monitoring ions discharged on the electrode(s); and    an additional detector provided in the instrument to detect background ion levels, the additional detector comprising an electrode and an apertured element, the additional detector electrode being separated from the electrode by a guard plate.