Repairing an alpha beta counter

["Gary H. Kramer" <gkramer@hpb.hwc.ca>]

Greetings

We have a old Nuclear Chicago alpha-beta counter that has recently 
started to give us problems.  Problems that we seem unable to resolve.

The symptoms began just after a calibration check, and we thought that 
the elevated background (up to 47 cpm, should be 2 cpm) was due to a 
contamination problem.

The counter was thoroughly cleaned, the window was changed, the 
detector cavity was cleaned, electronic boards were swapped, anode wire 
removed and no counts observed, gas lines flushed with methanol (and 
purged for several days before recounting).  All resistances of the 
detector components and high voltage circuit components matched 
manufacturer's specifications.

Finally we rechecked the plateau, background was still high, and found 
that the instrument does not plateau.  The anode wire was cleaned and the 
plateau re-run.  The counts were all substantially increased at all 
voltages (by approximately 2) but still no sign of a plateau. 

Any help in diagnosing this problem (actually two problems:  high 
background and no plateau) would be greatly appreciated.

Regards
	Gary

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| H     H   M       M   L	|  Gary H. Kramer			      |
| H     H   MM     MM   L	|  Head, Human Monitoring Laboratory	      |
| H     H   M M   M M   L	|  The Canadian National Calibration Reference|
| H     H   M  M M  M   L	|  Centre for In-Vivo Monitoring	      |
| HHHHHHH   M   M   M   L	|  775 Brookfield Road, Ottawa, Ontario	      |
| H     H   M       M   L	|  CANADA K1A 1C1			      |
| H     H   M       M   L	|  Tel: 613 954 6668;  Fax 613 957 1089	      |
| H     H   M       M   LLLLLLL |  GKRAMER@HPB.HWC.CA			      |
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