ASI Model 1950

First, make sure that the HV is toggled to on via the switch on the back, and that the P10 gas is flowing into the chamber. Without those two things no counts will be detected.

If those are both in a satisfactory state, then there is likely a problem with either the HV circuitry or the internal structure of the counter itself. Unfortunately, XIA is no longer able to manufacture new circuitry or repair existing instruments. We encourage anyone who needs a new instrument for alpha particle measurements to consider the UltraLo-1800.

In order to calibrate a Model 1950 you simply need access to a alpha source with an activity of about 2 cps (you can use more active sources but risk issues with dead time). You place that source on the tray, adjust the height to be 1-2mm from the Mylar window (being very careful to not touch the window and damage it, as we can no longer repair broken windows), and count it for enough time for an accurate determination of the count rate to be made. (At XIA we do 10 windows of 10 minutes each, but this will vary with count rate.) You want the resulting efficiency to be 80-84%. If your efficiency is outside that range, you’ll need to adjust the Gain potentiometer exposed when you remove the cover on the electronics box (warning: high voltage inside).

For more detail on this procedure, see section 6 of the User’s Manual.

Determining the background of a Model 1950 is straightforward. Simply remove the tray and perform a count. The resulting CPH value will be the background rate. For more detail on this procedure see section 5 of the User’s Manual.

If the sample tray has become contaminated or soiled and needs to be cleaned, then please follow the instructions in the Technical Note titled Sample Tray Re-surfacing.