|Adjustment of the Phillips 715 Constant Fraction Discriminator Threshold Value
Occasionally changes to the MWPC detector require adjustments to the 715CFD threshold value. This is done with a voltmeter capable of reading milliVolts and a small flathead screw driver.
The attached page shows the front panel of the 715CFD. The 715CFD has five independent channels, four of which are used with the MWPC.
Each channel has a gold contact surrounded by a white insulator marked “Threshold”. Beside the contact is a hole into the unit that will accept a small flathead screwdriver.
Connect the negative lead from the voltmeter to the “Ground” terminal of the NIM Crate. Touch the positive lead to the Threshold contact. The voltmeter should read between -250mV and -2V. These values correspond to discrimination thresholds of between -25mV and -200mV due to the factor of 10 scaling inside the unit.
The preferred value of the discrimination threshold will be noted on a separate settings sheet. On this detector, it is -110.0mV
Insert a small flathead screwdriver into the hold and gently adjust the 10-turn potentiometer until the voltmeter reads the desired value.
Repeat this for all of the 715CFD channels.
Improper setting of the discrimination threshold can lead to too much noise (set too low) or lost counts (set too high).