G
George Herold
So someone was rewriting our manual for using a PMT.
(Used in photon counting mode.)
There was this confused procedure to set the discriminator. (The
threshold level on the comparator before counter.) So I said let’s
just take the data, and do it right. I took a PMT off the shelf that
had a fair amount of dark current (3nA at 1000V) We looked at count
rate with a light on versus the dark count rate at various
discriminator setting and PMT voltages.
Here’s a plot of the data,
http://bayimg.com/CAlfEaaEb
I must admit to being surprised. I’d always heard/ been told to
operate the PMT near the voltage recommended by the manufacturer
(1000 volts in this case.)
I said to my boss that the best way to find the largest signal to
noise is to set the discriminator to a tad above zero, then reduce the
voltage till the count rate drops to a bit more than 1/2 it maximum
value. I did this the voltage was 537 and the light on/ dark ratio
was 455...more than 5 times better than at 1000V!
George H.
(Used in photon counting mode.)
There was this confused procedure to set the discriminator. (The
threshold level on the comparator before counter.) So I said let’s
just take the data, and do it right. I took a PMT off the shelf that
had a fair amount of dark current (3nA at 1000V) We looked at count
rate with a light on versus the dark count rate at various
discriminator setting and PMT voltages.
Here’s a plot of the data,
http://bayimg.com/CAlfEaaEb
I must admit to being surprised. I’d always heard/ been told to
operate the PMT near the voltage recommended by the manufacturer
(1000 volts in this case.)
I said to my boss that the best way to find the largest signal to
noise is to set the discriminator to a tad above zero, then reduce the
voltage till the count rate drops to a bit more than 1/2 it maximum
value. I did this the voltage was 537 and the light on/ dark ratio
was 455...more than 5 times better than at 1000V!
George H.