Low Level Gamma Radiation (2023 Update)

The capacitor will have high voltage on it. This could destroy any electroncs that inadvertently got connected badly.

You don't need to couple to the anode. Just take the signal off the cathode like the old cathode followers of yesteryear.

There's no signal at the cathode to speak of--almost all the anode current comes in via the dynodes.

Cheers

Phil Hobbs

A PMT cathode is typically a multialkali evaporated coating on the end of the tube, has significant stray capacitance, and the signal there is weak (un-multiplied, no benefit from the PMT gain).

[...]

The CCFL inverter I got from Amazon runs at 40KHz.

A simple half-wave rectifier running at 40 KHz with a 1nF cap and 9e6 Ohm load will produce about 2V sawtooth ripple.

This is about 2/800 = 0.0025 * 100 = 0.25% ripple.

That should be good enough.

Totally. You can probably use one of the HV caps that came with it to couple the anode pulses to the outside world.

Cheers

Phil Hobbs

That is a problem. The RH Electronics MCA expects positive pulses. These won't come from the anode.

I shall write them and ask where they get those positive pulses.

Right. I got derailed by the positive pulses from the RH Electronics MCA.

Got to find out how they do that.

A transformer could invert pulses and take out the HV and the HV ripple. You'd need to take out ps ripple for pulse height analysis.

Probably from a TIA.

One approach is to use an old-fashioned low-gain MMIC--most are basically Darlingtons with resistive feedback, so they invert.

Cheers

Phil Hobbs

Never thought of doing it like this but sounds reasonable. OTOH for PHA with a scintillator there is not much energy resolution to speak of so just about any shaping method you come up with will do.

[...]

I begin to suspect a simple inverter. This shows the schematic of the Theremino about 1/3 of the way down. 100 Meg feedback to a flyback HV generator, 2 Meg PMT load, grounded cathode, pulse-shaping network to a grounded emitter inverter. No concern about linearity. One thing I don't understand is a 1N4148 connected betwen the base and ground to catch negative pulses. I wonder if this is a cheap log converter:

I think I'll run mine with a negative cathode to avoid mistakes wiping out the preamplifier. It's not going to be in operation long enough to worry about ion migration. Then find out how RH inverts their pulses.

Looks like it's for short circuit protection. A spark across the output terminals could be quite unpleasant for T3 otherwise.

Cheers

Phil Hobbs

[...]

That is precisely why I do not want to run mine with 800 Volts on the anode.

One could also hang an opamp on the anode to get some gain before AC coupling down to ground. A clever circuit would subtract out the power supply ripple up there, or couple into a grounded diffamp. There are some cheap, low-capacitance dc/dc converters to power the opamp.

T3 looks well protected to me. The problem is that it should be Q3. T means transformer.

The diode is there to prevent the base-emitter diode from zenering, and also to protect from large negative pulses.

No need to do it that complicated, there just is no energy resolution to worry about too much. They measure it in %, FWHM at the 661.6 137Cs peak divided by 661.6. NaI detectors can go down to around 6%, usually 8 or even 10 is still acceptable. This is 40keV FWHM at best, out of a spectrum well within 2 MeV, more like 1.5. People often measure

1k spectra with NaI where clearly 128 channels would be plenty. (Compare that to something like 1.3keV FWHM for the 137Cs line with a Ge detector, with these you need 8 or 16k to get the full picture).

I did a PMT circuit for photon counting a few years ago for a TLD reader I designed,

(no PHA). Since the PMT glass hangs in the air there was no issue biasing it negatively and coupling the anode into one of the modern day fast opamps ADI make, works pretty well - throughput being around 22 million photons/second.

But my first MCA card for NaI - designed >30 years ago, the so called TISA card - was expecting only detectors with negative bias. I had no idea what detectors could be etc., I just designed the MCA card - and it had to deliver up to 3 mA as the detectors it must have been meant for had a preamp bleeding the HV for power.... (it could do up to 1200V/3mA, programmable).

More recently I had a customer who wanted to connect an NaI detector to one of our netMCA-s. The detector had just one BNC, for both +HV in and signal out. So I had to build a tiny splitter box for it... Clearly one gets one more pole in such a configuration but it is too slow and is practically immaterial. Worked with the standard netMCA HV configuration (up to 5kV for Ge), though the HV had to be set to something like 2kV to compensate for the drop over around 30 megaohm in the output filter (Ge detectors consume practically no current).

[..]

Got a reply from RH. They don't. You do:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Hello Mike,

To flip the pulse polarity you can use a classic inverting OP-AMP circuit.

The chip op-amp you will use has to be able to process fast PMT pulses. Usually 10MHz chip is enough in most cases with NaI(Tl) probe.

Please note, the MCA input specifications require for input signal amplitude of 0mV-3300mV

Thank you Alex ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now I have to figure out the input impedance of the inverter. I'll have to fire up the PMT and see what amplitude the pulses are.

Fortunately, my Radium order just arrived so I will have some signals.

Unfortunately the decay steps are alpha and beta. The scintillator is gamma, so I don't think Ra will help, but it drives my Radiacode crazy. First time I have heard the alarm. No wonder it made the girls who painted it on their bodies glow in the dark.

I have some volcanic rock on order. I'll have to bug John and Dave and find out where they got their sources. This stuff is tough to find. They think you want to make a bomb or something.

Well, the PMT has a gain of a million already. ;)

BTW the gain of a PMT goes as some high power (like 8 or 11 or something) of the bias voltage.

Cheers

Phil Hobbs