PMT/Scint. divider topology differences

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ChristofferBraestrup
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PMT/Scint. divider topology differences

Post by ChristofferBraestrup »

Going though all the classic texts I have seen on PMT divider networks, and reference design for commercial PMT bases, there is one thing that is never fully made clear. I'd like to hear people's take on it. Mostly specific to scintillation probes but applicable to all fields I guess.

-Some probes has one connector that carries both bias and signal. Sometimes this is connected directly to the anode without anode resistor
-Some has separate HV and sig connectors, having the correct anode bias resistor, and signal coupled out directly for the anode.
-Some additionally has a signal out from the last dynode of the chain


One connector is obviously ideal for portable detectors and handheld instruments, but spectroscopy-wise, signal speed time etc. what is the benefit of the 2-connector layout? Is it worth it to convert one's probes?

The Dynode out, as seen on many Ortec bases is inverted from the anode signal (since electrons are ripped off instead of being deposited), and I can see they're usually used for fast timing applications, such as coincidence but why would dynode pulses be faster?

See attached schematic for some examples, based on a random schematic I drew long ago. Don't worry about component values, they're specific to a photonis PMT.
PMT divider differences.jpg
Frank Sanns
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Re: PMT/Scint. divider topology differences

Post by Frank Sanns »

They are essentially equivalent circuits. The location of the capacitive coupled pulse output is the only meaningful difference. It is present on both configurations but in the case of the two lead setup, it comes after the split.

Capacitors do not pass DC but they do pass a pulse. The key in both of these configurations is to leave the HV intact while sensing a pulse that appears in the circuit. In this case it is the photon induced avalanche of electrons at the end of the dynode string.
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ChristofferBraestrup
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Re: PMT/Scint. divider topology differences

Post by ChristofferBraestrup »

That's true, and for just counting applications with a NaI(Tl) scintillator you'd probably never see the difference.

What I'm wondering is if there is a timing or impedance difference between the two configs, if we assume the resistor is the same value.
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Joe Gayo
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Re: PMT/Scint. divider topology differences

Post by Joe Gayo »

I see the difference as noise immunity. Separating the bias and signal allows high impedance lines to be short parasitics to be minimized.
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Richard Hull
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Re: PMT/Scint. divider topology differences

Post by Richard Hull »

I have always made up my PMT devices as separate. I have several Ludlum scint. probes that are single and they attach to walk-around survey meters. The single connection is fine for that purpose. For Gamma spec work, my 3 shiny chromed Bicrons and others like Harshaw always separate the HV from the signal. It depends on what you are doing if you are rolling your own scintillators. I'm with Joe, its noise. No one wants a kilovolt on a signal line no matter how quiet you think your HV line is thought to be or how pretty you think your coupling cap might be. Don't let them run through the same cable if you are in control.

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John Futter
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Re: PMT/Scint. divider topology differences

Post by John Futter »

You have all the answers

but yes single outputs are usually used for counting
and the dual ( less noise etc ) is used for pulse height info ie to go into an MCA
ChristofferBraestrup
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Re: PMT/Scint. divider topology differences

Post by ChristofferBraestrup »

That sounds very reasonable, I'm buying that explanation!

I guess the main reason for the single cable thing is that you get a detector that's completely anologous to a GM or proportional probe, so a ratemeter/survey meter can cater all three.

It might be worth converting my single connector probes if they're for stationary use.

thanks!
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