Introduction and a few questions

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wbelisle
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Introduction and a few questions

Postby wbelisle » Sat Jan 14, 2017 5:40 pm

Hello everyone,

My name is Bill Belisle and I live in the San Francisco Bay Area. Formerly, I worked at a company that manufactured linear accelerators for medical oncology use, which is how I developed an interest in radiation and gamma spectroscopy. I'm an amateur machinist, dabble in electronics, and am currently in the process of building a scintillator to attach to my GS-1100 Pro.

I have a used NaI(T) crystal that is 32-mm wide and 122-mm long (I think it was used in oil-well logging equipment). I've mated the crystal with a Hamamatsu R9420-20 photomultiplier tube (PMT) that I got from ebay. I wrapped mu-metal (also from ebay) around the PMT.

I had a few questions that I was hoping someone could answer:
1) Should the mu-metal be grounded? I thought it might act like a capacitor if it wasn't grounded so I soldered a wire from it to a ground lug on the BNC connector.
2) The datasheet for the PMT says it runs at 1400 volts. Should I assume that is the correct voltage to run at for gamma spectroscopy? If not, how do I determine the optimum voltage?
3) I am able to adjust the voltage on my GS-1100 up to 1400 volts, but that is beyond the 1100 volt specification. Is it okay to run at that voltage?

I have attached a couple of photos of the probe. The body is stainless steel, the end cap with the BNC and SHV connectors is machined aluminum, and the black piece is 3-D printed PLA.

Thanks in advance for any help you can provide. I'm sure I'll have many other questions in the future.

Regards,
Bill Belisle
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Bill Belisle
San Francisco, California

luuk
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Joined: Fri May 01, 2015 9:00 pm
Location: the Netherlands
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Re: Introduction and a few questions

Postby luuk » Sat Jan 14, 2017 7:02 pm

Hi Bill,

Yes you must always ground your mu-metal shield.
The specs from you pmt are maximum values so it may work perfectly on e.g. 500 V for your purpose it all depends if the pmt is still good.
Start with 500V and use a “standard” source so any source you have preferable an energy between 600 and 1500 KeV.
The calibration all depends on what you want to measure, most time up to 1600KeVwill do or maximum 2500KeV adjust the high voltage so that you have the maximum energy that you want to measure is at the end of you spectrum.


Luuk

wbelisle
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Joined: Thu Oct 13, 2016 3:59 pm
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Re: Introduction and a few questions

Postby wbelisle » Thu Feb 02, 2017 2:20 pm

The scintillation probe I am using runs on a positive supply voltage. The pulse I get is negative. The negative pulse is not a problem because I set my PRA height threshold to a negative number. I am curious, however, because it seems that most scintillator probes send a positive pulse (perhaps I am mistaken). Is there an inverter or a buffer of some sort inside the scintillator probe? If so, is there any advantage for me to add one?
Bill Belisle
San Francisco, California

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Steven Sesselmann
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Joined: Mon Apr 27, 2015 11:40 am
Location: Sydney
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Re: Introduction and a few questions

Postby Steven Sesselmann » Thu Feb 02, 2017 2:31 pm

Bill,

The pulse you see in PRA can have any polarity.

The pulse from the detector is almost always negative, remember a PMT is an electron multiplier, so when a bunch of negatively charged electrons hit the anode it's going to give a negative pulse through the cable, but most amplifier circuits are inverting, so the pulse will flip each time it goes through an opamp.

Inside the GS-1100A or PRO, the pulse is amplified through a single stage opamp, this flips the pulse once, so the output is positive, but inside your sound card it might go through another single or double opamp and be flipped another once or twice.

The simple solution is to run PRA and look at the pulse through the audio input window.

Steven
Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | rephopper.com | beejewel.com.au |


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