34-Basic Radiation Detection: Detector lab 3


Let’s move on to our next detector, which
is a gamma detector. This is a sodium iodide detector, which is
a gamma detector. This is a sodium iodide scintillation detector
system. This particular one has a built-in voltage
supply, and without further ado, let’s begin to set it up. The first thing we need to do is to hook up
the connector that supplies power to the tube. This goes here, and there’s several ways
to do this, so I could just go around back and hook it up, but you won’t see very much. So I can pull this out of the NIM-BIN, and
feed this through, and it hooks up in the back right here, and this feeds the preamps. This is traditionally how it’s done. The preamp that I used before, the power to
it was fed the same way. There are little slots that this fits in,
which I will get in there sooner or later. And now, I can turn on the power
to the sodium iodide detector. Let’s move over here and look at what the
pulses look like coming out of the detector. Okay, so we can take the signal from the sodium
iodide. We can go into our friend, the o-scope, and
there they are. We can see the gammas there. They are relatively slow pulses. This is set on 200 microseconds, so you can
see it takes quite a while for them to come down. Let’s see. Can I slow it down? Nope. So from here, we’re going to take our signal
and go into our amplifier, and then we’re going to take out of our amplifier, and let’s
see what it looks like coming out of the amplifier. Oh my gosh, they’re fast! Okay, and they’re bigger. So I want to go up to a couple of volts. Let’s change the level. There they are. We’re going to spread them out, and there’s
our gamma signals. It looks a whole lot like the same signal
that came out of our BF3 tube, and that is kind of the joy of NIM-BINs. They all look the same when it’s said and
done. So now let’s feed this into our MCA, and
we’ll begin to calibrate the sodium iodide detector. I’m going to unhook this. I’m going to hook our MCA up. Okay, we’ve got it hooked up. We’ve got it headed to our ADC .. uh … our
MCA, and I have a cobalt-60 check source here, so we have some gammas to play with, and let’s
start the count. It’s going to scan for a second. It will now come up. There’s cobalt-60. This is 1.3MeV, 1.17MeV, and you can watch
them grow in. So there’s two points, if I wanted to do
an energy calibration. Here I have cesium-137, so let’s add it
near here. Ah, there’s cesium-137. And so now I could find the channel, the centroid
of each of these peaks, and I could do a crude, or even fairly good, energy calibration. To do an absolute calibration, I would have
to have a much more fixed geometry than this, and it would have to be like whatever it is
I’m trying to measure. So this shows you kind of how it works. Now, let’s go down and use a high purity
germanium detector, and we’ll make these peaks so narrow they look like a single line
coming up, just about. And the best and the gold standard equipment,
of course, is HPGe. We’re going to go downstairs into our counting
lab, and I’ll see you down there.

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