Needing more than a spark test?

[graham-xrf]

I am thinking that if we later want to cool the detector, the right angle board attachment would be better? I am not sure how important cooling will be.
If you go with PLA and plaster, (like lost wax) you would burn out the PLA in a kiln or oven and then pour in the lead. It is not like lost foam casting where the foam stays in place for the pour. I think any of the approaches would be fine: lead bucket shield embedded in plastic, the entire front end cast in lead, or machining this from a lead cylinder. Having no experience machining lead, I personally might try casting.

OK - Let's look through this. Is it worth cooling the diode?
So far as I can tell, for photons, the PIN diode will respond to incoming x-ray photons as normal, regardless the temperature.
Then, there is a temperature-related noise current, that being the reverse bias dark current.

NOTE: That is a DC current on which the pulse is superimposed, and the pulse is taken off (in my circuit), with a carefully chosen capacitor.
There is wideband white noise associated with that current, from temperature jangling about the molecules in the diode material. It is not the same value as the DC dark current

The noise current for the PIN diode is listed as 6.1e-14/√Hz, making it about 20nA for a pulse with 300kHz content waveshape.
The temperature coefficient at -12V bias is 13%/K change of dark current. Again, not the same as wideband noise

The above must be worst case of something, because it is at odds with the graph for 23 °C


Dark current is about 2.5nA at 10V reverse bias. On a coldish day, say 15C, that compounded 13% drop with every degree brings the dark current down to 0.71nA. Still, it's a DC current, not a noise racket, although what racket there is would be related to, and driven by it. For all this, the noise figure of the semiconductor first stage in the TIA is below all that

OK - it might be worth having, and we can experiment with some ice, but building in active cooling, like Peltier or something would cost more than the whole thing, and at about 2% efficiency, needs it all radiated off somewhere else, even if heat pipes were used. Maybe a "cold finger" thingy, I don't know. There are two physical builds anyway, so you get to choose.

 

[rwm]

So my lead cup could be useful if I machine a little off the inside or maybe file a hair off the corner of the detector housing.
I was thinking of a simplified shield like this:

The sources are as close to the front of the ring as possible without breaking the plane. The ring may be too tall but could be shortened on the bottom. The disc holding the ring could be plastic or lead sheet. This simple geometry would be easy to build without lost wax casting or extensive machining.
Graham- Might be worth experimenting with diode cooling way later in the process after we have some viable data to benchmark against?

 

[homebrewed]

OK - Let's look through this. Is it worth cooling the diode?
So far as I can tell, for photons, the PIN diode will respond to incoming x-ray photons as normal, regardless the temperature.
Then, there is a temperature-related noise current, that being the reverse bias dark current.

NOTE: That is a DC current on which the pulse is superimposed, and the pulse is taken off (in my circuit), with a carefully chosen capacitor.
There is wideband white noise associated with that current, from temperature jangling about the molecules in the diode material. It is not the same value as the DC dark current

The noise current for the PIN diode is listed as 6.1e-14/√Hz, making it about 20nA for a pulse with 300kHz content waveshape.
The temperature coefficient at -12V bias is 13%/K change of dark current. Again, not the same as wideband noise

The above must be worst case of something, because it is at odds with the graph for 23 °C

View attachment 433432
Dark current is about 2.5nA at 10V reverse bias. On a coldish day, say 15C, that compounded 13% drop with every degree brings the dark current down to 0.71nA. Still, it's a DC current, not a noise racket, although what racket there is would be related to, and driven by it. For all this, the noise figure of the semiconductor first stage in the TIA is below all that

OK - it might be worth having, and we can experiment with some ice, but building in active cooling, like Peltier or something would cost more than the whole thing, and at about 2% efficiency, needs it all radiated off somewhere else, even if heat pipes were used. Maybe a "cold finger" thingy, I don't know. There are two physical builds anyway, so you get to choose.

Most of the current flowing through a reverse-biased junction is thermally generated so it definitely is not noise-free. So I would expect a drop in the noise current. It might be worthwhile to play around with active cooling, at least as a part of the development process.

Our standard diode equation sez: Id = Is(exp(qVd/kT) - 1). For negative Vd, the exponent rapidly goes to zero so for Vd << q/kT, Id can be accurately approximated as : Is. So what IS Is? It is not a constant, it contains yet another exponent that has a temperature relationship, based on the bandgap of our semiconductor in question. In addition to temperature, Is has an area dependency -- makes sense, we expect a really big diode to have a larger reverse current than a small one.

 

[WobblyHand]

Personally, I think dealing with active cooling at this point is premature. It can help, but it is difficult to do right. We don't want frosting or condensation on the active element, or anywhere else. It's hard to make well sealed areas that are moisture tight. Yes, anyone who is determined enough can do it, but it would force a lot of design changes at this point. Let's get something working that appears to be "believable". By that I mean it basically works, but could use some sensitivity improvements. At that point one could decide to go about having a chilled detector or not.

 

[homebrewed]

So my lead cup could be useful if I machine a little off the inside or maybe file a hair off the corner of the detector housing.
I was thinking of a simplified shield like this:
View attachment 433434
The sources are as close to the front of the ring as possible without breaking the plane. The ring may be too tall but could be shortened on the bottom. The disc holding the ring could be plastic or lead sheet. This simple geometry would be easy to build without lost wax casting or extensive machining.
Graham- Might be worth experimenting with diode cooling way later in the process after we have some viable data to benchmark against?

I made a punch for making lead rings. Much easier than machining lead.

 

[WobblyHand]

Forming or punching is a good approach for dealing with lead. If that can't be accomplished than I suppose one is back to casting.

 

[homebrewed]

Personally, I think dealing with active cooling at this point is premature. It can help, but it is difficult to do right. We don't want frosting or condensation on the active element, or anywhere else. It's hard to make well sealed areas that are moisture tight. Yes, anyone who is determined enough can do it, but it would force a lot of design changes at this point. Let's get something working that appears to be "believable". By that I mean it basically works, but could use some sensitivity improvements. At that point one could decide to go about having a chilled detector or not.

Water condensation will be an issue much sooner than frost will be. For instance, if the dew point is 13C (to use Graham's example of cooling), that would come to an RH of only ~24% at 25C. That's desert-dry, probably not too likely for most of us.

 

[graham-xrf]

Most of the current flowing through a reverse-biased junction is thermally generated so it definitely is not noise-free. So I would expect a drop in the noise current. It might be worthwhile to play around with active cooling, at least as a part of the development process.

Our standard diode equation sez: Id = Is(exp(qVd/kT) - 1). For negative Vd, the exponent rapidly goes to zero so for Vd << q/kT, Id can be accurately approximated as : Is. So what IS Is? It is not a constant, it contains yet another exponent that has a temperature relationship, based on the bandgap of our semiconductor in question. In addition to temperature, Is has an area dependency -- makes sense, we expect a really big diode to have a larger reverse current than a small one.

This is, I think, my point. The wideband noise racket (currents) is associated with the DC dark current, and gets bigger with bigger dark currents, which get bigger with temperature, but that wideband noise is not the same thing as the DC dark current. Knowing that we can see a noise floor lower that what comes out of a reverse biased diode, I think we can have this gadget work as well as any at the kind of workshop temperatures most of us would tolerate on a hot day.
BUT..
I do have one of those "flat" heat pipes about15mm wide, and 3mm thick. If simply put in there, as if it was part of the EM shielding, one might put the end of the heatpipe sticking out of the machine into a glass of ice.

I don't think we will need to go for cooling, but there is easily the potential to accommodate it.
Mark - thanks much for the Id equation. I know I have it somewhere, but that was handy.

 

[homebrewed]

It's hard to make well sealed areas that are moisture tight

For sure. But I did outline an approach that wouldn't require a super-sealed enclosure in an earlier post. "All" that's required is a second Peltier that gets a bit colder than the one cooling the detector. The other one would act as a moisture scavenger. Eventually (that's the most obvious downside).

 

[graham-xrf]

@WobblyHand
I screwed up with the FreeCAD colours.
I need some way to set them back to standard, or something, so I can see the sketches I "revolved"