Dear Randall,
Yes. You're right about the Hamamatsu - it is for a SiPIN. I have a vague memory that I saw some articles about the A-250 being deployed on satellites and I seem to remember the detectors being proportional counters - but I could be wrong. Looking at the data sheet for the A-250, it mentions proportional counters as a possible detector.
To a certain degree this is more of an academic exercise - to understand the underlying circuit theory of these "state of the art' preamps. I have a Amptek SiPIN on its way in the mail right now. But I figure that since I have the proportional counter, why not try to get it working as I think it will be more sensitive in the higher energy regions than the SiPIN.?
It's a little hard to compare noise specs as the measures seem to be different. The Hamamatsu is listed as having a "550 Electron/FWHM noise characteristic." The A-150 instead provides a chart indicating that with certain front end FETs, such as the 2SK152 (of which I just received 5 in the mail), at 2pF capacitance the noise is about .5kev FWHM. I'm not sure if those are just different terms for the same thing. The patent claims to have measured "equivalent noise charge of less than 20 electrons r.m.s." But as I'm a bit new to this, I'm not sure if these measures are all the same.
Earlier you mentioned
Now to compare the op amp specs with the Ortec 142PC
There may also be other, higher performance alternatives.
With such a low capacitance in the Porp Chamber, stability, slew rate and
settling time (at a good accuracy level) will be important characteristics if the goal
is energy discrimination, not just detection.
Comparing the gain bandwidth product of a couple options, we get;
- AD823:??3 dB bandwidth of 16 MHz, G = +1
- Amptek A-250:?300 MHz with 2N4416 FET
Having just learned what slew rate and gain bandwidth product are about 4 days ago, I'm not sure that I'm equipped (read: too lazy to do the math) to compare other characteristics.
But I know that the are much better op amps out there such as the AD829, which appears to be what Amptek is using in their PA-230 preamp. This has a 230V/?s slew rate and a much better settling time than the AD823. The AD829 and the AD823 are about the same price ¡Ö$7 but you'd need two of the AD829s since its only a single op amp chip.
A quick DigiKey search yields some other options;
- LTC6253 - $7 - 280V/?s - 720MHz
- OPA2690 - $6 - 1800V/?s - 300MHz
- LMH6626 - $6.5 - 360V/?s - 1.3GHz
- THS3062 - $13.5 - 7000V/?S - 2.2GHz
I imagine that slew rate is more important as the count-rate goes up. And gain bandwidth product is more important and one needs to amplify smaller and smaller signals? Is that right or have I gotten my wires crossed?
Then there is the ADA4530-1 which has an input bias of 20fA. But that might be for different applications altogether.
I know with SiPIN low capacitance is a good thing (hence the use of high reverse breakdown voltage PINs). What are the issues with a low capacitance proportional counter?
All the best,
Soren
