Needing more than a spark test?

[rwm]

I do not have a clear understanding of the problem? Are you saying you have too low of a count rate with a target sample in place? I would expect the count rate to be very low with no fluorescing target.
Can you post some pics of your setup so we can see the geometry?

 

[graham-xrf]

@rwm Rob speculating about Chinese lightweight neutrons is thankfully not it, but messing with what we have at this level, is getting close to that region of what happens with matter and energy being known (to me) as "wierd sh*t". We cannot really know the photon is even there until the current comes into existence, and if it encounters diffraction edges and gaps, we maybe end up doing our fractured version of the double slit experiment?

Looking to why the "low" count.
Starting from the diode, we have the X100-7 data sheet.
The probability of a 60keV incoming X-ray doing something other than passing straight through is shown as near 3%.
We can't know what X-ray source was used to test the diode, but we keep in mind that the graph is about probability in a 10mmx10mm area. To get the 0.03 number, the tester must have known the flux onto that area. However many glows we can make coming out of stuff the Am241 is pounding on (every now and then), and also direct from the Am241 speck of oxide, they are going to be going all over the place.

By our mechanical surrounds, we have it that most of "all over the place" means encountering shielding, where it wastes itself, except for some extremely rare photon that might eventually make it to the other end of the galaxy. We can only work with those that are kind enough to make it to our detector, and some get lucky enough to encounter some diode stuff. Those will be 3% of photons in a 100mm^2 area fraction of a sphere area 4π*r^2. I thought r=50mm, or about a couple of inches, if we were trying to get something to happen from direct 60keV. One could, I suppose, put up to maybe four Am241 sources right down onto the diode

Putting aside, for the present, all the glow X-rays it might splatter when it encounters metals, consider first the direct 60keV stuff we hope to see. In my sketch from post #35, the Am241 is denied any opportunity to have it's 60keV photons make it to the diode, but suppose we change things to allow it.

Calculating the pulses
Since 1978, the average amount of Am241 Oxide in a smoke detector changed from 3 microCuries to 1 microCurie. This is equivalent to 37,000 decays per second. Those be numbers are for smoke detectors in USA.
According to Wikipedia, 85% of those decays are 5.486MeV alpha, 13% is 5.443MeV of (something), and 5.338MeV for the remaining 2%.
Umm.. that adds up to 100%. Where are the 59.5409keV Gammas? Where are the 13.9keV, 17.8keV and 26.4keV other bits?
Out of the 37,00 decays, is it going to be "hardly any"??

37,000 per second is 37kBq.

PDF jackpot!
I am still reading it, and unfortunately, I have to go to attend something else.
From just a quick look, this guy's germanium detector was 10mm diameter.
His source was 3.7E9 Bq. That is 100,000 times stronger than the smoke detector.
His source was a whole ring, instead of six spots.
His detector area was 78.5mm^2. The X100-7 diode is 100mm^2
His collection time was 2000 seconds, ie. three minutes more than half an hour.


He was getting about 19 counts per second direct from the Am241
While looking at Zinc, 49.1KeV, counting at 3 or 4 per second, he was getting about 3 per second from Am241.
While looking at Molybdenum, he was getting 4 to 5 counts per second from Am241.

We do know that the pocket-geiger was used to do this trick. Something must be wrong!
Please help me check, and reconcile the numbers, because at this rate, with 6 smoke detectors, we would have to wait 13.8 hours to see one count originating from the Am241?? This is very roughly speaking. Detector area, strike probability, aperture fraction etc. not included yet.
There is a gross error somewhere. We should be able to predict quite closely the counts we see. We should be able to tell if the smoke detector is doing anything at all, We should see other pulse events also, like some background we can see, and stop with shielding.

Maybe his germanium detector, when this was done, has a much lower detect efficiency than a X100-7 diode??

We have to look for more papers. Maybe there is a typo about the Bequerels from Amersham International source.

 

[rwm]

I could be wrong about this but let me throw this out. I think the 59 Kev gamma actually comes from Np during decay. When Am241 gives off an alpha to become Np the Np is in an excited state and immediately gives off the 59Kev gamma. Also, when measuring activity, I think they typically measure the gamma emission. So 1 Bq equals one gamma per second or 2.703×10−11 Ci. Therefore 1 micro curie would be 3.7 x 10 E7 Bq? Check my math.
Also, I would put the target as close as possible. Say use an R of 1 or 2 cm.

Edit: Yeah the math is wrong. 1 micro Ci is only 3.7 x 10E4. Graham already did the math in his post anyway. Still that is a decent count rate even if we only capture 3% of a fraction of a sphere.
R

 

[graham-xrf]

I could be wrong about this but let me throw this out. I think the 59 Kev gamma actually comes from Np during decay. When Am241 gives off an alpha to become Np the Np is in an excited state and immediately gives off the 59Kev gamma. Also, when measuring activity, I think they typically measure the gamma emission. So 1 Bq equals one gamma per second or 2.703×10−11 Ci. Therefore 1 micro curie would be 3.7 x 10 E7 Bq? Check my math.
Also, I would put the target as close as possible. Say use an R of 1 or 2 cm.

3.7 x 10^7 , or 3.7E+7, or just 3.7E7, however you want to write it, is the same as 37 x 10^6, which is 37E6 Bequerels (decays per second).
This is 100x the number I used.

Our problem is about our expectations when considering the C.S.Chong Malasian University paper.
He said his source was 3.7 x 10^9 Bq. That is 100,000 times greater!
I would doubt that a Am241 annular source from Amersham International would be no stronger than a smoke detector.
Therefore, we have a problem!
We need to calculate exactly what to expect. On the face of it, it does not look good.
BUT
If it was going to be so poor, how come the pocket-geiger works? What exactly is it counting?
We are not the first to try this. Others have, though using a scintillation photo-detector.

Yes, Am241 does decay to Neptunium, by alpha decay. It will be about 4% of the mass after about 30 years.
Neptunium is very long-lived. I think the Am241 decays to Neptunium, losing two Alphas (Helium nuclei), and delivers the big Gamma pulse in the process. I think the added up mass of the Neptunium, and the Helium is short of the mass of the Am241 that it started out with by 60KeV/c^2.

 

[homebrewed]

There's a significant difference between detector efficiency and count rate. The fact that the detector's efficiency is just 3% of its max for a 60Kev photon doesn't mean it is only "seeing" 3% of them. It means it's outputting current pulses that are 3% of its maximum possible amplitude.

I can vouch for this because my detector outputs many counts per second when it is directly exposed to my Am241 sources. The count rate also is pretty high when my source-aperture plate is turned around, where the gammas have to go through the sources' steel substrates and .25 inches of aluminum!

I need to do a better job of characterizing the pulse height I'm getting from those 60Kev photons. Using the efficiency curve, I should be able to predict the pulse amplitude for an Iron K-alpha. If nothing else, I can check to see if there's a chance of overloading my electronics (the PocketGeiger and my signal conditioning board). Just something else to eliminate in the path to finding out what's really going on here

I will take a photo of my setup and post it when I have a chance. This morning (PST, USA's west coast) is gonna be too busy, maybe this afternoon.

 

[graham-xrf]

There's a significant difference between detector efficiency and count rate. The fact that the detector's efficiency is just 3% of its max for a 60Kev photon doesn't mean it is only "seeing" 3% of them. It means it's outputting current pulses that are 3% of its maximum possible amplitude.

I did not read it like that!
The Y-Axis is labeled "Absorption Probability", not efficiency

I can vouch for this because my detector outputs many counts per second when it is directly exposed to my Am241 sources. The count rate also is pretty high when my source-aperture plate is turned around, where the gammas have to go through the sources' steel substrates and .25 inches of aluminum!

That is a great relief, and corresponds to my expectations.

I need to do a better job of characterizing the pulse height I'm getting from those 60Kev photons. Using the efficiency curve, I should be able to predict the pulse amplitude for an Iron K-alpha. If nothing else, I can check to see if there's a chance of overloading my electronics (the PocketGeiger and my signal conditioning board). Just something else to eliminate in the path to finding out what's really going on here

I will take a photo of my setup and post it when I have a chance. This morning (PST, USA's west coast) is gonna be too busy, maybe this afternoon.

The one thing I have been striving for, for a long time, is to find exactly how much current pulse will happen, to be seen by the transconductance amplifier, when ONE photon makes it, (with a 3% probability unrelated to the energy it delivers to the circuit when it encounters a diode atom).

We have detector efficiency factors in the available equations, which only let one know the current from a FLUX, because we here are back to probabilities of a photon doing stuff. We are right back into quantum physics, and our problem is that we don't have a "flux" to put into the equations. We have single photon events.

We have the knowledge that when a photon DOES strike (with the certain probability) then the entire energy as released will end up as a current pulse. It might miss, but If it does do it's thing, we get 100% of it. I know we conventially count electrons per second as current, but in the circuit, the field energy is moved around the circuit at a large fraction of the speed of light, whereas a given electron might take hours, or weeks to go around.

From my previous calculations, (which could be seriously messed up), I figured we can expect a current pulse into the transconductance amplifier of around 10 nA peak, lasting for 13nS to 20nS, and that was from a approximately 400eV photon. I do have to go through these again, but clearly, we cannot reconcile the stuff from the Malasian university with our direct experience. Much relief

 

[rwm]

Graham- I think you may have replied before my edit. I did have a math error. Micro vs milli duh. Anyway it is hard to understand the CPS claimed in the paper. I do think it will be fine at the count rate we are seeing.
R

 

[graham-xrf]

Graham- I think you may have replied before my edit. I did have a math error. Micro vs milli duh. Anyway it is hard to understand the CPS claimed in the paper. I do think it will be fine at the count rate we are seeing.
R

Yep - feel OK about it
Even so, I do not have a complete grip on getting design calculations right. I do also skip by factors of 1000
Counts per second when exposed to the source, is what I wanted to hear.
We have to give kudos to Mark. He is at the sharp end of all this.

 

[homebrewed]

Regarding absorption probability vs. efficiency, it gets complicated. We want some sort of output pulse-height proportional to the photon energy, regardless if 100% of them are absorbed or 3% are. So what is it, higher energy makes more carriers or higher energy is less-absorbed? Both? Perhaps the latter statement is true, since the "efficiency" curve really is a curve with a single maximum. But it should be obvious that the count rate is not useful when it comes to differentiating between iron and nickel.

I know I'm getting a fairly wide variation in pulse height, ranging from a few tens of millivolts to somewhere in the volt range. I also know that specialized silicon, germanium and CZT detectors (the latter are the current darlings of the detector world) do output current pulses that are proportional to the photon energy, up to the point where the material's absorptivity fallof messes up the proportionality. This fallof, by the way, also applies to scintillators -- they have a maxim-useful energy, mostly determined by how thick they are.

 

[graham-xrf]

I think you have got it right!
When you show the detector the Am241 source, you see a definite increase in numbers of pulses. We know where they come from!
You expect pulse height to be proportional to energy, and I think that may well be what you are seeing.
What emanates directly from Am241 is the 59.54keV, and 26.4keV, and 17.8keV, and 13.9keV, and there may be others.
The counts of these various photons is biased well in favor of the lower energy photons.
The 59.45keV photons may be more numerous to start with, but the diode admits 10x more of the fewer crowd.

About the number of counts - of various height
The incoming that happen to be 59.54 signature gamma photons, then 3% of them. If some other energy, like 17.8keV, then 30% of those.
The count amounts will be following the absorption probability profile.

About the amplitude(s) of the pulses
The energy from any given photon will entirely (100%) be given over to making it's corresponding size pulse - those that actually make it.
This has to be. The whole principle of the device depends on this being so.
The height of the pulse delivered cannot be randomly variable. The gain of the amplifier is fixed. A pulse is repeatable.

Capturing 100% of a photon is not like "100% efficiency". It is in the nature of a photon you get the whole quantum, or nothing.
How well the diode bulk material then delivers a current, vs how much it wastes in heat, etc. does involve an efficiency, of the kind that can reduce the size of the pulse. But it would not be a variable thing that modifies pulse sizes in a contrary way.

.. is what I think. I also think you are looking at a much better situation that you give it credit for!