Natural uranium ore from New Mexico

[stamasd]

I just returned from vacation, and a package was awaiting at home, with some uranium ore samples from New Mexico. There are many samples, but I chose one for a quick test - both of the ore, and of my "mobile" gamma spectroscopy setup.

The setup first:
Acquisition on the laptop described here: viewtopic.php?f=15&t=534 using the built-in sound card at 192kbps, 24-bit.
Using PRA22 with settings as in image below, detector is homemade from a NaI(Tl) 1" by 1" crystal (USSR stock) with a Hamamatsu R6095 PMT (from IRadInc on ebay), driver GS-2000 Pro at 700V. Recording volume 100%, no volume boost.

Before the test I did a 5-point calibration in PRA using: Lu-176 source (20mm cube of LYSO) at 202 and 307keV, Cs-137 check source (662keV) and Co-60 check source (1173 and 1332keV). I used interpolation as this one seems to work best with my detector. Parameters pictured below.

The spectrum was obtained from one of the samples that was labeled "medium activity", an approx 50x50x10mm rock which I wrapped in a paper towel and then placed in a ziplock bag. As such it gave me approx 270cpm on a pancake Geiger counter with the alpha shield on. I have many more samples, but this is the one I chose for this quick experiment and I'm sticking to it.

I acquired data for 3600s, I wasn't patient to wait for more. No shielding was used, my background is pretty low (also remember this is a "mobile" unit) and so there are no lead fluorescence peaks.

I did a tentative identification of some of the peaks as labeled in the picture (interpolated energy, tentative isotope with expected energies and Ig):
1 - 86.7 keV ?
2 - 150.4 keV ?U-235 (163.3, 5%)
3 - 188.7 keV ?Ra-226 (186.2, 3.6%) or U-235 (185.7, 57.2%)
4 - 242.7 keV ?Pb-214 (241.997, 7.43%)
5 - 295 keV ?Pb-214 (295.224, 19.3%)
6 - 360.3 keV ?Pb-214 (351.9, 37.6%)
7 - 612.5 keV ?Bi-214 (609.3, 46.1%)
8 - 774 keV ?Bi-214 (768.35, 4.9%)
9 - 823 keV ?
10 - 880 keV ?
11 - 1229 keV ?Bi-214 (1238.1, 5.8%)
12 - 1376 keV ?Bi-214 (1377.6, 4%)
13 - 1539 keV ?Bi-214 (1509.2, 2.1%)
14 - 1620 keV ?
15 - 1805 keV ?Bi-214 (1764.5, 15.4%)

This is a quick-and-dirty test, will play more with the many samples I have. I'd appreciate feedback, and possible candidates for the extra peaks marked "?"

[Go-Figure]

I did a tentative identification of some of the peaks as labeled in the picture (interpolated energy, tentative isotope with expected energies and Ig):
1 - 86.7 keV ?
2 - 150.4 keV ?U-235 (163.3, 5%)
3 - 188.7 keV ?Ra-226 (186.2, 3.6%) or U-235 (185.7, 57.2%)
4 - 242.7 keV ?Pb-214 (241.997, 7.43%)
5 - 295 keV ?Pb-214 (295.224, 19.3%)
6 - 360.3 keV ?Pb-214 (351.9, 37.6%)
7 - 612.5 keV ?Bi-214 (609.3, 46.1%)
8 - 774 keV ?Bi-214 (768.35, 4.9%)
9 - 823 keV ?
10 - 880 keV ?
11 - 1229 keV ?Bi-214 (1238.1, 5.8%)
12 - 1376 keV ?Bi-214 (1377.6, 4%)
13 - 1539 keV ?Bi-214 (1509.2, 2.1%)
14 - 1620 keV ?
15 - 1805 keV ?Bi-214 (1764.5, 15.4%)

Hi there.

#1 - There are X-rays peaks expected from the decay of Pb212 and Pb214 (which is consistent with U238 decay chain) at 87 and 89 keV, but I usually detect the one at 77 keV. It depends on how the calibration is at such low energies.

#2 - I would suggest it's U235's peak at 143.76 keV rather than the one at 163.3. First because it has a higher emission probability, and second because your calibration seems to give you peaks at an energy slighly higher than they are supposed to have, and that would be the only one clearly going the other way around.

#9 - Bi214 has a peak at 806 keV which could be sort of consistent with that one given the calibration drift. But I never observed that peak so I would not bet on it.

# 14 - Bi212 from Th232 series has a peak at exactly that same energy, but there are no other signs of Th232's presence, besides that would be too right on the dot, given the slight drift observed in the surrouding peaks.

Massimo

[stamasd]

Thank you Massimo for your comment.
#1 yes the Pb-214 peaks at 74.8 and 77.1 should go there, and it's probably what that peak is. I'm not too confident about calibration of my small detector at low energies so a large non-linearity can be expected; it's a 1-inch crystal after all (and TBH not a very good one)
#2 I agree with your assessment there

Further comment: my goal here, if it wasn't evident from my description above, is to see "how low can you go" with a homemade system that can be packed in a small backpack to perform gamma spectroscopy on samples in the field with reasonable results. So far I'm not too displeased with the performance. The whole system weighs less that 3kg (4kg with the power brick).

[Sparky]

Depending on the thickness of your housing, a one inch probe should do very well in the range 20-100 keV. In this range characteristic x-rays become important. L peaks are generally too low to be seen with a standard gamma probe, but K alpha and K beta lines are often seen, with K alpha being more prominent. In fact the peak at 32.2 keV with Cs-137 is actually a Barium K alpha (Ba-137m) peak.

Lead x-ray peaks are quite prominent in Radium and Uranium (in equilibrium with daughters) samples. Ka is at 75 keV abd Kb at 85 keV. With a good shielded (NaI) spectrum they can be seen separately, but most of the time they blend together at about 77 with a bulge to the right.

With Theremino, you can adjust for non-linearity in this range by using multiple calibration sources. I have used Ag Ka (22 keV), Ba Ka (32.2), Am241 (59.5), and Th234 (92).

http://hyperphysics.phy-astr.gsu.edu/hb ... xrayc.html

https://www.amptek.com/resources/period ... okup-chart