A comparison of two (low-activity) rock samples

[gwgw]

Those are two little (about 100g) rock samples I collected some time ago and recently analyzed. My original intention was actually to have a comparison between a very low-activity sample (limestone) and a higher activity one, with more naturally occurring radionuclides in it (granite). In the process, I got a little surprised by the results.

The very low sample was easy, my hometown is at Danube river, it used to be an ancient seabed millions of years ago and so it's all built on a sediment base - limestone and loess. It's known for being very low-background, the gamma background in the region rarely exceeding 0.10 uSv/h. The most radioactive place in town is one of the downtown squares, paved with granite blocks. I was lucky to be in town at the same time a major rennovation of the river bank area was underway and there were hundreds of tons of limestone being transported there by trucks. The limestone was used to pave new alleys and to reclaim some land from the river for a new quay wall and a marina. Construction works though were halted in the weekend and there was noone at the place, so I took the opportunity to errr.....steal....gather some little samples. By chance, I was also told the exact location the limestone quarry was located, it was a place about 40 kilometers away. The geiger counter did not measure any noticeable difference even at close contact with the samples.

The other sample I got recently from a short hike in the nearby Vitosha mountain, which is just in the outskirts of Sofia, the city I live in. The mountain is a fairly young one, geologically speaking, built of mostly igneous and metamorphic rocks with some sediments in the lower part. I didn't even go high, I was walking in the lower parts (perhaps less than 1000m above sea level) but I was lucky enough to find some granite (actually sienite) among the prevailing sandstones. The geiger counter (using a Soviet SBT-10 alpha/beta/gamma tube) did show an increase in CPS for gamma, but went rather high when I removed the metal lid and measured alpha/betas. So I was happy to find the other sample for my comparison.

Limestone was about 103g and sienite was about 106g, so the mass is quite close for both samples.

When I measured them with the spectrometer, I was rather surprised. I did several measurements with increasing measurement times and finally settled on 8h measurement and background subtraction of 12h of background. So after the 8h measurements, I was surprised to find out that the limestone actually had some gamma activity, although very low, 0.9 CPS. I was also surprised by the low activity of the sienite sample, just 4.2 CPS, despite registering so many alphas/betas. As a comparison, the room background was about 124 CPS and the background measured inside my poor lead castle was 25.3 CPS. Thus I got the following two spectra.

I was surprised that the limestone activity is not due to K-40, for some reason I believed that together with calcium there should be some potassium as well. It appears to be caused by minor amounts of thorium. The sienite sample clearly gave thorium peaks.

Finally a question to experienced members - what do you think would be more appropriate to get better spectrums of such low-activity specimens - increase measurement times (e.g from 8h to 24h) or improve my shielding?

[cicastol]

CUT
what do you think would be more appropriate to get better spectrums of such low-activity specimens - increase measurement times (e.g from 8h to 24h) or improve my shielding?

Hi,to take better spectrums for low Activity samples you need both, better shielding and longer integrations times !

Nice results