A University Graduate Student Measures A Few Granite Samples
This is a message on one of the radiation enthusiast forums from a gentleman that I had sent some low level granite samples. Nothing over 60 uR/hr measured with our PM 1703 meters. He used aluminum shielding, so this is all Gamma radiation. Each “count” is a Gamma ray hitting the meter. His testing method is far more accurate than out hand held meters, showing that our meters are indeed catching a small percentage.
On the Bordeaux, our meters registering 40 uR, his machine measured 8,492, 21.2 times more.
The Niagara Gold was at 50 uR/hr, Steve measured 19,604, or 39.2 times more radiation.
The Four Seasons was at 30 uR/hr, Steve found 6,466, or 21.5 times more radiation.
No doubt that as the stone gets past 40 uR/hr on our PM 1703 meters, the readings become way low. The meters are still useful since anthing over 40 should not be sold under any circumstances. Steve measured background at 4,000 cpm wich is about 66.6 times our background levels, of course he no doubt had a different radiation level in Pennsylvania.
These readings roughly support one of our scientists measurements that say our meters are catching only 5% of the radiation. That would be 20 times more on the lower level granites, close to what Steve found.
Al & others
My measurements so far on some of the offending granites:
(all gamma counts are 30 second averages w/ background subtracted,
measured with a 2″ dia x 2″ h NaI(Tl) crystal 1/2″ from the polished
surface, with 1/8″ aluminum shielding.)
Bordeaux G-3 4/8/08:
8,492 cpm gamma
Silver-gray opaque mineral noted – several 1-2 mm grains – this is
probably the radioactive species
Niagra Gold N6 4/8/08
19,604 cpm gamma
~1 cm silver-gray opaque grain exposed on one cut surface + some smaller
grains – again, this is probably the radioactive species (I cut off this
edge and it shows higher cpm gamma than pieces without visible opaque
grains)
4 Seasons P9 4/8/08
6,466 cpm gamma
Fracture surfaces show greenish/colorless secondary U mineral –
fluoresces bright green under UV light, is clearly visible in daylight
but not visible under incandescent. Possibly autunite? Primary U
mineral(s) are almost certainly present in this granite but I haven’t
found any yet.
Steve
2 Responses to 'A University Graduate Student Measures A Few Granite Samples'
Leave a Reply
You must be logged in to post a comment.
on August 29th, 2008 at 9:35 pm
Using a large (2″ by 2″) sodium iodide (NaI) detector is an effective way to quickly locate areas or items with a higher level of radioactivity. Because the detector medium is a solid, rather that a gas (as in a GM tube), there are a lot more atoms available for a gamma to interact with. Also the volume of the detector is greater. Steve compares results between granite surveyed with the same method, which is valid. However, comparing results in CPM from his SPA-3 or equivalent detector with microRad/hour (uR/hr) from a GM detector is not. His detector will over respond (read high) to low level radiation from radon daughters. An uncompensated GM tube will also over respond. Thus while his method is much more ’sensitive’ and can detect small differences better that your GM tube. It is not any more accurate.
on August 29th, 2008 at 10:43 pm
Gary,
Thanks for the comments.
I am unaware of any uR/hr (micro Roentgen per hour) versus cpm comparisions from Steve. All I read was the cpm readings that he reported. I also didn’t see any reference to a Gieger Mueller tube (Geiger Counter) in his response.
Could you point out where either or us used a GM tube that might have over responded? I see where I compared our small scintillator reading with his larger scintillator readings, but surely you aren’t saying that the scintillators are over responding.
From the research I have done, his 2″ x 2″ scintillation crystal can only be surpassed by a larger crystal or a HPGE type detector. I would say his cpm readings are very accurate unless you can point to a flaw in his methodology.
If you have any questions for Steve, ask them and I’ll forward them on.
Thanks.
.