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Uranium glass again and how to make a radiometric measurement I

Dear Reader,

I have checked most of the uranium glass with a geiger counter, the geiger counter uses a tube which is known as a Geiger-Muller tube which is a gas filled high voltage discharge tube which uses an avalanche effect to increase the number of free electrons and ions formed in the tube after the absorption of radiation by the gas inside the tube. It is important to understand that GM tubes are not all born equal, it is possible through careful design to optimise a tube for an application. I borrowed a GM tube based device to check my uranium glass, this first device was a bit of a disappointment.

It has a tube with a thin mica end window and it has some beta sensitivity, but it is not very sensitive. It was intended as a gamma / beta detector which has a full scale reading of 1000 rem per hour. I think that such a device is the tool of choice when dealing with a high dose rate event. It would be very suitable for nuclear warfare use assuming that the fragile GM tube survives the bomb detonation, it could be very useful when dealing with a industrial radiography accident such as a lost source or a radiotherapy accident which involves a lost source. This detector has a rate meter for GM tube events which goes as low as 1 count per second.

With this high dose rate meter I could not get any reading from my uranium glasses, except for a very dark green one which gave 2 counts per second, the background in my house was 1 count per second. It is hard to work out if 2 cps is different to 1 cps as radiation from radioactive decay and cosmic rays is occurring randomly.

So I then tried a different GM tube based device, I choose one which is optimised for looking for low to moderate levels of beta emitting radioactive contamination. Note while from personal experience I know it works for carbon-14 it will never work for tritium. The device is a “Radiation Alert Inspector” made by S.E. International INC (Summertown Tennessee). This device can give the count rate or it can be set to record the number of counts over a given time.

As I was dealing with very weak sources I choose a counting time of five minutes, I measured the background in my house three times. The results for the background were 248, 224 and 263 counts. The total number of counts in these three determinations of the background count rate were 735 counts, which makes the count rate in my house to be 0.82 counts per second, the ESD on this count rate is 0.03 counts per second which is about 3.7 %. This ESD is based on the number of events observed. So based on this count number we should expect 245 events (give or take 9 events) in five minutes.

I measured a glass object which does not fluoresce when exposed to UV light, I got a total of 224 counts in five minutes. The difference between the count number for this object and the background is 21 counts, the sum of the ESDs is 24, so this difference is unlikely to be significant in a statistical sense.

I then went and measured a green glass milk jug which fluoresces nicely with UV light, this gave a count of 580 events. The difference between this count number and the background count number is a staggering 335, while the sum of the ESDs is 33, as the difference is ten times the sum of the ESDs it is very real which suggests that the milk jug does contain something which is radioactive.

The lowest count number I got in five minutes on uranium glass was 441 on a pale green thin blown glass vase, this value still suggests that it is radioactive. The difference between the background and the sample is 196 and the sum of the ESDs is 30. This is still very convincing.

Now while I have done quite a trivial experiment I would like to ask other people who are considering doing independent radiation measurements to up their game a bit. I sometimes see data shown on the internet where the people making the measurements do not explain their experimental method fully or state the number of counts which they use to estimate the dose rate or radioactivity level. For example Greenpeace have been using NaI spectrometers in Japan, they used a thing called a “Georadis RT-30″ which is a nice bit of kit. The only problem is that they did not give full details of how they obtained dose rates with these machines.

While I know that a NaI spectrometer will never give as good energy resolution as a high purity germanium detector this type of NaI detector can distinguish between different radionuclides (based on the gamma photon energy), what I would like Greenpeace to report are the gamma spectra and all the details such as the counting time, details of the dead time correction. This would allow the contribution of Cs-137 to the dose rate to be separated from the gamma rays from the uranium decay chains.

I would also like spectra obtained at different distances from a known cesium-137 source at different distances. This could be used to calibrate the spectrometers. I would also like to see the spectra obtained using natural uranium, natural thorium and uranium ore samples. This would allow me to see how well the machine is able to separate the signals from the different gamma photons.

When I write gamma spectrum I always mean a table or chart of counts per channel against channel number. The Greenpeace NaI spectrometer has 1024 channels so with some luck it should be able to separate the photopeak for Cs-137 (662 keV) from all the other gamma photons or at least allow a partial cleanup of the data.

Now I will not pretend that Greenpeace are neutral regarding the question of “should the world have nuclear power ?”. I know that they are opposed to nuclear power, the fact that they are opposed to it does not either disqualify them from commenting on nuclear issues or make them more trustworthy. I hold the view that if Greenpeace put in extra effort into their radiometric measurements then in the long run it will be good for them and the rest of society.

Firstly it would make their results more trustworthy, people would be more willing to accept their results as true.

Secondly it would avoid problems such as “I will not trust it until I have checked to see if another explanation exists for their observation”. For example if an antinuclear activist claimed that hot spot exists in Aberdeen (Scotland) as a result of a discharge from a Scottish nuclear reactor, then I would want to know that they had not been fooled by the high gamma background due to the rocks in Aberdeen (Granite). One way of proving to me that a high gamma level was not due to the granite is to show me the gamma spectrum.

In recent times the higher background radiation levels on some beaches in the western part of the USA have provoked great excitement. The radiation has been blamed by some on the Fukushima accident, however a close examination of the site indicates that the radiation is coming from daughters of uranium / radium rather than cesium 134 or cesium 137. As the Fukushima event released mainly cesium and iodine this radionuclide signature is not reasonable for the beach.

A person or group which has a track record of making hasty statements will carry much less weight than a group which takes its time and makes sure that its statements are correct.

The problem with making a statement which is quickly shown to be false is that the person or group which made the statement will lose credibility, so by taking additional care to improve the quality of the work which is behind a statement then in the long run you will be more persuasive. I will get onto another point about radiometric measurements soon.

More uranium glass

Dear Reader,

I have been in the second hand shops today of Lidköping, I have managed to find some more uranium glass. The glass does glow a nice green / yellow when you expose it to UV light. I will have to make a radiometric examination of the glass to determine if it is uranium glass or some other glass which has similar optical properties.

I think in future I will carry a UV torch in my pocket when I am out and about, here is a photo of the glass under normal light.

Mark's new uranium glass

Mark’s new uranium glass

Here is a photo when the only light is UV light from my UV torch.

My new uranium glass with UV light, notice it is glowing green

My new uranium glass with UV light, notice it is glowing green

I will examine the glass as soon as possible, then I will post the results of my tests.

Candlestick III

Dear Reader,

A lady from the USA who collects glass has been in communication with me regarding the uranium glass candlesticks, she has a pair of very similar candlesticks to my uranium glass one. It is possible that they all came from the same maker.

I did collect a gamma spectrum from my candlestick, as the activity level in a candlestick is low it took many hours to get a good spectrum. Here is the full range gamma spectrum from 10 keV photon energy to about 3 MeV photon energy. This covers almost all of the gamma spectrum. OK I can think of gamma events which are outside this range like the n + N-14 reaction to form C-14 which emits photons at about 10 MeV but such a high energy photon is very rare.

Full range gamma spectrum of the candlestick

Full range gamma spectrum of the candlestick

You should be able to see that almost all of the peaks are at low energies (below 500 keV), I have stretched the y axis by using a log scale so you can see some more of the minor peaks.

Candle stick full range log scale for counts

Candle stick full range log scale for counts

Now I have replotted the graph to show the lower energy end only, it should be clear that a series of peaks are present due to the gamma rays from the candlestick. Also on the left hand side is a big peak due to X-ray formation. What happens is that high speed electrons (both beta particles and photoelectrons) fly around inside the shielding and form X-rays by hitting the copper inner shielding on the lead castle in which the detector is placed. The copper inner shield reduces the fraction of photoelectrons which are ejected from the lead surfaces that are able to strike materials close to the detector crystal.

Also any X-rays formed in the surface layer of the lead nearest the radiation detector and the object being measured will tend to be stopped by the copper metal. By experience is that 1 mm of copper sheet is able to stop almost all copper K x-rays. And I imagine that it will stop many moderate energy photons like the moly K x-rays which are used for single crystal crystallography work and for mammograms. In the ideal world we would also have an aluminium layer inside the copper shield to further reduce the back ground at low energies due to secondary X-rays, and then a layer of a plastic like PMMA to finish the job. This is an example of graded Z shielding.

Even if the shielding is designed in such a way to prevent X-rays reaching the detector, it is possible to create X-rays inside the sample. The ideal sample for gamma counting would be a very small sample with a very high specific activity (radioactivity per mass or unit volume). If this was wrapped in plastic then any beta particles which come from the sample would lose their energy gently in the plastic without making horrible X-rays. But in our case the volume of the sample is large and the glass contains plenty of elements such as silicon which can form bremsstrahlung as the electrons pass close the atomic nuclei.

But we will not let a little bremsstrahlung spoil our fun. Here is the lower end of the gamma spectrum.

Candle stick spectrum lower end

Candle stick spectrum lower end

What we need to do next is to work out the relative efficiency for the detector for the different gamma lines, what we need to do is to measure the size of the peaks which we can identify and then use the fact that the yield of the different gamma lines are known. This will allow us to reconstruct the relative efficiency for the detector. In case anyone is thinking that we could do it with more easy by using a mixed photon energy radioactive source such as a radium-226 source. There is a problem, the odd shape of the sample makes it have a sum of many different counting geometries and a lot of self adsorption for the lower energy gamma photons.

I could make it more simple by crushing or melting my candle stick to make it into a more simple shape, but that would spoil my candle stick so I am not going to do that.

X-ray energy and getting the terms right

While reading the article entitled “the art detectives” in the RSC’s Chemistry World magazine I saw the statement that high energy X-rays are used for XRF of elements such as zinc. I strongly suspect that a misunderstanding has occurred, for example the zinc k lines will come at 8.6 keV which is hardly high when compared with the X-ray photons commonly for the industrial radiography of steel objects. To excite an atom in a X-ray fluorescence (XRF) experiment only moderate energy photons are needed (tube voltage of 40 kV is acceptable) while for industrial X-ray radiography it is common to use much higher accelerating voltages (100 kV and higher). For very thick metal objects photons in the MeV range are used.

What I think the article should have stated is that the object in XRF was illuminated with a high intensity of x-ray photons, to my mind intensity (photons cm-2 s-1) is very different to photon energy. But why would anyone use an expensive intense x-ray source rather than a weaker and cheaper one ?

If we assume that the increase measured above background is directly proportional to the concentration of an element and the intensity of the incoming exciting x-ray beam, then if the background is 10 cps, then with a weak x-ray source then we could get a reading of 20 cps on a spot on a painting. As for a random events the standard deviation on the count number is the square root of the count number after 1 second then the sum of the two SDs is 7.634 which is close to the difference between the two count numbers. If we were to use a source ten times brighter then the sum of the standard deviations (10.49 + 3.16 = 13.65) is small compared to the difference in counts after 1 second.

The great problem is that people writing about science sometimes tend to throw words about, almost randomly, without thinking about the fact that the word already has a meaning. To write clearly about science we must first avoid confusion.

SSM and ISA

Dear Reader,

Those of you who live in Sweden may have heard of a body called SSM (StralSakerhetsMyndigheten), this is a state body in Sweden which has the task of protecting people and the environment from the adverse effects of radiation both today and in the future. In common with the now defunct NRPB their task includes X-rays, “Nuclear” radiation (α, β, γ and neutrons), radiowaves (cell phones etc) and UV light.

One of the great problems we as a society is that for the good radiological protection of future generations living far in the future we need to make predictions based on experiments which only last a short time.

One important issue is the formation of organic complexing agents in low and intermediate level radioactive waste. If a substance forms which is able to bind to metals and form water soluble complexes which do not bind to mineral surfaces then the rate at which radioactivity leaks out of a waste store could be increased.

One such compound which has been considered by many people is isosaccharinic acid (ISA) which is formed from cellulose when it is exposed to calcium hydroxide. The cellulose can come in the form of wood, paper or cloth while many cements contain calcium hydroxide.

The classic way to make ISA is to treat lactose with calcium hydroxide, I have done this several times and the mixture soon turns brown and after boiling it down you are rewarded with a dark brown mixture which smells strongly of cooking. By careful filtration of the dark brown mixture a brown solution can be obtained which is then evaporated to a dark solid. This is then extracted with water and recrystalized to give a white solid, due to the insolubility of the calcium salt of the alpha isomer of ISA this is possible. The calcium salt of the beta isomer is water soluble and stays in the mother liquor with a lot of other compounds.

As a result it is relatively easy to obtain alpha ISA, the beta ISA is harder to obtain, so as a result almost all work done on ISA has been done with the alpha isomer. As the properties of the two isomers are not exactly the same it may not be safe to assume that alpha ISA can be used to model a mixture of alpha and beta ISA. Within this project we will explore beta ISA and determine if it poses a special threat in nuclear waste stores.

Now you might wounder why I mention SSM, the reason I mention them is that they are funding this research. One of the ways that SSM protect society is to fund research which allows them to make better predictions about the future. Now my SSM work is about to start, the plan is that I will try to publish papers as well as writing a report for SSM. I also want to bring you some updates about the work here on my blog.

Candlestick II

Dear Reader,

I took my candlestick to work and I quickly found it was radioactive, it was emitting beta particles according to a quick check with a contamination meter. As it was emitting that nice yellow/green light when exposed to UV light and it was emitting beta particles I quickly decided it was genuine uranium glass.

The next step in the characterization of the candle stick was to use gamma spectroscopy on it, now before we get going I would like to point out that gamma spectroscopy is not a press the button and get the result type of machine. For those of you who are proper traditional chemists / scientists you will be aware that for a new type of sample it is very hard with most machines to create a method with a spectrometer where you just put in the sample and press go before getting the final answer.

One of the problems is the issue of self adsorption, for the lower energy gamma lines many of the photons will never escape from a large sample. The ideal sample for gamma spectroscopy would be a tiny spec (a point source) which would be at a well defined distance from the detector.

The candlestick is anything but well defined in distance from the detector and it is far from being a point source. I did not want to melt it down to make a lump with a more simple shape so I decided that we should measure it in its native form.

One of my questions about the candle stick was “is the uranium a depleted uranium, or is it a natural uranium which is likely to predate the nuclear age ?”

I reason that as DU is less valuable than natural uranium it would be the logical uranium to use if you were making a uranium glass candlestick in the 1950s or later. But if it was a more early candlestick then it would be more likely to have a natural isotope signature for its uranium.

We need to consider three uranium isotopes

238U which is the bulk of natural uranium, this does not have any useful gamma lines but its daughter (234Th) which emits gamma rays, as the half life of 234Th is short when compared with the age of the candle stick it can be treated as an extension of the radioactive decay of the parent 238U. 70% of the 234Th will decay to the meta stable state of 234Pa (234mPa). It is important to note that the 234Pa (both forms) give a forest of gamma lines (hedgehog spectrum).

Nuclide Half life Decay mode Main gamma lines
238U 4.468 x 109 years alpha No gamma
234Th 24.1 days beta 63.3 (4.8 %), 92.4 (2.8 %) and 92.8 (2.8 %)
234mPa 1.17 minutes beta 258.3 (0.73 %), Hedgehog spectrum
234Pa 6.7 hours beta Hedgehog spectrum

 

If the uranium had been a depleted uranium then I would expect that almost all the 234U and 235U would have been removed. As the 234U has a long half life it serves to block the decay chain of 238U if the sample is not old on a geological time scale.

I reasoned that by looking for the decay products of 234U that I could test the hypothesis that the uranium was a prenuclear age natural mixture of isotopes.

This uranium will decay to form a long lived radium (226Ra) which will then slowly on the timescale of the candlestick’s age decay further.

234U –> 230Th –> 226Ra –> 222Rn –> 218Po –> 214Pb

Nuclide Half life Decay mode Main gamma lines
234U 245500 years alpha No gamma
230Th 75380 years alpha 67.7 (37 %)
226Ra 1600 years alpha 186 (3.6 %)
222Rn 3.8 days alpha No gamma
218Po 3.1 minutes alpha No gamma
214Pb 26.8 minutes beta 242 (7.4 %), 295 (19.3 %), 352 (37.6 %),
214Bi 19.9 minutes beta Forest of lines
214Po 0.1643 ms alpha No gamma

 

The 214Pb will decay by beta emission to form 214Bi and then 214Po which then decays to form 210Pb. As after 226Ra no nuclide has a half life longer than a few days until you reach 210Pb we can treat these decays as extensions of the radium decay if we make a kinetic model of the candlestick.

The fissile 235U does have a useful gamma emission of its own, this can be used to confirm if the uranium was natural or depleted.

It will decay by alpha emission according to the following mechanism.

Nuclide Half life Decay mode Main gamma lines
235U 703800000 years alpha 109 (1.5 %), 144 (11 %), 163 (5.1 %), 186 (57 %), 205 (5%),
231Th 25.52 hours beta No gamma
231Pa 32760 years alpha Forest of lines
227Ac 21.773 years beta No gamma
227Th 18.72 days alpha Forest of lines

 

I hope to now be able to go through the spectrum and then hunt for lines, I recall that the 186 keV line for 235U was present. So far I think the uranium is from before the nuclear age.

Uranium glass

Dear Reader,

For some time I have been looking for some uranium glass, this is a type of glass which contains uranium. because of the uranium it is green in colour. The glass also fluorescent, when exposed to UV light it emits green light. I was in a second hand shop today and I spotted some green glass, I happened to have a long wavelength UV light in my pocket so I did a quick test of the glass with UV light. As the 20 SEK candlestick emitted green light I bought it.

I have made a short film of the candlestick being exposed to UV light. I hope to upload to youtube soon a film of the candlestick in a dark room being exposed to a UV light which I turn on and off.

20 SEK (circa £ 2) candlestick made of uranium doped glass

20 SEK (circa £ 2) candlestick made of uranium doped glass

I also hope to take the glass candlestick to work and measure its radioactivity. When I do I will post the details in another blog post.

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