Answer to Question #8671 Submitted to "Ask the Experts"

Category: Radiation Basics

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Q

I am an element collector and one of the items in my collection is a 24 g piece of 238U metal. I also have a few small uranium and thorium minerals (all with much less measured radioactivity than the metal) and an inexpensive Geiger counter (DX-1) that measures gamma and beta plus gamma radiation, but not alpha radiation.

It was explained to me that in the case of my minerals, radium exists in equilibrium—equal amounts created and destroyed—and thus these will emit radon. For this reason, I normally keep the minerals (and the uranium metal) in plastic bags.

My understanding is that most of the beta radiation observed by my Geiger counter from my minerals comes from the beta-emitting radon daughters on their way to 210Pb and 206Pb.

In looking at the 238U decay chain, I don't understand why this would be the case with my 238U metal, which my Geiger counter shows as yielding 50 µSv h-1 gamma and greater than 100 µSv h-1 (the Geiger counter's limit) beta plus gamma. Considering the half-life of 238U and the 234U and 230Th daughters, wouldn't there be negligible amounts of radium?

The 238U decay chain shows 234Th and 234Pa as beta-emitting daughters on the way to 234U. Is this what my Geiger counter is seeing? Or is there also radium in equilibrium with my uranium metal?

Also, is it possible to calculate how much beta and alpha radiation my uranium metal emits, given its mass (24g) and my observation of roughly 50 µSv h-1 gamma?

A

You state that the uranium metal is 238U. If this is so, the implication is that the source of the metal was depleted uranium. Natural uranium, containing radioactive progeny in equilibrium with the parent 238U, contains an amount of 234U that has an activity approximately equal to that of the 238U along with all the other major shorter-lived progeny, also with activities approximately equal to that of the 238U.

Small amounts of 235U, typically about 0.7 atom percent, are also present, and this adds a small amount to the observed radiations coming from the material. The process of depletion of natural uranium involves partially depleting it in the 235U isotope, usually in the process of making fuel for nuclear reactors or in making materials for nuclear weapons, both of which use uranium enriched in the235U isotope.

In the process of removing the 235U, the 234U isotope is also removed to a large extent. When the metal is made from this depleted uranium through a chemical reduction process, little of the original 234U would be present, and the shorter-lived progeny, including radium and the various lead and bismuth isotopes that are major gamma emitters, would be removed. Because most of the 234U would not be present, the subsequent progeny could not grow into the metal for a very long time (until a significant amount of the 234U had again grown into the material).

If your piece of metal was made from natural uranium, then the 234U would likely be present in equilibrium with the 238U. Note, however that the first progeny species following 234U in the decay chain is 230Th, which has a half-life of about 80,000 years. This thorium isotope would expectedly have been removed during the chemical processing to produce the uranium metal, and it would require a long time to grow into the metal in significant amounts through the decay of the 234U.

The end result is, as you have inferred, that you would not expect to see major contributions to the gamma and beta emissions from short-lived progeny, especially 214Pb and 214Bi, both significant beta-gamma emitters. So where are the radiations you measure coming from?

Your conclusion about the potential source of some of the radiation you measure is correct. If you look closely at the decay scheme for 238U, you will see that the uranium initially decays through alpha emission to 234Th, which has a 24-day half-life and decays by beta emission to 234mPa (metastable protactinium-234), which has a very short half-life of 1.17 minutes, decaying by beta emission to 234U.

Thus, within about five months following production of the metal, the 234Th and the 234mPa would be in activity equilibrium with the 238U. The 234mPa beta radiation is quite high in energy, having a maximum energy of about 2.3 MeV. This is the beta radiation that is contributing most heavily to the readings you observe. In fact, depending on the amount of thickness covering the detector, some of the beta radiation may be interpreted as gamma radiation. It requires about one centimeter of a plastic such as Lucite to stop all the beta radiation from the 234mPa. You might want to try placing about this much material between the metal source and the detector to see whether there is a noticeable decrease in what you are referring to as the gamma exposure rate.

There are some low-abundance gamma rays emitted by the 234Th and 234mPa, and some gamma rays from 235U may contribute a small amount to the gamma reading, but you should also be made aware that a significant fraction of the 234Pa beta particles lose their energy in the high-atomic-number uranium through interactions that lead to bremsstrahlung x-ray production. These x rays may also add a noticeable amount to the observed gamma readings. I believe, however, that the readings you are seeing are caused largely by the beta radiation from the 234mPa and that a significant part of what you are calling gamma is actually beta radiation.

The above uncertainties as to what radiations are being measured make it unlikely that we can correlate the readings you obtain with the actual emission rates of the specific radiations. Additionally the geometries of the radiation detector and of the piece of uranium metal may be such that the detector volume is not being irradiated uniformly when the detector is close to the metal surface, and this results in readings that do not represent the true dose rate or exposure rate.

If we assume that your metal was made from depleted uranium and there is no significant 234U or 235U present, and there are no significant short-lived progeny present, as is likely the case, and that the mass of your piece of metal is all 238U, which is also reasonable, we could calculate the production rates of the various radiations associated with the decay of 238U, 234Th, and 234mPa.

For example, 24 grams of 238U represents a total 238U decay rate of 1.78 x 107 disintegrations per minute. Since each decay is associated with the emission of one alpha particle, the total alpha production rate is also 1.78 x 107 alpha particles per minute. Since the 234Th and 234mPa each emit essentially one beta particle per disintegration, the total expected beta production rate would be 3.56 x 107 beta particles per minute.

The production rate of the individual radiations, however, are not the emission rates from the metal surfaces, since most of the alpha and beta radiations are stopped within the metal. There are some crude estimations of the fractions of the produced alpha radiations and beta radiations that escape the surface of the metal that could be made by considering the respective ranges of the particles in uranium metal, but such estimations would require specific knowledge of the geometry of the piece of uranium metal.

I wish you well in your search for elements to add to your collection.

George Chabot, PhD, CHP

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