Miles Pickering
and Richard Davis Reed College Portland, Oregon 97202
Alpha SpeCtr~SC~py Using Thorium Daughter Products
The teaching of nuclear chemistry a t many institutions is handicapped by lack of access to nuclear reactors or other sources of short-lived nuclides. Experiments are definitely needed for nuclear programs which involve the use of natural radioactivity. The advantages of simplicity and safety are also present with naturally occurring isotopes. The experiment to be described was originally designed as an attempt to make a "thin" alpha source to he used in a demonstration of alpha spectroscopy with a small surface barrier detector recently purchased. Detectors of this type have become quite common in radiochemistry laboratories, both for research and teaching. The requirement of "thinness" is to prevent self absorption of the alpha particles, and consequent tailing of the spectra on the low energy side. This requirement necessitated a carrier-free preparation start-
Figure 1.
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The thorium series, showing half4ves.
Journal o f Chemical Education
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ing from one of the natural alpha emitters. The specific activity of thorium or uranium is far too low to give enough events within a reasonable time. Consequently, the short-lived alpha emitting daughters were separated for the experiment. The thorium series is shown in Figure 1. The longlivcd 232This in secular equilibrium with the 6.7 y 228Ra,which undergoes a succession of beta emissions to reach the 1.9 y %"Th. Hence, a preparation of natural thorium that has not been purified recently will contain some 228Thand its daughters, 224Ra(3.64 d), 220Rn(54.5 sec), and 2L6Po(0.558 see) all of which are alpha emitters and will be in equilibrium, and the beta emitter, 2'2Pb,which decays further to 2'2Bi and Z12Po, all of which decay partly by alpha emission. Hence, if the radium and other daughters are separated from the thorium, an alpha spectrum should show five peaks corresponding to 224Ra,220Rn,216Po,2L2Bi,and 2'2P0, as shown in Figure 2. The alpha particles are emitted monoenergetically, since the parent nucleus makes a transition to a definite state of the daughter nucleus. Therefore, a plot of frequency occurrence versus particle energy will he a set of narrow lines. Unless there is branching decay, the peaks will all be at the same height, since a condition of radioactive equilibrium is rapidly established, and
Figure 2. sample.
The alpha spectrum of the Th daughters, in an infinitely thin
the activities of the daughters should be equal to the activities of the parents. Such a plot can be obtained as the output of multichannel analysis of pulses from a surface harrier detector. This type of detector is a solid state device, using ion collection in a semi-conductor, rather than in a gaseous medium. It is ideal for alpha particle counting because it has no window, and all of the energy of the particle will be deposited in the "depletion region," the active volume of the semi-conductor. Since only about 2.5 eV are required to produce an ion pair in Ge, instead of the 30-35 eV required in most gaseous media, the same alpha particle will produce about ten times as many ion pairs in the former case. This will reduce the importance of statistical fluctuations and lead to sharper energy resolution. The counting is done in vacuo to prevent energy loss by the a particles passing through air. Pulses from this detector are fed into a 400-channel analyzer in the same way as pulses from a NaI crystal. The separation of the daughter is performed by extraction of an aqueous solution of "old" thorium nitrate (old means several years since the last purification). The solvent used is tributyl phosphate (hereafter TBP) and chloroform, saturated with concentrated nitric acid. This mixture removes the thorium from the solution and leaves behind a carrier free aqueous solution of the daughter products, which can be evaporated on a glass planchet (nitric acid will dissolve a metal one) to give an essentially weightless sample. Experirnenlal 1) One hundred ml of a 507' solution of T B P in chloroform is shaken with 50-100 ml of conc. HNOs in a 250-ml sepsratory funnel, and the aqueous phase discarded. This process is repeated with fresh HNOI three times, and saturates the TBP solution with HNOa. 2 ) A 1-g sample of Th(NOah is dissolved in 10 ml of cone. HNOX,and extracted with 10 rnl of the TBP solution prepared above. After extraction the organic phase is discarded into a. radioactive waste container, and the aqueous phase saved, since it contains the T h daughters. The extraction must he repeated a t least three times.
ENERGY OF ALPHA PARTICLE (MeV)
Figure 3. An actual d p h o spectrum of the T h daughters, as m e o w r e d by a student.
as near the detector as possible, and the chamber is pumped down until a vacuum is achieved. Detector voltage must he off during this process. Counting time will be 30-60 min using a. small surface barrier detector and a 400-channel analyzer. If a. thin absorber is placed over the sample, the expected shift of the peaks to lower energies will be seen.
Results
The actual experimental spectra such as the one shown in Figure 3, exhibit "tailing" of the peaks. This is caused by the fact that the sample is not completely weightless, but some self-absorption is taking place. Not all the alpha particles travel through an equal amount of matter, and some will be slowed down more than others even before leaving the sample. Absorbers placed between the detector and the sample produce a lowering of alpha energies. There is no marked change in spectral shapes, but the whole spectrum is shifted toward lower energies. The slight inhomogeneities in the foils also tend to smooth out the jaggedness of the peaks, and widen them. Disposal of the sample is no problem. The half-life of 224Rais only three and a half days, so within a f e ~ weeks the sample is dead. The Th(N0,)d solution can be collected and re-used for other purposes. It should not be dumped into the sewage system.
Volume 49, Number 6, June 1972
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