Answer to Question #9383 Submitted to "Ask the Experts"

Category: Accelerators

The following question was answered by an expert in the appropriate field:

I am a radiological control technician working on the decommissioning of a major national laboratory accelerator. I have been informed that titanium-44, among others, is one of the major activation products we encounter in activated steel and concrete. I understand the creation of the other activation products I encounter, however, I am unable to understand the creation of titanium-44 under typical accelerator circumstances. Would titanium-44 indeed exist as an activation product within an accelerator and its surroundings? Could its energies be confused with something else?

There are several ways that you could produce 44Ti in steel and concrete. Let me first say that the decay chain for each radionuclide; half-lives, mode of decay, energy of the decay product, decay gamma-ray energies are quite unique. If you have the right detector and the correct energy, efficiency, and geometry calibrations, it will be very difficult to misidentify a radionuclide.

44Ti has a half life of about 60 years and decays by electron capture, which in itself is very difficult to detect, but it decays to the excited state of 44Sc nucleus which decays by gamma emission. This is followed by the decay of 44Sc, which has a much shorter half life, with its own telltale gamma and positron emissions.

Steel, although mainly iron, comes in many different compositions. Many of the elements composing steel are heavier than 44Ti. It is common to see in activated steel the radionuclides 3H, 7Be,11C, 18F, 22Na, 24Na, 42K,43K, 44Sc, 46Sc, 47Sc, 48Sc,48V, 51Cr, 52Mn, 54Mn, 56Co,57Co, 58Co, 60Co, 55Fe, and 59Fe, and more if there are heavier elements are added to the composition. Note most of these radionuclides have half lives less than a year. Only 22Na, 55Fe, and 60Co have longer half lives. There are even longer lived isotopes produced, but their activity is so low that falls below the certain level of detection, 44Ti is usually one of these. Stainless steels have various fractions of natural chromium (50Cr, 53Cr, and 54Cr). 44Ti can be easily produced from these isotopes in spallation and fragmentation reactions.

None of the main elements in the composition of ordinary concrete can produce 44Ti, even by direct irradiation by electron, photon, or proton beams. Even on 44Ca, you have to replace two neutrons by two protons, which does not easily happen. However, if you use a heavy-ion beam, it can produce 44Ti on concrete. The other more probable reason is that 44Ti is produced from impurities in the concrete. Or, it could be produced in heavy concrete which contains some iron.  

A longer half life means a lower specific activity. For example the specific activity of 60Co is about million times higher than 44Ti. You usually see 44Ti/44Sc in the radioactive materials that have been cooling down for several years, where the shorter-lived activity have been reduced significantly.

Finally, if some natural titanium, vanadium, or chromium are in the form of impurities in the materials that you have, it is possible to produce 44Ti in spallation and fragmentation reactions.

Kamran Vaziri, PhD

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