This is a description of an experiment to measure the electron-capture branch of 100Tc we're developing here at CENPA.
Motivation
This experiment is interesting for three reasons.
1.)The electron-capture branch hasn't been measured very precisely. The only published measurement (Phys. Rev. C 47, 2910-2915 (1993)) has a 50% uncertainty on its result, BREC=(1.8 ± 0.9)10-5.
The level diagram to the right shows the predominant modes of decay. 100Tc β-decays to 100Ru 99.99x% of the time. The object is to measure the value of x.
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2.)The A=100 system is ideal for testing theoretical models of double-β decay. Several experiments, including Majorana and MOON, are currently in development to search for neutrinoless double-β decay (0νββ). An observation of 0νββ would indicate that the neutrino is its own antiparticle. Furthermore, the decay rate associated with 0νββ is dependent on the neutrino's effective mass.
It is important to test the theoretical models of 0νββ because the interpretation of these up-and-coming experiments will require the use of nuclear models to separate the nuclear contributions from the leptonic contributions.
The picture to the right shows the level scheme with 100Tc as a virtual state. A very general equation for the nuclear contribution from time-dependent perturbation theory is given below, assuming that the leptonic part of the matrix element can be disentangled from the nuclear part. The transition is strictly Gamow-Teller. Only 1+ intermediate states contribute to the two neutrino double-β decay. The principle of detailed balance allows us to deduce the ground-state contribution to the two neutrino transition from a measurement of 100Tc's electron capture.
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3.)The electron-capture branch, once again through detailed balance, also determines the efficiency of 100Mo as a solar neutrino detector. The small Q value of 168 keV would allow for the absorption of solar neutrinos from the reaction p+p->D+e++νe, along with other low-energy solar reactions. This is an exciting possibility, but it's not being pursued at the moment (Please correct me if you know otherwise!).
The SAGE experiment was sensitive to pp neutrinos, but not spectroscopically; SAGE only measured an integral.
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NOT TO SCALE!
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Challenges
The first problem associated with studying 100Tc is that it's radioactive, with a short half-life (t1/2≈15.8 s). The wonderful IGISOL facility at the University of Jyväskylä, Finland solves this problem. They are able to extract a beam of 100Tc from a 100Mo(p,n)100Tc reaction. A separator magnet helps remove contaminants from other reactions, such as 100Mo(p,2n)99Tc. Unfortunately, during our last visit there the separator magnet did not completely remove the 99Tc contamination.
Another difficulty is the large background of βs from the decay to 100Ru. The plot to the right compares a raw spectrum to a β-vetoed spectrum from 100Tc, in which the vetoed spectrum has been multiplied by a factor of about 6 for comparison. Vetoing βs shouldn't remove any of the electron-capture events. The large peak that dominates the spectrum is from the K-α x rays of 100Ru.
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To the right is a fit to the vetoed spectrum. The peaks from 99Tc and 100Ru don't help resolve the miniscule 100Mo peak. The spectrum would look much better if we could remove all the 99Tc contamination and veto more βs, which would also help by getting rid of the 100Ru x rays that come with the βs.
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Current Efforts
The goal is to obtain a much cleaner spectrum, via vetoing βs and avoiding 99Tc contamination. To get rid of the contamination, we plan to return to Jyväskylä and use their trap (JYFLTRAP) in addition to IGISOL. The trap's resolving power should eliminate all 99Tc.
Previously, we had a planar plastic scintillator between the radioactivity and an HPGe detector. This setup can veto less than 50% of βs, at most. We're currently designing a three-dimensional scintillator into which we'll deposit the activity, allowing us to veto >90% of βs. The sketch below this paragraph shows the scheme. A cylindrical cavity in a cube of scintillator allows implantation of the pure 100Tc beam from the trap onto a foil near the opposite end of the cube. The HPGe detector abuts the cube. This way, we can both maximize the solid angle of the Ge detector and veto >90% of βs.
More to come...
Questions? Comments? Useful suggestions? E-mail skykiloWHENCE?u.washington.edu
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