TRIAC Progress Report - KEK

supernovae explosion [3-11]. Therefore, it is quite important to accurately determine the
cross section at Ecm ≈ 0.3 MeV corresponding to the stellar temperature of interest. The
direct measurement of the cross section at Ecm ≈ 0.3 MeV, however, is not possible with
current experimental techniques, because the cross section is too small, around 10 -17
barn. Hence one has to estimate the cross section by extrapolating the cross sections
measured at energies higher than 1.0 MeV into the region of corresponding energies,
and the extrapolations rely on theoretical calculations that gave scattered results
according to their theoretical approaches. Consequently, the estimated cross section at
Ecm ≈ 0.3 MeV has a large uncertainty, which turned out to be largely due to the poor
experimental determination of the relative strengths of E2- to E1-radiative capture
reactions (σE2 / σE1) at higher energies.
Recently, it has been shown that the ratio of σE2 / σE1 could be determined with a
good accuracy by using high-efficiency anti-Compton NaI(Tl) spectrometers to detect
the γ-rays from the reaction, where a pulsed α-beam was employed to discriminate true
events from background events induced by neutrons from 13 C(α,n) 16 O reaction with a
time-of-flight (TOF) method and the target thickness and α-beam intensity were
monitored during the measurement by the Rutherford backscattering spectrum of
α-particles from the target [3-12, -13]. In order to provide a stringent constrain to the
extrapolation down to Ecm ≈ 0.3 MeV, the present proposal aims at measuring the ratios
at several energies, where large discrepancies have been observed in the values of the
ratios predicted by various theoretical calculations, by using the experimental set-up in
Ref. 3-13. The measurements would provide good insight into distinguishing between
different calculations, and thereby help reduce the large uncertainty in the
extrapolations.
In the proposed experimental set-up, the use of the pulsed intense α beam is
essential for reducing the background γ-rays induced by neutrons originating from the
13
C(α,n) reaction; the background γ-rays can be removed because they have a delayed
detection time as compared to the prompt γ-rays of interest. The pulsed beam with a
width of less than 10 ns and an interval between 250 ns and 500 ns was developed for
the experiment at the TRIAC. We fabricated a beam-bunching system consisting of a
pseudo saw-tooth wave pre-buncher and a multi-layer chopper, and tested the
pre-buncher with a repetition frequency of 2 MHz using 16 O 4+ and 12 C 3+ beams with the
same mass-to-charge ratio as 4 He 1+ from the TRIAC. A clear bunched structure with a
width of 2 ns in standard deviation was observed at a target position 10 m away from
the exit of TRIAC [3-14]. Incomplete beam bunching, observed as a remainder of the
26-MHz operation of SCRFQ was removed by the operation of the chopper installed
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