Nuclear Seminar

Direct measurement of ⁴He(¹²C,¹⁶O)g cross section near stellar energy at KUTL

Professor Kenshi Sagara
Kyushu University Tandem Laboratory
Fukuoka, Japan

Monday, March 19, 2007   4:00 p.m.   NSH 124
(Refreshments served prior to seminar in NSH 124)

As is well known, ⁴He+¹²C®¹⁶O+g reaction is very important in helium burning in stars, however, the reaction rate at the stellar energy of E_cm = 0.3 MeV has been measured yet in spite of many attempts in the world for about 40 years.  Due to resonances in ¹⁶Othe cross section (S-factor) varies sharply around 0.3 MeV.  Hence we have to measure the cross section at energy as low as possible, e.g., down to 0.7 MeV where the cross section is about 1 pbarn. 

In Kyushu University tandem accelerator laboratory (KUTL), direct measurement of ⁴He+¹²C ® ¹⁶O+g reaction cross section is in progress.  A ¹²C beam is injected onto a ⁴He windowless target, and all the ¹⁶O recoils in a charge state are separated from the ¹²C beam by a recoil mass separator and are detected by a Si detector.

To measure the ⁴He+¹²C®¹⁶O+g cross section down to 0.7 MeV, we need 1) to increase the beam intensity, target thickness and detection efficiency, because ¹⁶O counts at 0.7 MeV are estimated as about 5 counts/day at most, and 2) to thoroughly reduce backgrounds, because the number of ¹⁶O recoils at 0.7MeV is 10^-18 smaller than the number of ¹²C beam particles.

I will talk on several methods and instruments at KUTL, such as a) a blow-in type windowless gas target and its cooling, b) a new accel-decel strong-focus operation of the tandem accelerator, c) a recoil mass separator, and d) a long-time chopper to reduce backgrounds by 10^-3.