Mass and Half-Life ofC9

Abstract

The mass of C has been measured by counting delayed protons near the Be('He, n)'C threshold. The data are consistent with an s-wave threshold at E3H, =8980 +5 keV, giving a C mass excess of 28907+4 keV, and confirming the reported deviation of the A=9, lowest T = 2 quartet from the quadratic mass formula. The half-life of ' C. measured with a multiscaler, is 126.5~2 msec.The ground states of 'C and 'Li together with the T = & excited states at 14.392 MeV in 'Be and at 14.655 MeV in 'B have been the subjects of considerable investigation because they form one of the few presently accessible isospin quartets. This experiment was undertaken to attempt to verify and improve the measurement of the 'C mass.The experimental method of deducing that quantity from the threshold energy of the 'Be('He, n)'C reaction had been used previously by Barnes et al. 'A 'BeO target, prepared for an earlier experiment' by evaporating under vacuum a-', -in. -diam spot of 'BeF, onto a 1.2-mm-thick platinum bar and then oxidizing the layer in air, provided a surfa, ce density of 7x10" 'Be atoms per cm'. This number represents about half of the 'Be present when the target was first prepared, the other half having decayed to 'Li, which, along with the oxygen and other impurities, contributed to a measured target thickness of 10 keV for the 9-MeV 'He beam supplied by the Office of Naval Research-California Institute of Technology tandem Van de Graaff accelerator near the reaction threshold.The target was mounted in a chamber' equipped with a solenoid-operated arm capable of switching the target between a beam line and a counting position in an 873-msec cycle. For the first 290 msec of each cycle, the target spot was exposed to --, ' p. amp 'He. If 'C was formed, it would be expected to P' decay into the broad particle-unstable states of 'B, which almost immediately break up into a proton plus 'Be. Thus, in the second part of the cycle, after the beam had been deflected, the target was moved directly in front of a~+E telescope consisting of an 11and a 26p, m silicon detector, with a geometrical efficiency of -15%. This arrangement of counters was necessary in order to be able to sort out the protons accompanying 'C decays from the intense background of 478-keV y rays emitted by the 50 mCi of 'Be, as well as from the n particles and other activity induced by the 'He bombardment. When a pulse from the~counter consistent with a proton energy . loss of 0.27-0.55 MeV arrived in coincidence with a pulse from the E counter large enough to be distinguished from the yray background, the sum was recorded by a multichannel analyzer. The natural radioactivity of the target restricted both the choice of detector volume and the smallest (largest proton energy) that could be accepted. Two successive counting periods of 229 msec each were recorded separately to provide a check of the half-life. Figure 1 shows the sum of the spectra obtained during the 16 threshold runs. The broad group of protons with 0.75-3.0-MeV energy loss in the counters is clearly separ...