Cosmic radiation during the solar flare of February 23d and its effect on 14C age measurements

✍ Scribed by Hl. De Vries

• Publisher: Elsevier Science
• Year: 1956
• Weight: 76 KB
• Volume: 22
• Category: Article
• ISSN: 0031-8914
• DOI: 10.1016/s0031-8914(56)80048-7

✦ Synopsis

Cosmic radiation during the solar flare of February 23 d an~i its effect on ~4C age measurements

During the solar flare of February 23d an important increase of cosmic radiation occurred. The most pronounced effect was recorded by the BF3 neutron counter. It detects neutrons produced by a soft component of cosmic radiation in a 10 cm thick lead shield surrounding the counter. Neutrons from external sources are absorbed in a shield consisting of a layer of 20 cm of paraffin wax. Normally the counting rate is 15 c/rain, but the average count between 21.30 G.M.T. Febr. 22 and 7.20 Febr. 23 was 55 c/rain. Since this counter is used only as a check on irregular fluctuations in the neutron production by cosmic radiation, no intermediate readings during the night have been made. In Amsterdam the on-set of the increased activity was observed at about 3.45 G.M.T., ten minutes after the flare had been observed in Tokio. Consequently our average count between 3.45 and 7.20 is nine times normal. But since the counting rate at 7.20 G.M.T. (or 8.20 local time) was only twice the normal value the peak value must have been much higher than nine times normal. The main peak of the soft component in Amsterdam lasted only 45 minutes. By assuming the same time course for the neutron producing component it is possible to estimate a peak value of about 27 times the normal activity. In the course of the day the counting rate gradually decreased to the normal level. The extra effect decreased by a factor of about two every two hours.

The three CO 2 counters at 3 atm used for 14C age measurements are shielded by 38 cm of iron and 20 cm of paraffin wax on top of the iron. The total count, between 3.45 and 7.20, showed an average increase of about 12 per cent. The medium sized counter showed arise of 18 c/min from 155 c/rain to 173 c/min. The rise in t~e residual activity, not suppressed by the anticoincidence ring, however, was appreciable. Norreally the medium sized counter would have counted 1.6 c/min after subtracting the effect of the radiocarbon, but between 3.45 and 7.20 the average count was now 9.1 c/rain. In the other two counters the increase of the background was larger, or smaller, in proportion to the size of the counters.

According to an earlier analysis 1)2) the normal background is mainly due to neutrons or to gamma's produced by capture of these neutrons, it is reasonable to assume that the present increase of the background is also due to neutrons. However, the neutron count in the BF 3 counter increases 9.0 times whereas the corresponding factor for the background is only 5.7. This difference may be due to absorption of the neutron producing component in the iron shield.

When the whole pattern of effects of the solar flare is taken into consideration, protons seem to be the most probable neutron producing component present. These protons are probably absorbed in the 38 cm of iron. This may be deduced from the fact that out of the total increment of 18 c/min in the 14C counter only 10.5 c/min are due to charged particles suppressed by the anticoincidence ring. This number of particles is not sufficient to produce 7.5 c/rain, due to neutrons, because of the small neutron efficiency of the counter. Thus the neutrons are produced in the top layer of the iron shield which supports the assumption of the previous paragraph.

The importance of continuous neutron monitoring is illustrated by the fact that if the effect of the solar flame would not have been taken into account, the age calculated for the sample in the counter considered would have been 6000 years instead of the correct 12000 years.