R.C. Finkel

Measurements of 36Cl in Antarctic meteorites and Antarctic ice using a Van de Graaff accelerator

Abstract Cosmic-ray produced 36 Cl( t 1/2 = 3.0 × 10 5 years) has been measured in four Antarctic meteorites and one sample of Antarctic ice using a tandem Van de Graaff accelerator as an ultrasensitive mass spectrometer with the extremely low background level of 36 Cl/Cl −16 . Results from this ion counting technique (applied here to extraterrestrial materals for the first time) are used to support a two-stage irradiation model for the Yamato-7301 and Allan Hills-76008 meteorites and to show a long terrestrial age (0.7 ± 0.1 m.y.) for Allan Hills-77002. Yamato-7304 has a terrestrial age of less than 0.1 m.y. The 36 Cl content of the Antarctic ice sample from the Yamato Mountain area implies that the age of the ice cap at this site is less than one 36 Cl half-life.

Depth profile of53Mn in the lunar surface

Abstract Measurements of cosmic-ray produced 53 Mn are reported for a series of lunar surface samples down to a depth of 416 g/cm 2 . These results clearly illustrate the decrease in activity with depth as the incident galactic cosmic rays are absorbed. Below 60 g/cm 2 the production rate decreases exponentially with a mean length, λ, of about 220 g/cm 2 . These results indicate that, at the Apollo 15 site, the lunar regolith has been unmixed, on a meter scale, for the last 5 my. The neutron activation technique for 53 Mn, which allowed samples smaller than 200 mg to be used for these measurements, is described.

Pattern of Bombardment-Produced Radionuclides in Rock 10017 and in Lunar Soil

A large number of radionuclides have been measured as a function of depth in lunar rock 10017 and in bulk fines. Data are reported on 10Be, 22Na, 26Al, 36Cl, 49V, 53mn, 54Mn 55Fe, 56Co, 57Co, and 59Ni and on upper limits for 46Sc, 48V, 51Cr, and 60Co. The results for several nuclides show striking evidence of excess surface production attributable to solar flare particles. Data for short-lived species, 56Co, 57CO, 54Mn, 55Fe, and 22Na, appear consistent with fluxes from known recent events. Long-lived species demonstrate the existence of solar flare protons and alphas at least for the last 105 to 106 years, at fluxes comparable to those now observerved.

Cosmic-ray-produced36Cl and53Mn in Allan Hills-77 meteorites

Abstract Cosmic-ray-produced53Mn (t1/2 = 3.7 × 106years) has been determined by neutron activation in nine Allan Hills-77 meteorites. Additionally,36Cl (1/2 = 3.0 × 105years) has been measured in seven of these objects using tandem accelerator mass spectrometry. These results, along with14C (t1/2 = 5740years) and26Al (7.2 × 105 years) concentrations determined elsewhere, yield terrestrial ages ranging from 0.1 × 105 to 7 × 105 years. Weathering was not found to result in53Mn loss.

226Ra,210Pb and210Po in the Red Sea

Abstract Profiles of 226 Ra and dissolved 210 Pb have been measured at several stations in the Red Sea. At one station in the central Red Sea an expanded profile was measured including 226 Ra and dissolved and particulate 210 Pb and 210 Po. These profiles show several distinct features: (1) 226 Ra displays a mid-depth maximum of about 13 dpm/100 kg at about 500 m; (2) dissolved 210 Pb concentrations are uniformly low at about 2 dpm/100 kg with little lateral or vertical variation; (3) the surface-water 210 Pb excess which is commonly observed in low-latitude open ocean regions is entirely lacking; (4) 210 Pb and 210 Po activities are essentially identical to each other in both particulate and dissolved phases although 210 Po activities appear somewhat lower; (5) about 20% of the 210 Pb and 210 Po in the water column residues on particulate matter. Assuming the atmospheric 210 Pb flux to be in the dissolved form and at the lower level of the normal range i.e. 0.5 dpm/cm 2 yr, the residence time of the dissolved Pb is about 1.5 years. However, if the same atmospheric flux is entirely in particulate form, then the residence time of the dissolved Pb is about 5 years. The residence time of Pb in the particulate phase is less than 0.4 years if all the Pb is removed only by sinking particles.

60Co in lunar samples

Abstract Concentrations of the (n, γ)-produced radionuclide 60 Co were measured in lunar samples at various depths from the surface down to 360 g/cm 2 . By comparing the data obtained to calculated production rates (based on the work of Lingenfelter et al. [8]) we determined the present day lunar neutron production rate, which was found to be (12 ± 3)neutrons/cm 2 sec ( E

Radon emanation from the lunar surface

Abstract The radon daughter 210 Pb in surface samples of lunar soil and rock is in equilibrium with its parent uranium to within five percent. The lack of a surface excess in these samples, if real, implies an effective diffusion coefficient for radon of less than 10 −6 that observed in terrestrial soils.