New attempts to understand nanodiamond stardust

We report on a concerted effort aimed at understanding the origin and history of the pre-solar nanodiamonds in meteorites including the astrophysical sources of the observed isotopic abundance signatures. This includes measurement of light elements by secondary ion mass spectrometry (SIMS), analysis of additional heavy trace elements by accelerator mass spectrometry (AMS) and dynamic calculations of r-process nucleosynthesis with updated nuclear properties. Results obtained indicate: a) there is no evidence for the former presence of now extinct 26Al and 44Ti in our diamond samples other than what can be attributed to silicon carbide and other "impurities"; this does not offer support for a supernova (SN) origin but neither does it negate it; b) analysis by AMS of platinum in "bulk diamond" yields an overabundance of r-only 198Pt that at face value seems more consistent with the neutron burst than with the separation model for the origin of heavy trace elements in the diamonds, although this conclusion is not firm given analytical uncertainties; c) if the Xe-H pattern was established by an unadulterated r-process, it must have been a strong variant of the main r-process, which possibly could also account for the new observations in platinum.Comment: Workshop on Astronomy with Radioactvities VII; Publications of the Astronomical Society of Australia, accepte

Needle in a Haystack: Finding Two S-band CubeSats in a Swarm of 64 Within 24 Hours

In the past few years we have witnessed the ascension of rideshare missions, breaking records again and again for the total number of satellites released on a single launch. Such large swarms of spacecraft make it difficult for the Combined Space Operations Center (CSpOC) to identify satellite orbits until days to weeks after launch. For the SSO-A launch in December 2018, it took 11 days to catalog all 64 objects. While satellites should be designed to survive without ground contact for that long, for most missions, making contact and assessing vehicle state of health during early orbit operations is critical, and waiting for object cataloging is simply too risky. Furthermore, as CubeSats take on more operational roles, the amount of data needed to both uplink and downlink requires moving away from the traditional L-band frequencies to S-band and higher. While higher frequency bands allow faster data transmission it comes at a cost of smaller ground antenna footprints, requiring an order of magnitude better pointing knowledge in order to establish communications lock. With typical canister ejection speeds, spacecraft can drift away from the launch vehicle, whose orbit is typically known and provided by the launch integrator. Depending on ground antenna size, this implies the spacecraft will no longer be in the ground antenna field of view within a day or so of launch. This makes establishing communications with the spacecraft within the first 24 hours after launch paramount. This paper discusses how the ORS-7/DHS Polar Scout mission successfully achieved contact with its two 6U CubeSats and determined their orbital ephemerides in less than 24 hours after launching on the SSO-A mission on December 3, 2018. We present our spacecraft acquisition plan, which encompassed a number of different strategies that can be employed depending on the capabilities and equipment at the ground site

Ultrasensitive search for long-lived superheavy nuclides in the mass range A=288 to A=300 in natural Pt, Pb, and Bi

Theoretical models of superheavy elements (SHEs) predict a region of increased stability around the proton and neutron shell closures of Z = 114 and N = 184. Therefore a sensitive search for nuclides in the mass range from A = 288 to A = 300 was performed in natural platinum, lead, and bismuth, covering long-lived isotopes of Eka-Pt (Ds, Z = 110), Eka-Pb (Z = 114), and Eka-Bi (Z = 115). Measurements with accelerator mass spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA) established upper limits for these SHE isotopes in Pt, Pb, and Bi with abundances of <2×10-15, <5×10-14, and <5×10-13, respectively. These results complement earlier searches for SHEs with AMS at VERA in natural thorium and gold, which now amounts to a total number of 37 SHE nuclides having been explored with AMS. In none of our measurements was evidence found for the existence of SHEs in nature at the reported sensitivity level. Even though a few events were observed in the window for Ek293a-Bi, a particularly strong pileup background did not allow a definite SHE isotope identification. The present result sets limits on nuclides around the center of the island of stability, essentially ruling out the existence of SHE nuclides with half-lives longer than ∼150 million years

Upper limits for the existence of long-lived isotopes of roentgenium in natural gold

A sensitive search for isotopes of a superheavy element (SHE) in natural gold materials has been performed with accelerator mass spectrometry at the Vienna Environmental Research Accelerator, which is based on a 3-MV tandem accelerator. Because the most likely SHE in gold is roentgenium (Rg, Z=111), the search concentrated on Rg isotopes. Two different mass regions were explored: (i) For the neutron-deficient isotopes Rg261 and Rg265, abundance limits in gold of 3×10-16 were reached (no events observed). This is in stark contrast to the findings of Marinov, who reported positive identification of these isotopes with inductively coupled plasma sector field mass spectrometry in the (1-10)×10-10 abundance range. (ii) Theoretical models of SHEs predict a region of increased stability around the proton and neutron shell closures of Z = 114 and N = 184. We therefore investigated eight heavy Rg isotopes, ARg, A=289, 290, 291, 292, 293, 294, 295, and 296. For six isotopes no events were observed, setting limits also in the 10-16 abundance range. For Rg291 and Rg294 we observed two and nine events, respectively, which results in an abundance in the 10-15 range. However, pileup of a particularly strong background in these cases makes a positive identification as Rg isotopes-even after pileup correction-unlikely

Stellar and thermal neutron capture cross section of ⁹Be

The neutron capture cross section of ⁹Be for stellar energies was measured via the activation technique using the Karlsruhe Van de Graaff accelerator in combination with accelerator mass spectrometry at the Vienna Environmental Research Accelerator. To characterize the energy region of interest for astrophysical applications, activations were performed in a quasistellar neutron spectrum of kT = 25 keV and for a spectrum at En = 473 ± 53 keV. Despite the very small cross section, the method used provided the required sensitivity for obtaining fairly accurate results of 10.4 ± 0.6 and 8.4 ± 1.0 μb, respectively. With these data it was possible to constrain the cross section shape up to the first resonances at 622 and 812 keV, thus allowing for the determination of Maxwellian-averaged cross sections at thermal energies between kT = 5 and 100 keV. In addition, we report a new experimental cross section value at thermal energy of σth = 8.31 ± 0.52 mb.This work was partly funded by the Austrian Science Fund (FWF), Projects No. P20434 and No. I428, and by the Australian Research Council, Projects No. DP140100136 and No. DP180100496

Measurement of the radiative capture cross section of the s-process branching points 204Tl and 171Tm at the n-TOF facility (CERN)

The neutron capture cross section of some unstable nuclei is especially relevant for s-process nucleosynthesis studies. This magnitude is crucial to determine the local abundance pattern, which can yield valuable information of the s-process stellar environment. In this work we describe the neutron capture (n,γ) measurement on two of these nuclei of interest, 204Tl and 171Tm, from target production to the final measurement, performed successfully at the n_TOF facility at CERN in 2014 and 2015. Preliminary results on the ongoing experimental data analysis will also be shown. These results include the first ever experimental observation of capture resonances for these two nuclei.The authors acknowledge financial support by the Spanish FPA2014-52823-C2-2-P project, by the EC Marie Curie Action “NeutAndalus” (FP7-PEOPLE-2012-CIG- 334315), by the ARGOS scholarship of the Spanish Nuclear Safety Council (CSN) and the Universitat Politècnica de Catalunya, and by the University of Sevilla via the VI PPIT-US program

Determination of the stellar (n,gamma) cross section of 40Ca with accelerator mass spectrometry

The stellar (n,gamma) cross section of 40Ca at kT=25 keV has been measured with a combination of the activation technique and accelerator mass spectrometry (AMS). This combination is required when direct off-line counting of the produced activity is compromised by the long half-life and/or missing gamma-ray transitions. The neutron activations were performed at the Karlsruhe Van de Graaff accelerator using the quasistellar neutron spectrum of kT=25 keV produced by the 7Li(p,n)7Be reaction. The subsequent AMS measurements were carried out at the Vienna Environmental Research Accelerator (VERA) with a 3 MV tandem accelerator. The doubly magic 40Ca is a bottle-neck isotope in incomplete silicon burning, and its neutron capture cross section determines the amount of leakage, thus impacting on the eventual production of iron group elements. Because of its high abundance, 40Ca can also play a secondary role as "neutron poison" for the s-process. Previous determinations of this value at stellar energies were based on time-of-flight measurements. Our method uses an independent approach, and yields for the Maxwellian-averaged cross section at kT=30 keV a value of 30 keV= 5.73+/-0.34 mb.Comment: 8 pages, 3 figure