Thursday, 15 January 2015

Samarium from the stars

Over the Christmas period, I enjoyed time with my family, sharing presents, eating more food than normal, and suchlike.  One of the perks of my job is that the University shuts down between Christmas and new year, and we are not expected to be in the office.  Not so the stakhanovites at the American Physical Society.  They beavered away producing a regular weekly edition of Physical Review Letters, and the edition for the week ending 31st Dec came out as usual, with online articles being populated during the week in question. 

One of the articles in that edition is entitled Isomer Decay Spectroscopy of 164Sm and 166Gd: Midshell Collectivity around N=100.  It may not sound like the catchiest title, but it does the job, and its lead author is none other than University of Surrey PhD student Zena Patel.

The purpose of the work was to explore nuclei in the large unknown region of the nuclear chart - the rather neutron rich isotopes of elements in the middle of the mass range.  Isotopes that only ever exist in nature fleetingly in supernovae or other fast astrophysical processes.  The elements mentioned in the title, Sm = Samarium and Gd = Gadolinium, both exist on the earth, but only in certain isotopic forms.  Samarium has atomic number 62, so the nuclei have 62 protons.  They can in principle have any number of neutrons to help bind the nucleus together, but only certain quantities are sufficiently stable to last long enough to be found on Earth.  The most common isotope is 152Sm (having 90 neutrons because 90+62=152), which is observed to be stable, and 154Sm (92 neutrons) is also observed to be stable.  The isotope made in the experiments described in the paper is 164Sm, with 102 neutrons.  They make it not by trying to add neutrons to lighter isotopes, but by smashing uranium nuclei against a target, causing them to split up, and selecting those cases in which one of the fragments is a isotope of 164Sm.

The fragments are made in highly excited states, which soon decay to their ground state by emitting gamma rays.  Only, sometimes they don't emit those gamma rays quite so quickly, if the nuclei get caught in isomeric states -- states effectively defined as those that don't decay as quickly as you might expect.  The reasons usually translate into details of the structure of those states;  they could be spinning in such a way that makes them a bit more stable, for example.  My colleagues here at Surrey, with their collaborators from around the world, not least at RIKEN in Japan where the experiment took place, deduced from the experiments that having 100 neutrons conferred an extra stability to nuclei in that region corresponding to what we call a "magic" number.  Their results help explain why certain elements are more common than others;  all to do with how nucleosynthesis processes work in stars.  They worked it out not by going to stars and making experiments there, but by going to Japan.

The researchers here at Surrey, with the help of the marketing department, did a good job of writing a press-release that got picked up by a lot of places.  The snapshot attached to this post is from a YouTube channel called DNews, who presented a story about the research.

citation details are below:

Patel, Z., Söderström, P., Podolyák, Z., Regan, P., Walker, P., Watanabe, H., Ideguchi, E., Simpson, G., Liu, H., Nishimura, S., Wu, Q., Xu, F., Browne, F., Doornenbal, P., Lorusso, G., Rice, S., Sinclair, L., Sumikama, T., Wu, J., Xu, Z., Aoi, N., Baba, H., Bello Garrote, F., Benzoni, G., Daido, R., Fang, Y., Fukuda, N., Gey, G., Go, S., Gottardo, A., Inabe, N., Isobe, T., Kameda, D., Kobayashi, K., Kobayashi, M., Komatsubara, T., Kojouharov, I., Kubo, T., Kurz, N., Kuti, I., Li, Z., Matsushita, M., Michimasa, S., Moon, C., Nishibata, H., Nishizuka, I., Odahara, A., Şahin, E., Sakurai, H., Schaffner, H., Suzuki, H., Takeda, H., Tanaka, M., Taprogge, J., Vajta, Z., Yagi, A., & Yokoyama, R. (2014). Isomer Decay Spectroscopy of Sm164 and Gd166: Midshell Collectivity Around N=100 Physical Review Letters, 113 (26) DOI: 10.1103/PhysRevLett.113.262502

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