Now a total of 14 laboratories around the world are running the titanium-50/californium-249 experiment, all looking for the first evidence of element 120. There have been three major centres involved in the discovery of past elements: that in Russia, the Lawrence Livermore, California, and the GSI in Darmstadt, Germany. But to raise the probability of success, the focus is on the even-numbered element 120. Similarly, an isotope of element 119 might be expected from the impact of titanium-50 on berkelium-249. Unfortunately, it would be on the low side of the feasible proton:neutron ratio, so the half-life of such a nucleus would be very low - probably in the microsecond range. Therefore impacting californium-249, it should enable the synthesis of an isotope of element 120. It has the same magic number of neutrons as calcium-48 (thus it is “singly magic”) with two more protons. Titanium-50 (abundance 5.2%) has been proposed. So the only alternative is to find a more massive projectile. Such a procedure would not be as easy with, for example, einsteinium with the longest-lived isotope of 1.3 years. It took four months of bombardment to finally have a suitable impact to form Uuo-294 at which time most of the californium still existed. Californium-249 has a half-life of 351 years. So how can we get any farther? We have run out of long-lived targets. Some neutrons are always lost in the process, these carrying away much of the excess energy from the impact: On to element 119 and 120? Instead of californium-249, berkelium-249 was used as the target, while calcium-48 was again the projectile. The greater difficulty of synthesizing element 117 compared to 118 is largely explicable in terms of the nucleus containing an odd number of protons. It was not until 2010 that atoms of element 117 were synthesized. The research was undertaken at the Flerov Laboratory of Nuclear Reactions in Dubna, Russia. The first synthesis of element 118 was accomplished in 2002 - one atom, in fact - followed by two more atoms in 2005. Also, the filled nucleon shells confer added stability to the nuclei as projectiles. But calcium also possesses the next doubly-magic isotope, calcium-48 (0.2% natural abundance) with 28 neutrons, giving a comparatively high ratio, that of 1:1.4 - making that isotope favoured for such syntheses. Of course, we can explain the stability of that isotope in terms of the nucleus being “doubly-magic,” with 20 protons and 20 neutrons. The common isotope, calcium-40 (96.9% natural abundance), has an abnormally low proton:neutron ratio for its location in the periodic table, that of 1:1. Calcium is an interesting element from the perspective of nuclear chemistry. To synthesize the later Period 7 elements, calcium-48 has been the projectile of choice. Thus in seeking a projectile, we need an isotope with an exceptionally high proportion of neutrons. For nucleons, filled shells are attained with 2, 8, 20, 28, 50, 82, and 126 particles (these are referred to as “magic numbers”). Magic numbersĪnother important point is that the rules governing the shell-filling are different for nucleons from those for electrons. In addition, elements with even numbers of protons tend to have many stable isotopes, while the odd-number elements have few stable isotopes. In fact, of the 273 stable nuclei, only four have odd numbers of both neutrons and protons. In “normal” chemistry, electron-pairing is important, but for nuclei, proton-pairing and neutron-pairing within the nucleus is even more important. That is, we can consider protons and neutrons filling shells just like we do electrons. Stable element isotopes reflect two phenomena both related to the nuclear shell model of the atom. The preference for even numbers of nucleons One can use a simple model that increasing quantities of neutron “glue” are needed to hold together the positively-charged protons. The reason is that the proton:neutron ratio in stable (and long-lived radioactive) isotopes increases from about 1:1 for the early elements to 1:1.6 for uranium-238. Not only must the projectile have a high enough atomic number, but just as important, it must have as many neutrons as possible. The target element must have as high an atomic (proton) number as possible. The new elements are synthesized by firing beams of one element at a target of another element. When will the first element of Period 8 be synthesized? Which element will it be? Which country will first accomplish the task? Now that each element in Period 7 has been synthesized, these are the questions on the minds of every nuclear chemist and physicist.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |