NEWS AND VIEWS NUCLEAR PHYSICS-------------------------------- case since it has one proton more than the Proton decay at the drip-line magic number Z = 82, corresponding to the closure of a major nuclear shell. In Philip Woods fact the observed proton decay comes from an excited intruder state configura­ WHAT determines whether a nuclear ton decay half-life measurements can be tion formed by the promotion of a proton species exists? For nuclear scientists the used to explore nuclear shell structures in from below the Z=82 shell closure. This answer to this poser is that the species this twilight zone of nuclear existence. decay transition was used to provide should live long enough to be identified The proton drip-line sounds an inter­ unique information on the quantum mix­ and its properties studied. This still begs esting place to visit. So how do we get ing between normal and intruder state the fundamental scientific question of there? The answer is an old one: fusion. configurations in the daughter nucleus. what the ultimate boundaries to nuclear In this case, the fusion of heavy nuclei Following the serendipitous discovery existence are. Work by Davids et al. 1 at produces highly neutron-deficient com­ of nuclear proton decay4 from an excited Argonne National Laboratory in the pound nuclei, which rapidly de-excite by state in 1970, and the first example of United States has pinpointed the remotest boiling off particles and gamma-rays, leav­ ground-state proton decay5 in 1981, there border post to date, with the discovery of ing behind a plethora of highly unstable followed relative lulls in activity. Now we proton decay from 185Bi, which has 24 nuclear species. Only a small fraction of are entering a renaissance of proton drip­ neutrons fewer than the only stable iso­ these, about 10-5 to 10-1, may be the line research, in which we can aim to fully tope of bismuth, 2~i. Nuclear proton decay occurs when 83 there is a severe deficit of neutrons com­ ,., "' 182 ISJ pared with stable nuclear species. Neu­ 82 Pb Pb Pb Pb 178 179 180 Ill 182 trons provide the glue that prevents the 81 Tl Tl Tl Tl Tl nucleus blowing itself apart under the mu­ 175 176 111 171 119 180 181 80 H"'g Hg Hg Hg Hg Hg Hg Hg tual electrostatic repulsion of the protons. 172 173 175 176 111 IO<J IOI If too many neutrons are removed from a Au Au '"Au Au Au Au Au"' 17 1 172 l7J 175 176 99 nucleus it eventually becomes energeti­ Pt Pt Pt Pt"' Pt Pt cally unstable to the spontaneous emis­ 173 'I~ \'l Ir 97 98 sion of a proton, as nature protests against w INJ 95 96 the extremes to which it is being pushed. Os ~ This ultimate limit to nuclear stability is known as the proton drip-line. The time taken for a proton to cross the nucleus is - 10-22 s, and state-of-the­ Number of] art techniques for detecting nuclear pro­ protons, Z ton decay are limited to lifetimes in excess of 10-7 s, so one might ask how it is possi­ Number of ble to study this phenomenon. In the nu­ neutrons,N clear surface region, the combination of the attractive strong nuclear force and the repulsive electrostatic force gives rise to a potential energy barrier that the proton must penetrate for the decay to occur. If a The proton drip-line, as currently established, between thulium and bismuth. The red isotopes decay by proton emission, yellow by ex and 13 only, and pink by 13 only. All the proton emitters nucleus has a large number of protons have odd Z, corresponding to an unpaired, energetically unbound proton. Proton emission from (the atomic number, Z) then this barrier even-Z isotopes has yet to be identified, because attractive pairing interactions push the drip­ can become high enough for the proton to line beyond current isotope production limits. In future it is hoped to observe two-proton emis­ be restrained inside it long enough for the sion from even-Z nuclei, in cases where single-proton emission is energetically forbidden. isotope to be observed. Proton decay therefore represents a proton-unstable isotope of interest. Ultra­ map one of the fundamental limits to beautifully simple example of a quantum sensitive techniques, involving in-flight nuclear stability. Furthermore, new tech­ tunnelling process in which, before electromagnetic separation devices and niques have been developed to study in emerging, the proton must traverse -100 sophisticated detection systems in which detail gamma-ray de-excitation cascades femtometres ( - 10-13 m) of the zone clas­ radioactive ions are implanted before de­ from excited proton-radioactive nuclei6• sically forbidden because its energy is too caying, have been developed to filter out With the advent of radioactive beam low. Ironically, alpha-decay- a more and identify these extremely rare isotopes. facilities allied to these techniques, the familiar and common nuclear decay Consequently, we can now explore ex­ prospects for nuclear drip-line research mechanism - is inherently a much more tended regions of the proton drip-line in look highly promising. D complex process, involving the formation detail for the first time. of 4He clusters in the nuclear surface re­ Particularly dramatic progress has been Philip Woods is in the Department of gion. The simplicity of the proton decay made in the region of odd-Z elements be­ Physics and Astronomy, University of Edin­ mechanism, requiring only the emission of tween thulium (Z=69) and gold (Z=79) burgh, Mayfield Road, Edinburgh EH9 3JZ, a single constituent nucleon, can be ex­ for which a continuous series of proton­ UK. ploited to provide a unique window on the decaying isotopes has been discovered2•3. properties of nuclei at the extreme edge of Most recently, work by Davids et al. has 1. Davids, C. N. et al. Phys. Rev. Lett. 78, 592- 595 (1996). existence. The potential energy barrier is established the current extreme outpost 2. Woods, P. J. et al. Nucl. Phys. A653, 485--488 (1993). critically influenced by the centrifugal po­ for such studies with the identification of 3. Davids, C. N. et al. Proc. Exotic Nuclei and Atomic Conf., 185 Masses Aries, 1995 (in the press). tential experienced by the proton, which proton emission from the isotope Bi, 4. Jackson, K. P. et al. Phys. Lett. B33, 281- 284 makes proton decay rates extremely sensi­ which represents by far the heaviest point (1970). 5. Hofmann, S. et al. Proc. 4th International Conf. Nuclei tive to the orbital angular momentum of at which the drip-line has been crossed. far from Stability CERN 81-09, 190-201 (1981). the proton in the host nucleus. Hence pro- This isotope is a particularly interesting 6. Paul, E. S. et al. Phys. Rev. C61, 7S-87 (1995). NATURE · VOL 381 · 2 MAY 1996 25 .