IL NUOVO CIMENTO VOL. 110 A, N. 9-10 Settembre-Ottobre 1997
Experimental situation in the field of cluster radioactivity
()
A. GUGLIELMETTI
Istituto di Fisica Generale Applicata dell’Universit`a di Milano INFN, Sezione di Milano, I-20133 Milano, Italy
(ricevuto il 31 Luglio 1997; approvato il 15 Ottobre 1997)
Summary. — A brief overview of the experiments performed up to now in the field of
cluster radioactivity is given, together with a description of the most widely used exper-imental techniques. The still unsolved problems and the possible related experiments are presented. A very recent result, connected to one of these open problems, is also discussed.
PACS 23.70 – Heavy-particle decay. PACS 01.30.Cc – Conference proceedings.
1. – Introduction
Cluster radioactivity has very intensively been studied both experimentally and theo-retically during the last thirteen years. Twenty cases of spontaneous emission of clusters, ranging from14
C to32
Si, from trans lead nuclei have been discovered, with partial half-lives from 1011
up to 1028
s and branching ratios relative to decay from 10 ,9
down to 10,16
[1]. The general features of this new radioactive decay mode have already been established, above all its strong dependence on the barrier penetration factor and conse-quently on theQ-value, which must be large in order to compensate for the small
prefor-mation factors typical of such complex clusters [2]. For this reason, all heavy residual nuclei resulting from cluster emission have been found so far to differ from the doubly magic208
Pb by three nucleons at most.
In this paper, after a brief description of the experimental techniques used in this field, both for sources and for detectors, I will concentrate on the unsolved problems and I will describe the measurements which are already being performed or which could be done in the next future to clarify these aspects and to reach a better comprehension of this new kind of radioactivity. Moreover, I will present the most recent result on cluster emission from114
Ba [3] and compare it with previous measurements [4, 5].
(
)Paper presented at the 174. WE-Heraeus-Seminar “New Ideas on Clustering in Nuclear and Atomic Physics”, Rauischholzhausen (Germany), 9-13 June 1997.
1040 A. GUGLIELMETTI
2. – Experimental techniques
2.1. Sources. – Two different types of sources can be used in experiments on cluster radioactivity: ”traditional sources” and ion-implanted ones. ”Traditional” sources are ob-tained by chemical separation and, generally, by electrodeposition of the active material and allow to perform experiments ”off-line”. On the other hand, ion-implanted sources are mostly used for experiments ”on-line” which are unavoidable when the half-life of the nuclide to be studied is short. Common features to all sources are a rather strong activity (up to few mCi), necessary for compensating the rarity of this phenomenon, a high purity or at least a very well known isotopic composition, to be able to disentangle contributions in cluster emission from adjacent isotopes, and a relatively low thickness (typically1
mg/cm2
), to enable clusters of2–2.5 MeV/a.m.u. to escape from the source with enough
energy to be detected and identified.
2.2. Detectors. – Small decay constants and small branching ratios relative to decay
demand for a high efficiency and high selectivity detecting apparatus. High selectivity means high rejection power of the huge flux of particles accompanying the rare events
searched for. So far, the most used detection technique has been the one of solid state nuclear track detectors (SSNTD). They are generally plastics or glasses which are able to record latent tracks of heavy ions whose reduced energy loss is above a given threshold, characteristic of the detector itself. A chemical etching is necessary for enlarging this damage up to dimensions visible under an optical microscope. What happens is that, while the non-irradiated surface is etched at a uniform velocityV
G, etching along the particle
trajectory might proceed at a higher velocity VT. In this situation, competition between
the two velocities produces a conical track appearing as a dark spot in the bright field of an optical microscope. The physical basis underlying particle identification by means of SSNTDs is the dependence ofV
T on the particle reduced energy loss which, in turn, is
related by means of the Bethe formula to the particle charge and mass numbers and to its energy. In practice, identification is achieved by measuring the sensitivityS =V
T =V
Gat
a given value of the particle range. This is essentially the same method as theE–Eone
on which a silicon detector telescope is based. The high flux of particles can be rejected
simply by choosing detectors which are not sensitive to alphas. Moreover these cheap plates can be tailored in whatever shape and a very high (100%) geometrical efficiency
apparatus can be built. The main disadvantage of such a technique lies in the poor energy resolution (1–2 MeV for ions with E'30–80 MeV) which does not allow fine structure
measurements in the decay spectra. The mass resolution (2–3 units forA =12–30) is
also not enough to distinguish between adjacent isotopes; usually the mass attribution is achieved on the basis of the maximumQ-value criterion.
3. – Open problems and recent results
As already outlined in the Introduction, all heavy residual nuclei resulting from cluster emission have been found so far to lie in the vicinity of208
Pb. Analogously, a new island of cluster radioactivity having residual nuclei close to the doubly magic100
Sn was predicted as early as 1989 by Greiner et al. [6]. In the subsequent years, two different experimen-tal groups have tried to measure the decay mode which was predicted to be the most favorable in this mass region, namely the12
C emission from114
Ba [4, 5]. However, due to different experimental difficulties and in particular to the presence of a background on the track detectors, both measurements gave only upper limits on the branching ratio for
EXPERIMENTAL SITUATION IN THE FIELD OF CLUSTER RADIOACTIVITY 1041
12
C emission, 3.810 ,4
and 10,4
, respectively, even if a few carbon tracks were detected in both cases. In view of this unclear and unsatisfactory situation, the measurement was very recently repeated [3] with improvements on the experimental conditions. By using the GSI on-line mass separator and a high temperature cavity source with on-line fluori-nation, it was possible to obtain a pure114
Ba beam out of the58
Ni(58
Ni,2n) reaction. The beam was then implanted, for almost one week, onto a thin catcher placed in the center of a spherical array of glass plates, used as track detectors. Before the irradiation, the detec-tors were heated for 32 h at250
C in order to completely eliminate any preexisting track. With such a procedure, called annealing, the experiment was run with background-free detectors. After having been etched, the glass plates were analyzed twice under an op-tical microscope and no evidence for12
C events was found, resulting in an upper limit of 3.610
,5
for the branching ratio. When comparing this result with the previous ones [4, 5], one finds consistency levels of only 7% and 510
,3
%, respectively. Such low values leave little room for a claim that this decay mode had actually been observed in previous mea-surements. In spite of all these efforts, cluster radioactivity around100
Sn is still an open problem which will probably remain unsolved until new experimental techniques for the production of a much more intense114
Ba beam will become available.
Another very important question to be addressed is the emission of heavy clusters. So far, the heaviest measured cluster is32
Si but an experiment aimed at measuring 34
Si emission from242
Cm is already being performed by a collaboration between Dubna and Milano. The interest in studying such heavy emissions lies in the possibility of discrim-inating among different theoretical approaches to the phenomenon. As a matter of fact, models describing cluster radioactivity as a generalization of decay (with preformation
of the cluster before its emission) [7] or as a very asymmetric cold fission [8], do give significantly different predictions only for clusters with mass numbers greater than 30. Unfortunately, the most probable emitters of such heavy clusters are all spontaneously fissioning nuclides such as transuraniums. Here, the problem of separating the clusters searched for from the background of similar mass fragments produced by ternary fission becomes very serious, even if the energies involved are different. In the experiment on
242
Cm, for solving this problem the track detectors were covered with a layer of polymide thick enough to stop binary and ternary fission fragments while allowing more energetic clusters to go through and be revealed. The analysis of these detectors is still in progress. The last, still open problem I will concentrate on is the fine structure in cluster decay. In close analogy with what happens for decay, also in cluster radioactivity the heavy
resid-ual fragment can be found in an excited state, even if this situation is not favoured from a mere energetical point of view, since part of theQ-value is lost in excitation energy of the
fragment itself. On the other hand, the similarity between single particle wave functions describing the unpaired particle in the mother and daughter nuclei can strongly influence the decay lifetime. In the case of14
C emission from223
Ra [9], it has been found indeed that the decay populates the first excited state of the residual nucleus with a80%
proba-bility. Another interesting case to be investigated could be14
C emission from225
Ac. This decay was already measured in 1993 [10] using track detectors, whose energy resolution is not enough to allow fine structure measurements, as already pointed out in section 2.2. A new measurement could be performed- and it is already planned by a collaboration among Milano, Cern and Orsay- by using the superconducting spectrometer Soleno (IPN, Orsay) in conjunction with aE–Esilicon telescope of150 keV energy resolution. By looking
at which level of211
Bi is populated by this decay, one could derive information also on the so far unknown ground state wave function of225
Ac. Cluster decay could be used as a spectroscopic tool.
1042 A. GUGLIELMETTI
REFERENCES
[1] BONETTIR. and GUGLIELMETTIA., in Nuclear Decay Modes, edited by D. N. POENARU (IOP, Bristol) 1996, pp. 370-392.
[2] POENARUD. N., in Proceedings of the 6th International Conference on Nuclear Reaction Mechanisms, Varenna 1991, edited by E. GADIOLI (Ricerca Scientifica ed Educazione Permanente, Milano) 1991, pp. 348-358.
[3] GUGLIELMETTIA. et al., Phys. Rev. C, 56 (1997) 2912. [4] GUGLIELMETTIA. et al., Phys Rev. C, 52 (1995) 740. [5] OGANESSIANYU. TS. et al., Z. Phys. A, 349 (1994) 341.
[6] GREINERW., IVASCUM., POENARUD. N. and SANDULESCUA., in Treatise on Heavy Ion Science, edited by D. A. BROMLEY, Vol. 9 (Plenum, New York) 1989, pp. 641-722.
[7] BLENDOWSKER., FLIESSBACHT. and WALLISERH., Z. Phys. A, 121 (1991) 1339. [8] POENARUD. N. et al., At. Data Nucl. Data Tables, 48 (1991) 231.
[9] HUSSONNOISM., LEDUJ. F., BRILLARDL. and ARDISSONG., Phys. Rev. C, 42 (1990) R495.
