Cluster decay of 230U via Ne emission

Nuclear Physics A 686 (2001) 64–70 www.elsevier.nl/locate/npe

Cluster decay of 230U via Ne emission R. Bonetti a,b , C. Carbonini a,b , A. Guglielmetti a,b,∗ , M. Hussonnois c , D. Trubert c , C. Le Naour c a Istituto di Fisica Generale Applicata, Università degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy b Istituto Nazionale di Fisica Nucleare, Sezione di Milano, Milano, Italy c Institute de Physique Nucleaire, Orsay, France

Received 26 September 2000; accepted 10 October 2000

Abstract By using glass track detectors in 4π geometry we measured the branching ratio of 230 U for emission of Ne clusters to be B = (4.8 ± 2.0) × 10−14 and an upper limit for 18 O emission from 226 Th to be B < 3.2 × 10−14 . On the basis of theoretical calculations, the most probable neon cluster comes out to be 22 Ne.  2001 Elsevier Science B.V. All rights reserved. PACS: 23.70.+j Keywords: Cluster radioactivity; 230 U; Solid state nuclear track detectors

1. Introduction After almost two decades of intense experimental investigation, a large amount of data has been accumulated on cluster radioactivity, mainly in the form of partial half lives or branching ratios relative to α decay for 21 nuclides with Z
87 [1]. The general features of such an exotic decay mode as well as its competition with the other two, more usual natural radioactivities such as α decay and spontaneous fission have been reviewed in several occasions [1,2] and will not be repeated here. In particular, the large systematics accumulated on uranium isotopes in recent years (A = 232–236) [1,3] has pointed out the different behavior of the partial half lives for spontaneous fission and cluster radioactivities with the mass number of the parent, the former being practically constant and the latter rapidly varying as a consequence of shell and other structure effects. In view of the additional feature that both spontaneous fission and cluster radioactivity have, for this particular isotopic series, partial half lives of similar order of magnitude, it is interesting to attempt to extend the systematics both from the light and heavy sides. This * Corresponding author. Tel: +39/02/70630854; Fax: +39/02/2665717

E-mail address: [email protected] (A. Guglielmetti). 0375-9474/01/$ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 4 7 4 ( 0 0 ) 0 0 5 0 8 - X

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Table 1 Comparison of experimental branching ratios for 22 Ne emission from 230 U and for 18 O emission from 226 Th obtained in this work with theoretical predictions of different models Bα = λcl /λα

Present work Blendowske et al. [5] Buck et al. [6] Zamyatnin et al. [7] Poenaru et al. [8] Barranco et al. [9,10]

230 U → 22 Ne + 208 Pb

226 Th → 18 O + 208 Pb

(4.8 ± 2.0) × 10−14 6.95 × 10−15 1.06 × 10−14 4.51 × 10−14 7.17 × 10−15 1.30 × 10−15

< 3.2 × 10−14 1.19 × 10−15 – 5.81 × 10−16 1.84 × 10−15 –

paper reports on the measurement of cluster decay of the lightest uranium isotope, 230 U, while the one on the heaviest, 238 U, is still in progress. We want to point out that in the course of preparation of the present paper we became aware of a similar experiment being published by a group of the Chinese Academy of Sciences [4]. However, we will report later on the reasons why we believe the present result continues to have its own value. The well-known sensitivity of the cluster decay rate to the Q-value strongly restricts the number of possible clusters being emitted by a trans-lead nucleus. For 230 U, it is found that the maximum Q-value, 61.40 MeV, is reached for 22 Ne emission (by leading to 208Pb); 24 Ne, the cluster measured for heavier (A = 232–234) U isotopes [1] also looks like a good candidate (Q = 61.36 MeV); other clusters like different isotopes of F or Mg give largely unfavorable Q-values. Several calculations have been performed on 22 Ne decay of 230 U; we have taken into account the cluster model of Blendowske et al. [5], Buck et al. [6] and Zamyatnin et al. [7], the superasymmetric fission model of Poenaru et al. [8] and the superfluid tunneling model of Barranco et al. [9,10] which predict a branching ratio relative to α decay, Bα = λcl /λα in the range 10−14 –10−15 (see Table 1). In order to be able to observe a few cluster decay events, such low branching ratios imply a strong 230U radioactive source, at least in the mCi range.

2. The experiment The preparation of the 230 U sources was similar to the one used in the production of 230 U for the study of 14 C decay of 222 Ra [11,12], its grand-daughter. In long (60 to 80 hours cumulated in 2 or 3 weeks) irradiations of metallic thorium targets, with intense (20 µA) proton beam, 230 Pa (T1/2 = 17.4 days) activities of some GBq were produced by the 232 Th(p, 3n) reaction. 230 U was subsequently produced by the β-branch decay of 230 Pa. An important problem is the presence of 232 Pa, produced in the course of the irradiations by the (p, n) reaction. This is expected to decay (T1/2 = 1.31 d) to 232 U, a well-known 24 Ne

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emitter [1]. However, the experimental technique we used (see below) would not allow to distinguish between such a cluster (from 232 U) and 22 Ne (from 230 U). The problem was therefore solved by starting the radiochemical separation of the Pa fraction only after an 18 to 20 day cooling time, in order to reduce by a factor larger than 104 the 232Pa produced during the last hours of irradiation. On the other hand, 233 Pa does not constitute a problem for our experiment, even if it β-decays to 233 U, another neon emitter. This is due to the very long half-life of this neon emitter (T1/2 = 1.6 × 105 y) which makes the number of its decayed atoms during the exposure time negligible. After dissolution of the target in 12 M HCl, a first step eliminated the bulk of thorium by percolation of the solution through a Dowex 1X-8 anion exchange resin (column sizes: Φ = 6 mm, l = 150 mm). Pa and U fractions and some fission products were fixed at the top of the resin, while Th and many fission products passed through. Successively, protactinium fraction (230Pa and traces of 232 Pa) was recovered by elution with 4.5 M HCl and uranium fraction (230 U and 232 U) was desorbed by 0.2 M HCl. After a purification from fission products, the Pa fraction was stocked. When the 230 U in-growth activity was at its maximum value (18 to 40 days after the last purification), 230 U was separated from 230 Pa on a small anion exchange resin column. Finally it was purified on a column filled with di(2-ethylhexyl) phosphoric acid sorbed on teflon powder from different stable impurities, mainly iron. As a final step 230 U was electroplated, from an ammonium chloride solution, on a platinum disk as a source of 0.5 cm2 area. All steps of the separation were checked by γ -spectroscopy measurements. The radioactive purity of 230 U final fractions was so good that not only its principal γ -rays (72.2, 154.2, 158.2 and 230.4 keV), but also the 235.3 keV line emitted with an intensity of only 0.01% could be measured. The activity of the 230 U sources was determined from the intensities of γ -rays (111.3 keV and 324.0 keV) of its daughters, 226 Th and 222 Ra, rapidly in radioactive equilibrium. As a final check, we can mention that the γ -spectra of the sources, measured 3 to 4 years after their preparation, presented an intense 46.5 keV γ -line due to 210 Pb, long-lived end of the decay chain of 230 U, but no appreciable activity of the 228Th decay chain, which would have been produced by 232 U. The task of detecting a few Ne ions within an enormous background of α particles was accomplished as in previous experiments [1] by solid-state nuclear track detectors. In comparison with similar experiments on cluster radioactivity, the present one was made difficult by the coexistence of a rather low branching ratio together with a relative light cluster (therefore having a relatively low ionizing rate). Indeed, Ne measurements performed up to now resulted in more favorable branching ratios, in the range 10−11–10−13 . On the other hand, low branching ratios like (or even smaller than) the one we are dealing with here are generally typical of heavier clusters, like Mg or Si. In addition, an appealing possibility was to detect an even lighter cluster, 18 O from 226Th in equilibrium with 230U, predicted to be emitted with Bα values of the order of 10−15 (Table 1). The solution to the problem of finding the track detector giving the optimum signal-to-noise ratio for this particular critical case was given by BP1, a phosphate glass

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we largely used in the past, etched under desensitizing conditions. This was achieved by using HF as etching solution instead of the more usual HBF4 [13]. Previous work has shown that such a glass can withstand an α integrated flux of the order of 1012 α/cm2 while allowing to detect ions with Z as low as 6. To maximize the detection efficiency while keeping the α integrated flux to the desired level we arranged the BP1 glass plates in a 4π geometry by covering the inner surface of an aluminum sphere, φ = 17 cm. The detectors were irradiated in two different runs. In the first one, the complete sphere was irradiated with a 1.3 mCi 230 U activity, while in the second one a 0.6 mCi source was used to irradiate only half sphere. Therefore one hemisphere received a total 1.9 mCi activity and the other only 1.3 mCi. The number of 230U atoms decayed were 1.83 × 1014 in the former and 1.25 × 1014 in the latter; the corresponding α integrated flux, taking into account the presence of α emitters in equilibrium in the decay chain were 1.00 × 1012 and 6.88 × 1011 , respectively. After irradiation, glasses were decontaminated with a diluted HNO3 solution and etched in HF, 50%, at 25 ◦ C for 5 h 30 . The task of scanning the large detector surface (≈ 700 cm2 ) was made easier by an automatic scanning device coupled to an image analyzer [14]. The system was calibrated by using accelerator-produced events (19 F with the same specific energy, 2.5 MeV/amu as the ions searched for). This allowed to optimize the crucial image analyzer parameters such as contrast, global brightness, central brightness, eccentricity, area, in order to reduce as much as possible the number of false candidates while keeping the detection efficiency for the decay events as high as possible. The scanning efficiency was checked by comparing the identified events with known accelerator track density and came out to be 100%. The scanning allowed to find 6 events in the complete sphere, 3 in the upper and 3 in the lower hemisphere. For each track a silicone replica was prepared in order to measure the track parameters under an optical microscope at 1000× in reflected light. As usual, identification was made possible by comparing the sensitivity, S = vt /vg measured for each track at several points along the particle trajectory, with accelerator calibrations (see Fig. 1). Here with vt we mean the etching velocity along the track and with vg the general etching velocity (for further details see for instance Ref. [15]). To take into account a possible variation of the track detector sensitivity with alpha background [1] and to allow a more precise identification of the detected events, the accelerator produced 19 F ions were placed on one of the glass samples irradiated with the 1.9 mCi 230 U source. Since neon beams are not available at Tandem accelerators like the one (in Legnaro) we used for our calibrations, the 22 Ne curve had to be extrapolated from the 19 F one by making use of the dependence of the sensitivity on the restricted energy loss rate [15]. The comparison shown in Fig. 1 allows to attribute to Ne the 6 events we found, and none to 0. A possible identification in terms of F clusters suggested by the correspondence of some of the measured sensitivity values with the calibration curve obtained for F ions is ruled out by simple Q-value considerations. Indeed, among F isotopes, even 21 F which leads to the magic 209Bi nucleus as heavy daughter would imply a much less favorable decay Q-value, 49.92 MeV. The corresponding decrease in the penetrability relative to 22 Ne emission would thus be of 8 orders of magnitude!

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Fig. 1. Comparison of the sensitivity S measured for the detected events at different points along the particle trajectory with accelerator calibration curves: Rr is the residual range, i.e. the part of the total track length which still needs to be developed.

As far as the mass number of the Ne clusters, it is well known that track detectors do not have enough A resolution to allow an unambiguous identification [16]. Therefore, in order to decide in favor of one of the two most probable Ne clusters, either 22 Ne or 24 Ne, we resorted to theoretical calculations. By using, e.g., the cluster model of Ref. [5] one finds that the former case is more favorable than the latter by two orders of magnitude, mainly because of the higher preformation probability of 22 Ne vs. 24 Ne. The same is true for another widely used model, the superasymmetric fission one of Ref. [8]. By taking into account the decayed number of 230 U atoms reported above and the detector coverage efficiency (≈ 78% of the geometrical surface) we obtained
 Bα 230 U → 22 Ne = (4.8 ± 2.0) × 10−14 and
 Bα 226 Th → 18 O < 3.2 × 10−14 (90% confidence level) and the corresponding partial half lives
 T1/2 230 U → 22 Ne = (3.7 ± 1.5) × 1019 s and
 T1/2 226 Th → 18 O > 5.7 × 1016 s.

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3. Discussion Our branching ratio for 22 Ne is a factor 3.7 higher than the one obtained in Ref. [4], although the two results would overlap at the 2σ level. However, we consider the result of Ref. [4] questionable for two reasons. First, it has been obtained with a kind of track detector, polyethylene terephthalate, which allows for an α background generally much lower than the one admissible for phosphate glasses like the one used in the present work. At an α integrated flux of ≈ 1013 α/cm2 , i.e. 1–2 orders of magnitude higher than the maximum tolerable dose, not only does the detector response strongly change but also the obscuring effect produced by α recoils becomes so important that any search and identification for cluster events becomes extremely difficult and ambiguous. A second reason is that no attempt was done to compare the two claimed cluster decay events with accelerator calibrations. When comparing the predictions given by a few theoretical models with our result, we see (Table 1) a satisfactory agreement for 22 Ne decay, generally within one order of magnitude. As far as 18 O, our limit is not stringent enough to rule out any of the predictions reported in Table 1. Our result for 22 Ne is shown in Fig. 2 together with those existing in the literature [1,3] or recently obtained [17] for the other U isotopes. It is interesting to see how the partial half life continues to decrease with decreasing neutron number of the uranium mother nucleus, an effect which has recently been pointed out elsewhere [17]. More specifically, by comparing the two lightest U isotopes (230,232U) which both decay with very similar Q-values (61.4 and 62.3 MeV, respectively) to the same bimagic 208 Pb daughter nucleus

Fig. 2. Logarithm of the half life for cluster radioactivity (squares) and spontaneous fission (circles) vs. neutron number for different even–even uranium isotopes.

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by emitting two different Ne isotopes (22,24Ne), and by using the preformed cluster model of Ref. [5], we can trace mainly such an effect to the higher preformation probability of the lighter cluster (22 Ne) in respect to the one of the heavier (24Ne). Furthermore, we would like to compare the two above decays with the predictions of the model of Ref. [5], generally quite a successful one in reproducing cluster decays of even–even nuclei. We start by observing that while the 232 U → 24 Ne case is rather well fitted by such a model [2], there is a factor of 7 discrepancy in the case of the 230U → 22 Ne decay (Table 1). This might be considered an odd result in view of the (apparent) strong similarity of the two above decays. However, they do differ in something, i.e. the deformation of the two clusters, 22 Ne and 24 Ne [18]. This might affect either the two-body potential used to calculate the barrier penetrability or the many-body wave functions needed in calculating the preformation probability, or both. No deformation effect has been explicitly included yet in the model of Ref. [5] or in other models for heavy cluster decay. It would be interesting to verify more quantitatively the above phenomenological conjectures.

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