8 He nuclei stopped in nuclear track emulsion
D. A. Artemenkov, A. A. Bezbakh, V. Bradnova, M. S. Golovkov, A. V. Gorshkov, G. Kaminsky, N. K. Kornegrutsa, S. A. Krupko, K. Z. Mamatkulov, R. R. Kattabekov, V. V. Rusakova, R. S. Slepnev, R. Stanoeva, S. V. Stepantsov, A. S. Fomichev, V. Chudoba, P. I. Zarubin, I. G. Zarubina
aa r X i v : . [ nu c l - e x ] O c t He nuclei stopped in nuclear track emulsion
D. A. Artemenkov, A. A. Bezbakh, V. Bradnova, M. S. Golovkov, A. V. Gorshkov,S. A. Krupko, N. K. Kornegrutsa, V. V. Rusakova, R. S. Slepnev,S. V. Stepantsov, A. S. Fomichev, P. I. Zarubin, ∗ and I. G. Zarubina Joint Insitute for Nuclear Research, Dubna, Russia
G. Kaminsky
Institute of Nuclear Physics, Polish Academy of Sciences, Krakow, Poland
R. R. Kattabekov
Institute for Physics and Technology,Uzbek Academy of Sciences, Tashkent, Republic of Uzbekistan
K. Z. Mamatkulov
Djizak State Pedagogical Institute, Djizak, Republic of Uzbekistan
R. Stanoeva
SouthWest University, Blagoevgrad, Bulgaria
V. Chudoba
Institute of Physics, Silesian University in Opava, Czech Republic (Dated: October 20, 2018) bstract The fragment separator ACCULINNA in the G. N. Flerov Laboratory of Nuclear Reactions ofJINR was used to expose a nuclear track emulsion to a beam of radioactive He nuclei of energyof 60 MeV and enrichment of about 80%. Measurements of decays of He nuclei stopped in theemulsion allow one to evaluate possibilities of α -spectrometry and to observe a thermal drift of Heatoms in matter. Knowledge of the energy and emission angles of α -particles allows one to derivethe energy distribution of α -decays Q α . The presence of a ”tail” of large values Q α is established.The physical reason for the appearance of this ”tail” in the distribution Q α is not clear. Its shapecould allow one to verify calculations of spatial structure of nucleon ensembles emerging as α -pairsof decays via the state Be . PACS numbers: 21.45.+v, 23.60+e, 25.10.+s ∗ Electronic address: [email protected]; URL: http://becquerel.jinr.ru . At the energy of a few MeV per nucleon, there is a possibility to study decays of radioac-tive nuclei by implanting them into a detector [1–4]. In particular, population of 2 α - and3 α -particle states is possible in decays of light radioactive nuclei. In this respect, the unique,although somewhat forgotten, possibilities of nuclear track emulsion (NTE) for the detectionof slow radioactive nuclei are worthy to be mentioned. The advantages of this method arethe best spatial resolution (about 0.5 µ m), the possibility of observing the tracks in a fullsolid angle and a record sensitivity starting with relativistic singly charged particles with aminimum ionization. In NTE, the directions and ranges of the beam nuclei, as well as slowproducts of their decays can be measured, which provides a basis for spectrometry. Morethan half a century ago, hammer-like tracks from the decay of Be nuclei through the firstexcited state 2 + of about 2.0 MeV were observed in NTE. They occurred in the β -decays ofstopped Li and Be fragments, which in turn were produced by high-energy particles [5].Another example is the first observation of the C nucleus from the decay 2 α + p [6]. Whenused with sufficiently pure secondary beams, NTE appears to be an effective means for asystematic study of the decay of light nuclei with an excess of both neutrons and protons.In March 2012 exposure of NTE to nuclei He of energy of 60 MeV [7] is performed at thefragment separator ACCULINNA [8] in the G. N. Flerov Laboratory of Nuclear Reactions,JINR. Features of decays of the He isotope are shown in Fig. 1, according to the compilation[9]. Fig. 2 shows a mosaic macrophotograph of a decay of a nucleus He stopped in NTE.It is typical one among thousands observed in this study. Video recordings of such decaystaken with the microscope and camera are collected [10].When scanning the NTE pellicle with a 20 × objectives on the microscopes MBI-9 aprimary search for β -decays of He nuclei was focused on hammer-like events (Fig. 2). Theabsence of tracks of the decay electrons in the event was interpreted as a consequence of anincomplete efficiency of observation. Often, in the events named “broken”ones gaps wereobserved between stopping points of primary tracks and subsequent hammer-like decays. Intotal 1413 ”whole” and 1123 ”broken” events were found. Decay vertices of 580 ”broken”events were found to be laying in a backward hemisphere with the respect to arrival directonsof ions. Corresponding to a half of the statistics this number indicate that the forward-backward asymmetry is absent. The ”broken” events were attributed to a drift of thermalized3
He atoms that arose as a result of neutralization of He nuclei. This effect is determinedby the nature of He and such events are identified them particularly reliably.The coordinates of stopping pointsof the ions He (as well as arrival directions), the decayvertices and stops of decay particles were determined for “hammers”of 136 “whole”and 142“broken”events. In “broken”events the decay points were determined by extrapolating theelectron tracks. The emission angles and the ranges of α -particles were obtained on thisbasis. The distribution of the opening angles of α -particle pairs has a mean value < Θ α > = (164.9 ± ◦ at rms = (11.6 ± ◦ . Some kink of “hammers”is defined by the momentacarried away by e ν -pairs. The dependence of the α -particle ranges L α and their energyvalues are determined by spline interpolation of calculations in the SRIM model [11]. Themean value of the α -particle ranges is (7.4 ± µ m at rms (3.8 ± µ m correspondingto a mean energy < E( He) > = (1.70 ± and L of α -particles in pairs is clearly manifested. The distribution of the range differencesL - L has rms 2.0 µ m.Knowledge of the energy and emission angles of α -particles allows one to derive the energydistribution of α -decays Q α . The relativistic-invariant variable Q is defined as the differencebetween the invariant mass of a final system M ⋆ and the mass of a primary nucleus M, thatis, Q = M ⋆ –M, M ⋆ is defined as the sum of all products of the 4-momenta P i,k of fragments,that is, M ∗ = P (P i · P k ). In general, the distribution of Q α (Fig. 3) corresponds to the Bedecay from the first excited state 2 + . However, the mean value < Q α > is slightly higherthan expected. This fact is determined by the presence of a ”tail” of large values Q α ,obviously not matched the description by a Gaussian function. Application of the selectioncriteria for ranges L and L less than 12.5 µ m and opening angles Θ > ◦ , provides avalue < Q α > = (2.9 ± ± + state. Ranges L and L stay to be well correlated above 12. 5 µ m. Therefore, enhancedranges L and L can not be attributed to fluctuations of ranges or recombination of ionsHe +2 .The targeted measurements are continued to saturate statistics in the high energy “tail”Q α and to establish its shape. The insertion in Fig. 3 shows Q α with additional 98 α -pairshaving L and L above 12.5 µ m. It should be noted that the highly energetic α -pairs areamong better measurable ones despite to relatively rare appearance. The physical reasonfor the appearance of the ”tail” in the distribution Q α is not clear. Probably, its shape4ill allow one to verify calculations of spatial structure of 8-nucleon ensembles emerging as α -pairs of decays via the state Be [12].In the 142 “broken”events the distances L( He- Be) between the stopping points of the He ions and the decay vertices as well as the angles Θ( He- Be) between directions ofarrivals of the ions and directions from the stopping points of the ions towards the decayvertices are defined (Fig. 4). Uniformity of distributions of events over these parameters andabsence of a clear correlation indicate on on a thermal drift of the atoms He. The meanvalue < L( He- Be) > amounting to (5.8 ± µ m at (3.1 ± µ m, can be associatedwith a mean range of atoms He. The low value of a mean speed of the atoms He definedas ratio of < L( He- Be) > to the half-life of the nucleus He supports a pattern of diffusion.Observation of the diffusion points to the possibility of generating of radioactive atoms He and pumping them out of sufficiently thin targets. Increasing of the mean speed anddrift length is achievable due to heating of the target. There is a prospect of accumulatingof a significant amount of He atoms. In particular, a He radioactive gas can be used tomeasure the half-life of the He nucleus at a new level of precision and for laser spectroscopyof this isotope. Applied interest consists in studies of thin films by pumping atoms He andtheir deposition on α -detectors. Such opportunities are developing intensively with respectof the He isotope [13], [14].To conclude, the result of this work is the demonstration of the opportunities of recentlyreproduced nuclear track emulsion in a way of exposure in a beam of He nuclei. Nucleartrack emulsion made it possible to identify decays of stopped He nuclei, estimate possibilitiesof α -range spectrometry and observe the drift of He atoms. The high quality of the beamof radioactive He nuclei at the ACCULINNA fragment separator was confirmed. Thepresented analysis of the decay of nuclei He can serve as a prototype for studying the decaysof stopped nuclei , Li, , B, C and N. Statistics of hammer-like decays found in this studyis a small part of the flux of He nuclei, and the measured decays - about 10% of this share.This limitation is defined by “reasonable expenses”of human time and labor. However,nuclear track emulsion in which radioactive nuclei are implanted provides a basis for theapplication of automated microscopy and image recognition software, allowing one to relyon unprecedented statistics. Thus, a synergy of classical technique and modern technologycan be achieved. This work was supported by the grants 12-02-00067, 11-02-00657 and11-02-00657a of the Russian Foundation for Basic Research and grants of Plenipotentiary5
IG. 1: Scheme of a major channel of the cascade decay of He isotope; light circles correspondto protons, dark ones -neutrons. representatives of Bulgaria, Egypt and Romania at JINR.6