KnE Energy | The 3rd International Conference on Particle Physics and Astrophysics (ICPPA) | pages: 33–40

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1. Introduction

The 17 Ne nucleus is located on the proton dripline, and it is relatively loosely bound with respect to the 2p breakup ( Eb=944 keV, see Fig. figlevels). There are several physical problems connected with this nucleus which are actually tightly interwoven. However, multiple efforts to investigate it, both theoretically and experimentally, still have not provided convincing clarity about this point. The 2p halo predicted for the ground state of 17 Ne and the true 2p -decay branch assumed for its first excited state rise interest to different aspects of its nuclear structure. We don't touch upon here the issue of the 17 Ne 2p halo structure which was discussed in the theory [1,2,3] and the experimental [4,5] works. The 2p -decay energy of the first excited state in 17 Ne is small: ET=-S2p(17Ne)=344 keV. This state belongs to the class of the so-called true 2p -emitters [6]. The true 2p decay from the ground state was observed for many nuclei lying close to the proton drip line, while the 17 Ne is only known nuclide where this decay can take place from the excited state. This point explains the considerable interest appearing for nuclear theory to the study of the 2p -decay branch which is assumed for this excited state of 17 Ne.

Experimental observation of 2p -emission from the first excited state of 17 Ne is complicated by concurrent channel of γ -decay which partial width is drastically greater one for 2p -decay. That make branching ratio of Γ2p/Γγ extremely small. Up to now only an upper limit of Γ2p/Γγ7.7×10-3 is experimentally achieved [7]. In the Same time theoretical estimates of 2p -decay partial width provide the value for Γ2p/Γγ about (0.9-2.5)×10-6 [8].

Problem of the observation of 2p -emission from the 3/2- state of 17 Ne has important astrophysical application. The 15 O nucleus is a “waiting point” in the astrophysical rp-process as its half-life T1/2=122.24 s is comparable to the timescale of the typical rp-process scenarios. The radiative 2p -capture is known to be a possible bypath for this waiting point [9]. Resonant particle radiative capture reaction rate of the selected resonant state at temperature T is proportional to

σ part ,γ(T)1T3n/2expErkTΓγΓ part Γ tot ,

(1) where Er is the resonance position, Γγ and Γ part are partial widths of the resonance Er into gamma and particle channels [10]. So far unknown 2p -decay width of the first excited state of 17 Ne ( E*=1288 keV, Jπ=3/2- ) makes a key point for solving the bypass problem of the 15 O waiting point. Taking into account this (previously omitted) state in the calculation of the resonant radiative capture rate strongly modified the corresponding rate in a broad temperature range around 0.15 GK [11]. This modification is as large as 3-8 orders of magnitude, where the variation corresponds to the uncertainty in 2p width of the 1.288 MeV 3/2- state predicted in theory works [11,8].

We can formulate following complex the 17 Ne problem of first excited state: the covering of the gap between experimental limit and theoretical predictions for the Γ2p/Γγ branching ratio as an “intermediate stage”, and experimental observation of the first excited state 2p -decay as a “final goal”.

Figure 1

The level schemes for 17 Ne, its one-proton subsystem 16 F, and decay scheme for 17 Ne states.

fig-1.jpg
Figure 2

The experimental setup and kinematic plot for the reaction products.

fig-2.jpg

2. Experiment

For experimental search of such rare events as the 17 Ne first excited state 2p -decay one should solve two general problems. The first one is the accumulation of enough statistics for the event of interest observation. The second problem is the separation of the 2p -decay events of the interested 3/2- state from other excited states located above which are also decaying by 2p -emission. One can see on Figure figlevels that above first excited state with Jπ=3/2- there are other excited states decaying by 2p -emission. Having only few events from state of interest we can separate them only in inclusive spectrum. There are two ways to reduce background in the inclusive spectrum: (a) reduce the population of states located above and (b) improve energy resolution.

We performed a dedicated search for the 2p -decay branch of the first excited 3/2- state of 17 Ne [12]. In comparison with work [7] two methodological improvement were applied. The neutron transfer reaction 1 H( 18 Ne,d) 17 Ne which was used to populate 3/2- state which provides significant background suppression from highly excited states and using of original experimental approach of “combined mass” method. The “combined mass” method is based on combination of the missing mass method and invariant mass method. In this approach recoil particle (the deuteron in our case) and the light fragment of the decay (protons) are detected. Obtaining center of mass momentum of the decaying nucleus from the recoil particle and momenta of light decay products allow one to reconstruct decay energy with relative good precision. This pick of detected particles provides several advantages: (a) one obtains full kinematic of the reaction, (b) the detection of deuteron provides information about population of the excited states of 17Ne (including bound states) in the reaction, (c) the detected particles can be easily separated from the beam that significantly reduce the detectors counting rate, and (d) in some kinematics condition (see [12] for details) the “combined mass” method provides relative good energy resolution even for setup with thick targets.

Above explained approach allowed us to maximize luminosity and to keep the resolution at appropriate level ( 500 keV) in the presented experiment. Figure figsetup shows a schematic drawing of the detector setup. The annular telescope ( d -telescope on Figure figsetup) detected the recoil deuterons from the 1H(18Ne,d) reaction. The telescope consisted of three position sensitive Si detectors with an inner (outer) radius of the sensitive area of 16 (41) mm and a thickness of 1 mm each. Particle identification was performed by standard ΔE-E analysis. Signals from any sector of the S1 detector triggered the data acquisition system. Another telescope located on the beam axis ( p -telescope) was intended for the detection of protons from the 17Ne*15O+2p decay. The telescope consisted of two square 6×6 cm 2 , 1 mm thick silicon detectors (Q1 and Q2). Following the pair of Si detectors installed was a wall of 16 CsI(Tl) crystals with PMT readout (CQ). To ensure the normal working conditions for the detectors a 1.4 mm thick aluminum filter was installed directly in front of the telescope. This was enough to stop all the nuclei making the beam cocktail while the protons from the decay of 17Ne* lost only a small part of their energy in the aluminum filter. Data presented in the experiment were collected in experiments carried out with Experimental data were collected using a secondary beam of total intensity of 2×105 pps at the target plane. The 18 Ne ions comprised about 18% of beam cocktail.

3. Width ratio evaluation

Figure 3

(color online) Excitation energy spectrum of 17 Ne: (a) Scatter plot showing the excitation energy of 17 Ne measured by the missing mass method ( E miss .* ) versus the energy obtained by the combined mass method ( E comb .* ). The ellipses shown by solid red, dashed blue, and dash-dotted magenta curves correspond to the loci where the observation of 68% of events for the 3/2- , 1/2+ , and 5/2+ states, respectively, is expected, (b) combined mass spectrum, (c) missing mass spectrum.

fig-3.jpg

Figures figthk-2p-hitsb and figthk-2p-hitsc show excitation energy spectra from the triple d-p-p coincidences associated with decay of 17 Ne excited states obtained by the “combined mass” method and the missing mass method, respectively. One can see a peak of 1/2+ state which dominates in both spectra. In the missing mass spectrum the location of 3/2- state is fully overlapped by 1/2+ peak tails. In the “combined mass” spectra due to better resolution only several events fall into location of 3/2- state. To clarify the source of this event (decay of the 3/2- state or 1/2+- state) the correlation plot in Figure figthk-2p-hitsa was derived from the data. The solid red ellipse corresponds to locus where 68% of events from 3/2- decay are concentrated. One can see eight events in this locus, however, all of these events are located in the top-right part of the ellipse and no one in the bottom-left. Note that the events from the 3/2- state should come upon both parts of the locus with approximately equal probabilities. Therefore most feasible source of this events is decay of 1/2+- state and no events which could be associated with 2p -decay from 3/2- state were observed. Thus, we can only claim new limit for Γ2p/Γγ ratio.

The Γ2p/Γγ value can be evaluated by the following equation Having number of 2p -coincidences N2p<1 , number of events with population of 3/2- N=38(6)×103 , and probability that 2p -decay event is observed at bottom-left part of the locus ε2p=0.16 one get limit for the ratio of Γ2p/Γγ<1.6(3)×10-4 .

4. Summary

A dedicated search for the 2p decay branch of the first excited 3/2- state of 17 Ne populated in the 1H(18Ne,d)17Ne was performed. Based on the experimental data the new upper limit Γ2p/Γγ1.6(3)×10-4 is established. This significantly (about a factor of 50) reduces the value of the limit defined in the previous work [7]. The strong improvement of the Γ2p/Γγ limit was achieved due to the choice of the transfer reaction used as a tool to populate excited states of 17Ne* and application of the novel “combined mass” method for the reconstruction of the 17 Ne excitation spectrum. The latter allowed us to improve significantly the instrumental resolution in the measurements made with the thick target. The measured limit for the rate value rules out the predictions made for the 2p decay width of the 17 Ne first excited state by the simplified di-proton decay model [13], but it is still insufficient to be restrictive for the realistic theoretical predictions [8].

We see prospects for a considerable (by 1-2 orders of magnitude) reduction of the Γ2p/Γγ upper limit in the proposed experimental method without revolutionary modification of the setup. Such improvements open a way to the direct experimental observation of the true, radioactive 2p -decay of the 17 Ne 3/2- state taking the theoretically predicted ratio of Γ2p/Γγ(0.9-2.5)×10-6 as a trusted aim.

There is a general issue of the development of methods applicable to the studies of weak particle (alpha, proton, or two-proton) decay branches of excited states which reside well below the Coulomb barrier and thus have extremely small Γ part /Γγ ratios. The possibility to derive directly such weak decay branches in one experiment makes promising the application of the proposed approach to the problems of nuclear astrophysics, in particular to problem of the explosive hydrogen burning.

Acknowledgments

A.A.B., S.A.R., T.A.G., A.V.G., I.A.E., S.A.K., A.G.K., and P.G.S. are supported by the Helmholtz Association under grant agreement IK-RU-002. This work was partly supported by the RSF 17-12-01367 grant and MEYS Czech Republic LTT17003 and LM2015049 grants.

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