High resolution study of24Mg(p, d)23Mg for the Ne-E problem

Abstract

Nuclear levels of 23Mg near and above the proton threshold were investigated with high resolution. A new level has been identified at 7.643 MeV (66 keV above the proton threshold) with a possible jn = (3/2, 5/2) +. Some other spin-parity and excitation-energy assignments are also made. These results enable the reaction rate estimate of 22Na(p,7)23Mg with much smaller uncertainties, which is critical for the Ne-E problem in nuclear astrophysics. PACS: 27.30.+t, 97.10.Cv, 97.10.Tk, 97.30.Qt, 98.58.Ca One of the interesting subjects in nuclear astrophysics is the so called Ne-E problem [1], where very high 22Ne enrichments were observed in some meteorites. The most plausible explanation is that the meteorites were produced from the ejecta of some stellar event with high temperatures. The nucleus 22Na, which has a long half-life time of 2.6 y, would have beta decayed afterward, giving a strong enrichment of 22Ne in the meteorites. Thus, the nuclear astrophysical problem here is to know the reaction rates which are associated with 22Na [1][2][3].The main production processes of 22Na would be 21Ne(p,~/) and 22Mg(l~+v ), and they are known rather well. However, the destruction process, 22Na(p,,f)23Mg, was not well studied. Above 290 keV this reaction was investigated by using a radioactive target of 22Na [4,5]. Nevertheless, the reaction rate had a very large uncertainty, several orders of magnitudes, especially at low temperatures [2].We report here the result of a high resolution study of the 24Mg(p,d)23Mg reaction to learn about the nuclear levels (excitation energy and spin-parity) near and just above the proton threshold in 23Mg. A doublet suggested before [3] at 7.63 MeV was resolved experimentally and studied separately.The experiment was performed with a 34.945 MeV proton beam from the sector-focusing cyclotron of the Institute for Nuclear Study, University of Tokyo. The reaction products were momentum analyzed by a high resolution magnetic spectrograph [6], and detected by a hybrid type gas-proportional counter [7] backed up with a plastic scintillator. A metallic foil of 24Mg (enriched to more than 99.9 %) of about 130 gg/cm 2 was used to separate the doublet at around 7.63 MeV in 23Mg: The excitation energy 5~

Resonant scattering of22Na + pstudied by the thick-target inverse-kinematic method

“…In 1972 Black found that in Orgueuil meteorites the abundance ratio of 20 Ne/ 22 Ne is less than 1.5, which is much smaller than that of 9.8 on earth; this is the so-called Ne-extraordinary (Ne-E) problem [1]. With the discovery and isolation of presolar grains from carbonaceous chondrites, it was soon realized that the highly enriched 22 Ne component carried by low-density graphite grains is dominantly from the extinct radioactivity of 22 Na [2,3].…”

“…The stellar sources of radioactive 22 Na are primarily created in neon-rich nova [6][7][8][9] and supernova explosions [10][11][12]. In neon-rich novae, 22 Na is produced via the sequence of reactions 20 Ne(p, γ ) 21 Na(β + ) 21 Ne(p, γ ) 22 Na, or 20 Ne(p, γ ) 21 Na(p, γ ) 22 Mg(β + ) 22 Na alternatively [3,6]. In core-collapse supernovae, 22 Na is mainly produced in the O/Ne zone by the 21 Ne(p, γ ) 22 Na reaction, where 21 Ne is produced by the 20 Ne(n, γ ) 21 Ne reaction and protons by 12 C( 12 C, p) 23 Na [10] or by the burst of neutrinos from the core [13].…”

“…Direct measurements with radioactive 22 Na targets were performed by several groups [15][16][17][18], in which absolute or relative resonance strengths were obtained. Moreover, the complicated level structure of the odd-mass 23 Mg also attracted many spectroscopic studies by transfer reactions, for example 25 Mg(p, t) 23 Mg [19], 24 Mg(p, d) 23 Mg [20], and 22 Na( 3 He, d) 23 Mg [21], and by the β + decay of 23 Al [22][23][24][25], as well as by the heavy-ion fusion evaporation reaction 12 C( 12 C, nγ ) 23 Mg [26,27]. In these studies new levels were identified in the energy region of astrophysical interest; however, most of the spectroscopic information and resonant properties are still missing for the 23 Mg levels above the proton threshold [28].…”

Espectroscopia do núcleo ²³Mg acima do limiar de decaimento de prótons com interesse para astrofísica nuclear

“…Uma possível explicação para esta disparidade é que estes meteoritos são gerados em eventos de alta temperatura, como o caso de explosões de nova. Dessa forma, o decaimento beta do 22 Na determinaria a abundância do núcleo 22 Ne [Toy94]. Em contrapartida, uma porção do 22 Na é consumida pela reação 22 Na(p,γ) 23 Mg, nos cenários em que a temperatura do meio estelar varia de T 9 =0.1 até 0.5 K, para eventos tipo nova.…”

“…O principal diferencial deste trabalho é o uso do detector de radiação gama denominado de Gammasphere, composto por 100 detectores de germânio hiper-puro cobrindo uma aréa de 4π da re-e com alta resolução. Os autores então corroboram os resultados obtidos por Kubono et al[Toy94] para os valores de spin dos estados [41] e [42]. Além disso propõem a existência de um dubleto de energias: 7.782 e 7.785 MeV([47]), cujos valores de , respectivamente.…”