Stopped Pion Absorption Research Articles

Formation of 8He in the 9Be(π–, p)X and 10B(π–, pp)X Stopped Pion Absorption Reactions

The structure of levels of the 8He heavy helium isotope has been studied in the 9Be $$({{\pi }^{ - }},p)X$$ and 10B $$({{\pi }^{ - }},pp)X$$ stopped pion absorption reactions. Measurements have been performed with a two-arm multilayer semiconductor spectrometer at the LANL. Indications to the existence of a soft dipole mode at the excitation energy $${{E}_{x}} \approx 3$$ MeV have been obtained. A state with a high excitation energy of 12.2(5) MeV has been observed for the first time.

Search for 10He in the stopped pion absorption 14C(π−, p 3He)X

The formation of the 10He states was studied in the reaction of stopped pion absorption 14C(π−, p 3He)X. Measurements were carried out using two-arm multilayer semiconductor spectrometer and “radioactive” target consisting of 76% 14C and 23% 12C. The contribution of uncontrolled impurities in the target was ≤ 1%. In order to determine the contribution of the 12C impurity measurements were performed on an isotope pure carbon 12C target. An indication on the excitation state with Ex ~ 6 MeV (Er ~ 7 MeV) was observed in missing mass spectrum of 14C(π−, p 3He)X reaction. Comparison with theoretical and experimental results obtained by other authors was performed.

Formation of the Heavy Isotope $${}^{\boldsymbol{6}}$$He upon Stopped Pion Absorption by Nuclei

A survey of experimental results on the level structure in the heavy helium isotope $${}^{6}$$ He that were obtained in reactions induced by stopped pion absorption in $${}^{9}$$ Be, $${}^{10,11}$$ B, and $${}^{12}$$ C light nuclei is presented. The respective measurements were performed on the low-energy pion channel of the accelerator at the Los Alamos National Laboratory (LANL) by means of a two-arm semiconductor spectrometer. A high statistical significance of the results was attained over a broad excitation-energy range extending up to 40 MeV. Several highly excited states, including $$t+t$$ cluster resonances, were revealed for the first time.

Spectroscopy of 7He in stopped pion absorption

Formation of the 7He heavy isotope was studied in the reactions of stopped pion absorption by light nuclei 9Be, 10,11B and 12,14C. Measurements were performed using the two-arm multilayer semiconductor spectrometer. Contrary to the results of other studies, we observed several narrow highly excited states (Ex > 16 MeV) of 7He. From the analysis of the continuous excitation spectrum in reactions 9Be(π−, d)X and 11B(π−, dd)X it was obtained first indication that the 4He+3n structure is not present in the ground state of 7He.

8He spectroscopy in stopped pion absorption by 11B nuclei

Level structure of 8He has been studied in the reaction of stopped pion absorption by 11B nuclei 11B(π−,pd)X. The experiment was carried out at the LANL with a two-arm semiconductor spectrometer. The missing mass spectrum in the range 0 MeV < MM < 50 MeV has been described by the superposition of phase-space distributions and the six states of 8He. Parameters of these states have been compared with data of other experimental and theoretical works.

Spectroscopy of 7He in the reactions of stopped pion absorption by 12,14C nuclei

The production of the 7He states was studied in the reactions of stopped pion absorption by carbon isotopes: 12C(π−, p4He)7He, 12C(π−, d3He)7He and 14C(π−, t4He)7He. Measurements were performed using two-arm multilayer semiconductor spectrometer. The ground and excited states were observed in all three reactions. Narrow (Γ < 2 M∍B) state with the high excitation energy ≈ 16 MeV was observed in the 14C(π−, t4He)7He reaction.

Charged Particle Production in Stopped Pion Absorption by Nuclei

The spectra of 3,4He charged particles produced in stopped pion absorption by nuclei were measured for 17 nuclei in the mass-number range of 6 ≤ A ≤ 209 by means of a semiconductor spectrometer. These data were analyzed on the basis of a phenomenological model where the absorption reaction in question is treated as a three-stage process: primary absorption by intranuclear clusters, preequilibrium stage, and evaporation stage. It is shown that a dominant contribution to the production of helium isotopes with energies not less than 20 MeV comes from the preequilibrium stage. The contribution of the knockout and pickup mechanisms to 3,4He production is estimated.

8He Spectroscopy with Stopped-Pion Absorption by 12C and 14C

The structure of the levels of heavy helium isotope 8He is investigated using the reactions of stopped-pion absorption by carbon isotopes 12С(π−, p3He)X, 14С(π−, d4He)X, and 14С(π−, t3He)X. Measurements are made with the two-arm multilayer semiconductor spectrometer at the LANL accelerator. Excited states of the 8He nucleus with energies of excitation of up to ~ 20 MeV are sought using the measured missing-mass spectra for these reactions. An excited state with Ex ≈ 6.4 MeV and Г ≤ 1 MeV is observed for the first time. Its structure is probably due to two valence nucleons on the (1p1/2)2 shell.

Charged Particles Evaporation in the Stopped Pion Absorption Reactions

In the present work we have analyzed evaporation spectra and yields of p,d,t formed in the reaction of stopped pion absorption. It is shown that the values of equilibrium temperature obtained through the usage of proposed model are in agreement with the values obtained in various other experiments. We also discuss the A-dependences of the evaporation yields and consider possible contributions of the “indirect” evaporation processes.

Spectroscopy of Helium Isotope 6He

The excited states of heavy helium isotope 6He were studied in stopped pion absorption in the reactions 9Be(

8He Spectroscopy in Stopped Pion Absorption By 9Be

Level structure of 8He has been studied in the reaction of stopped pion absorption by 9Be nuclei. The missing mass spectrum in the range 0 MeV ≤ MM ≤ 10 MeV has been described by the superposition of phase-space distributions and the three states of 8He. Parameters of these states have been compared with data of other experimental and theoretical works.

Yields of hydrogen isotopes in stopped-pion absorption by light nuclei

The spectra of p, d, and t charged particles produced in stopped-pion absorption by nuclei are analyzed. Respective measurements were performed for 17 nuclei by means of a semiconductor spectrometer. A phenomenological model developed previously for medium-heavy and heavy nuclei was applied to describing the spectra and yields of charged particles originating from light nuclei (6,7Li, 9Be, 10,11B, and 12C). The contribution of intranuclear clusters (such as pp and 3,4He) to pion absorption was estimated.

Formation of 3He in the reactions of stopped pion absorption

The results of the measurements of spectra and yields of helium isotope 3He formed in the stopped pion absorption by nuclei are presented. The contributions of different mechanisms of the helium formation in these reactions on medium and heavy nuclei were determined. The proposed phenomenological model reproduces the 3He spectra at energies >30 MeV satisfactorily.

Search for light neutron-rich isotopes in stopped pion absorption

The results based on the spectroscopy of superheavy hydrogen isotopes (4−7H), heavy helium isotopes (6,7He), and heavy lithium isotopes (7−12Li) produced in stopped pion absorption by light nuclei were analyzed. Search for nuclear states was performed in inclusive and correlation measurements of missing mass spectra. A broad range of excitation energies studied in correlation measurements provided the possibility of search for isobaric analog states and cluster resonances. A comparison with experimental and theoretical results of other authors was conducted.

Study of hydrogen and helium isotopes with A=5,6,7

The level structure of superheavy hydrogen and heavy helium isotopes with A = 5, 6, 7 was studied in stopped pion absorption by light nuclei. The measurements were performed at low energy pion channel of LANL with two-arm multilayer semiconductor spectrometer. Excited states of the 5-7H and 5-7He were observed in several reaction channels on the 9Be and 10,11B nuclei. Several excited levels were found only in our measurements. Candidate on isobar-analog state of the 6H ground states with Er ∼ 5.5 MeV was observed in the reaction 10B(π−,pt)X.

Search for heavy lithium isotopes 10-12Li in stopped pion absorption reactions

The experimental search for the production of heavy lithium isotopes 10-12Li was carried out in the stopped π−-mesons absorption reactions on the targets 11B, 12,14C. Study of 10Li was performed in the correlation and inclusive measurements of reactions 12C(π−,pp)X, 14C(π−,pt)X, 14C(π−,dd)X and 11B(π−,p)X. Three highly excited states of 10Li with the resonance energies Er = 6.1 ± 0.1 MeV, 7.8 ± 0.1 MeV and 10.1 ± 0.1 MeV were identified together with the well-known levels. The resonance parameters of three low-lying excited states of 11Li were determined in the 14C(π−,pd)X reaction. In the 14C(π−,pp)X reaction 12Li excited state with Er = 4.0 ± 0.2 MeV and an indication on a level with Er ≈ 7 MeV were found.

Formation of primary particles in stopped pion absorption reactions

The results from analyzing the yields of protons (p), deuterons (d), and tritons (t) formed in the absorption of stopped π– mesons by intranuclear clusters are presented. The contributions from cluster absorption to the production of hydrogen isotopes in stopped pion absorption reactions are determined. Phenomenological formulas are proposed for describing the mass number (A) dependences of the yields of the primary protons with ∼15% precision in the mass number range of 6 < A < 209 (and of the primary deuterons and tritons in the mass number range of 59 < A < 209). Evidence is observed of the constancy of the ratio between the elemental absorption widths for the absorption of pions by pp and pn pairs over the investigated range of mass numbers.

Yields of p, d, t formed in stopped pion absorption by intranuclear clusters

Results of the study of spectra and yields of p, d, t formed in stopped pion absorption by nuclei are presented. Data on charged particle formation have been obtained for 17 isotopes in wide atomic mass range (6 < A < 209). Empirical model was proposed to describe A- dependences of primary p, d, t yields.

Search for isobar-analog states of superheavy hydrogen isotopes5-7He

Search for isobar-analog states (IAS) of superheavy hydrogen isotopes 5-7H was performed among the high-excited states of helium isotopes 5-7He. The excited spectra were measured in stopped pion absorption by light nuclei. The experiment was performed at low energy pion channel of LANL with two-arm multilayer semiconductor spectrometer. Excited states of 5-7He were observed in three-body reaction channels on 10,11B nuclei. Several excited levels were observed for the first time. 6He excited state with Ex = 27.0(8) MeV observed in 10B(π-,pt)X channel is an IAS candidate for 6H with Er ∼ 5.5 MeV. 7He excited state with Ex = 24.8(4) MeV observed in 10B(π-,pd)X, nB(π-,pt)X and nB(π-,dd)X channels is an IAS candidate for 7H with Er ∼ 3 MeV.

Formation of hydrogen isotopes in reactions of stopped pion absorption

Results are presented from analyzing the yields of protons (p), deuterons (d), and tritons (t) in pre-equilibrium stopped pion absorption reactions. Phenomenological formulas are proposed for describing the mass-number (A) dependences of these yields accurate to within 10–20% in the mass-number range of 6 < A < 209 (59 < A < 209 for t). The final results do not contradict the hypothesis that the predominant role in the formation of deuterons and tritons is played by pick-up on the surface of nuclei.