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In a search for β-delayed charged particles from 14Be we have obtained an upper limit on β-delayed α particles, B(βα)

Beta delayed charged particles from Be-14 have been measured and give an upper limit on beta delayed alpha particles of B(betaalpha)

In β-decay experiments on 9C at CERN/ISOLDE the β-strength was determined to the ground state, the 12.2 MeV excited state and the Isobaric Analog State (IAS) at 14.655 MeV in 9B. A large β-strength asymmetry is deduced for the mirror transitions of 9C and 9Li to states around 12 MeV excitation energy. A satisfactory description of the three-body decay from a narrow energy region around the 12.2 MeV resonance is obtained within a sequential model involving the ground and first-excited states of 5Li and 8Be. From the study of angular correlations the spin of the 12.2 MeV state is determined as 5/2−. For the first time the population of the IAS is observed in β-decay and new information on the decay of this state is obtained. The advantages of a closely packed, highly segmented detector setup are demonstrated.

The magnitude of the Coulomb mixing parameter δ 1 has been experimentally deduced, for the first time, for the β decay of 74 Rb. The estimated magnitude is derived from the feeding of the non-analog first excited 0 + state in 74 Kr. The inferred upper limit of 0.07% is small compared to theoretical predictions. The half-life was measured to be 64.90(9) ms.  2001 Published by Elsevier Science B.V.

The use of a resonance ionization laser ion source at CERN/ISOLDE has made it possible to study the decay of very neutron-rich Sn135-137. The decay of Sn-135 is found to populate low-energy levels in Sb-135 via direct beta decay and the first excited state in Sb-134 by beta-delayed neutron emission. The level structure of Sb-135 Will be discussed and a possible signature for a more diffuse nuclear surface considered.

Neutral and charged two-pion production in p+d→ 3 He+2 π reactions has been studied at CELSIUS at a proton beam energy of 477 MeV. The total cross section for double pion production is 0.22±0.03 μ b. The ratio of the cross sections for the production of charged pion pairs with isospin T =1 and T =0 was determined to be σ ( π + π − ; T =1)/ σ ( π + π − ; T =0)=1.4±0.4.

The d(α,6Li ∗ 3.56 )π0 reaction has been studied at Ec.m.=1.2 and 1.9 MeV above threshold with an alpha-particle beam incident on a deuterium cluster-jet target in CELSIUS. Complete differential cross sections were measured at both energies, integrated to σ=228±6+70 nb and 141±12+42 nb respectively. Observed large anisotropies are discussed in relation to the cluster structure of the 6Li ∗ halo.

In near-threshold reactions the heavy reaction products are emitted within a narrow cone in the forward direction. To detect these a small-size charged-particle telescope has been built that is operated inside the vacuum chamber within the dipole magnets of the CELSIUS storage ring. The first aim has been to study two-pion production close to threshold. By measuring the d(d,2π) 4 He reaction at a beam energy of 569 MeV, corresponding to 29 MeV above the production threshold for two neutral pions in the c.m., we add a low-energy end point to the previous experiments concerning this reaction. The two pions are produced in a T=0 channel which is the signature of the so called ABC-effect.

The isobaric analogue state of the ground state of the halo nucleus6He,6Li* has been studied in a pionic fusion experiment at the CELSIUS storage ring facility in Uppsala, Sweden. This was the first of two proposed experiments with the aim to study the high momentum part of the wave functions of the halo nuclei6He and6Li (0+, T=1). The Li nuclei were produced in inverse kinematics reactions with alpha particles incident on a deuterium cluster jet target. The6Li ions were detected in a zero-degree spectrometer situated in the fourth quadrant of the CELSIUS ring. The measurement was done for three different beam energies corresponding to 5.4, 2.2 and 1.5 MeV above threshold in the c.m. The cross-sections slowly increase with beam energy from 95 nb at the lowest energy to about 250 nb at 5.4 MeV above threshold.

A small size spectrometer for detecting heavy reaction products emitted at small angles has been developed. It is installed at the synchrotron storage ring CELSIUS in Uppsala. It uses the focussing and bending magnetic elements of the storage ring itself in combination with a detector telescope, made of high-purity germanium, inserted into the ring vacuum. The spectrometer has been used for studies of pion production reactions close to threshold. The spectrometer is described together with results of ray-trace calculations and experimental tests.

The momentum spectrum of 4 He particles from d + d reactions at E lab = 569 MeV has been measured at zero degrees in the laboratory. The missing-mass spectrum is consistent with a pure phase space behaviour for the isospin zero channel of two pions, corresponding to a total cross section of 43 ± 5 nb.

The properties of states in 12 C above the 3α-threshold are of high current interest for nuclear astrophysics and for the nuclear many-body problem in general. We have initiated a series of experiments aimed at elucidating this region by using the β-decays of 12 N and 12 B. By applying the ISOL method for producing these radioactive isotopes, in combination with modern segmented charged particle detectors, our approach has significant advantages over previous studies of this region.

Reactions of a 41 MeV/u beam of the radioactive halo nucleus Be-11 have been studied with a counter telescope coupled to an array of neutron detectors covering angles up to 97-degrees. The technique allows to determine single-neutron inclusive and exclusive angular distributions. The targets (Be, Ti and Au) were chosen to illustrate the relative roles played by nuclear and Coulomb mechanisms. The channels leading to Be-10, the dissociation channels, correspond to impact parameters larger than the sum of the radii of the target and the Be-10 core. It is shown that for the dissociation process it is possible to account almost quantitatively for the integral, single- and double-differential cross sections from models without free parameters including the Coulomb, Serber and Glauber (diffraction-dissociation) mechanisms. The neutron distributions from the nondissociative reaction channels show little individuality and it is convenient to group them together as the channel ''neutron plus anything different from Be-10''. We refer to these as ''restricted-inclusive'' reactions. These seem to be a promising tool for obtaining accurate information on the halo wave function in momentum coordinates.

7 pages, 1 table, 4 figures.-- PACS nrs.: 23.40.Hc; 21.10.Hw; 27.20.+n.-- Printed version published Dec 4, 2003., A new measurement of the β-decay of Li-9 has clarified the feeding to the highest accessible states in Be-9. It is found that the β-decay mainly populates the 11.8 MeV state, whose spin is determined as 5/2(-). The extracted B-GT value of 5.3(0.9) is a factor 4.4(1.0) larger than that of the mirror transition from C-9. A theoretical explanation of such a pronounced β-decay asymmetry is presently lacking., One of us (Y.P.) thanks the Spanish Ministery of Science and Technology for the 4-year grant assigned to the project AEN1999-1046-C02-01. The work was supported by the European Union Fifth Framework Programme “Improving Human Potential, Access to Research Infrastructure”.

Using $\ensuremath{\beta}$ decays of a clean source of $^{12}\mathrm{N}$ produced at the IGISOL facility, we have measured the breakup of the $^{12}\mathrm{C}$ (12.71 MeV) state into three $\ensuremath{\alpha}$ particles with a segmented particle detector setup. The high quality of the data permits solving the question of the breakup mechanism of the 12.71 MeV state, a longstanding problem in few-body nuclear physics. Among existing models, a modified sequential model fits the data best, but systematic deviations indicate that a three-body description is needed.

Using beta decays of a clean source of 12N produced at the IGISOL facility, we have measured the breakup of the 12C (12.71 MeV) state into three alpha particles with a segmented particle detector setup. The high quality of the data permits solving the question of the breakup mechanism of the 12.71 MeV state, a longstanding problem in few-body nuclear physics. Among existing models, a modified sequential model fits the data best, but systematic deviations indicate that a three-body description is needed.

For very exotic nuclei close to the drip-lines the basic decay characteristics can provide a first insight into nuclear structure. In this contribution we present recent experimental results on the β-delayed charged particle branches from 14Be. With the present data a complete account of the 14Be β strength has emerged.

The study of beta decay to final states with high multiplicity has recently become more feasible with the advent of highly segmented particle detectors adjusted to low energy nuclear physics experiments [1]. This development has made possible to study exotic nuclear decay modes and especially to follow the break-up of unbound light nuclei fed in the decay. The high detector granularity allows for detecting all particles in the break-up, determining energy as well as emission angle and therefore the possibility to fully reconstruct the excitation energy of the decaying states.

The low-energy βγ-decay from 11Li has recently been shown to be a sensitive tool for extracting information about halo wave functions [1]. Here we present a method for using the shape of Doppler-broadened γ-lines to extract information about excited states fed in β-decay [2]. The procedure has been used to extract new information about the β-decay of 11Li through the de-excitation of excited states in 10Be.

Observation of Deeply Bound Pionic States of Xenon Produced in Xenat(d, 3He)Xepi-bound Reactions

The decays of the very neutron rich Sn isotopes Sn135-137 were studied at CERN/ISOLDE using isotopic and isobaric selectivity achieved by the use of a resonance ionization laser ion source and mass spectroscopy, respectively. Neutron decay rates, gamma-ray singles, and gamma-gamma coincidence data were collected as a function of time. The half-life (T-1/2) and delayed neutron emission probability (P-n) values of 135 Sn were measured to be 530(20) ms and 21(3)%, respectively. For Sn-136, a T-1/2 of 250(30) ms was determined along with a P-n value of 30(5)%. For Sn-137, a T-1/2 of 190(60) ms and a P-n value of 58(15)% were deduced. Identification of low-energy transitions in Sb-135 was made possible by comparison of laser-on and laser-off gamma-ray spectra. Those data combined with gamma-gamma coincidence spectra were used to construct a level scheme for Sb-135 that includes an unexpectedly low first excited state at 282 keV. A ground state beta branch of 33.2% was measured by following the growth and decay of the Sb-135 daughter. Shell-model calculations are consistent with the observed Sb-135 level structure and can account for a lowered first excited state.

This paper is divided in three parts: (i) the measurement of yields and decay losses of Li and Be isotopes released from a thin foil tantalum target at the CERN/ISOLDE PS-Booster; (ii) results from beta-decay experiments on Be-12 and Be-14, an improved half-life of 21.49(3) ms has been obtained for Be-12; (iii) the beta-decay of C-9. An outline of the analysis procedure to determine the branching at high excitation energies is given. The ground-state branch has been determined to 54.1(15)%.

In this contribution examples of the application of new technologies to disentangle the mechanism of beta-delayed multi-particle emission are given. In particular the mechanism of β-delayed two-proton emission from 31Ar is resolved and proved to be sequential, both for isospin allowed and isospin forbidden transitions. The analysis of the 9C decay has been focused in the study of the 12.2 MeV state in 9B. The decay of this state to pαα occurs sequentially, partial decay branches through the 8Be(0+,2+) and 5Li(3/2−,1/2−) intermediate states have been determined. A BGT value of 1.20(15) is deduced, compared with the BGT-value of the mirror transition, a asymmetry factor δ=3.7(12) is obtained, the largest ever observed. A preview of β3α data from 12N decay is discussed.

International audience; The existence of β-delayed particle emission allows one to study the β-strength distribution, and thereby the ground-state structure, of exotic drip-line nuclei. A time-correlation method has been applied to recent data on the heaviest Be-isotopes to determine their neutron branches. Improved results were obtained for 12Be and new results were found for the very short-lived neutron-emitter, 14Be.

17 pages, 3 tables, 5 figures, 1 appendix.-- PACS nrs.: 23.40.Hc; 27.20.+n., The β-delayed neutron branches from neutron-rich Be-isotopes have been measured with a setup sensitive down to thermal energies. For Be-14 a new value of P(n) = 101 ± 4% has been found for the total neutron-emission probability and an upper limit of 2.4% (95% confidence level) is imposed on the combination P(2n) + 3P(3n) of multi-neutron emission probabilities. Moreover, an improved value of P(n) = 0.50 ± 0.03% has been obtained for Be-12. A time-correlation method, previously developed for a continuous radioactive beam to determine neutron branching ratios, has been revised and adapted to the pulsed secondary beam at ISOLDE, CERN.

Investigations of light nuclei close to the drip lines have revealed new and intriguing features of the nuclear structure. The occurrence of halo structures in loosely bound systems has had a great impact on the nuclear physics research in the last years, as intriguing, but not yet solved, is the nature of transitions with very large beta strength. We report here on the investigation of this latter feature by an accurate measurement of the beta decay asymmetry between the mirror nuclei in the A=9 mass chain.

For the realization of near threshold studies a small-size spectrometer has been developed by the Nuclear Physics Division at Stockholm University in conjunction with Institut fur Kernphysik in Julich. A particle telescope is mounted inside the CELSIUS vacuum chamber in the bend following the cluster-jet target. It is possible to vary the measuring position within the dipole field to cover different magnetic rigidities which makes it a versatile tool for studies of threshold reactions. The first aim has been to study two-pion production and we present preliminary results from measurements of the d(d, 2π)4 He reaction.

Neutron angular distributions from violent break-up reactions of Li-11 and Be-11 have been measured at 28 MeV/u and 280 MeV/u and at 41 MeV/u and 460 MeV/u, respectively. The derived neutron momentum distributions show a narrow component in transverse momentum that is within uncertainties independent of beam energy and target charge. This component is suggested to be simply related to the momentum distribution of the loosely bound halo neutron(s) in the projectiles.

We report a search for gamma rays emanating from Coulomb excitation of fast (30-46 MeV/u) radioactive projectiles He-8, Be-11,Be-12,Be-14 interacting with a lead target. These are clearly identified by their Doppler shift. The 320 keV 1/2(-) --> 1/2(+)gamma transition from Be-11 was observed with a cross-section of 191 +/- 26 mb which is noticeably less than expected from the known lifetime and in the perturbation limit of pure Coulomb excitation. In the other nuclei rather stringent upper limits of 0.01 to 0.2 Weisskopf units, are placed on the hypothetical transition to 1(-) states.

The halo nucleus 6He has been studied in a pionic fusion experiment at the CELSIUS storage ring facility in Uppsala. The 6He nuclei were produced in reactions with a deuteron beam incident on a 4He jet target 0.9–5.4 MeV above threshold in the center-of-mass frame. The 6He ions were detected in a ΔE-E solid-state detector telescope inserted into the CELSIUS ring. The aim of the experiment was to investigate, in particular, the high-momentum part of the halo wave function by measuring the total and differential cross sections of the reaction d+4He→6He+π+.

13 pages, 6 figures., The beta-delayed charged particle spectra from 6He and 8He were measured with a telescope consisting of a gas counter and a Si surface barrier detector. Beta-delayed deuterons and tritons are emitted in the two cases with branching ratios of (7.6 ± 0.6)×10−6, Ed > 350 keV and (8.0 ± 0.5)×10−3, respectively. Both spectra present anomalies that must be due to the special structure of these nuclei (an alpha core surrounded by several neutrons)., One of us (M.J.G.B.) has been partly supported by CICYT under contract AE-90-0037.

The angular distributions of the forward neutrons in the exclusive (Be-10 + n) channel have been measured. They can be accounted for quantitatively and without free parameters in terms of Coulomb and diffraction dissociation. The results show that the transverse momentum distributions result from an interplay between the tail of the wave function (the halo) and the reaction mechanism.

We are investigating the halo nucleus 6He and its isobaric analogue state at 3.56 MeV excitation energy in 6Li in a series of pionic fusion experiments at the CELSIUS storage ring facility in Uppsala. The 4He(d, 6He)π+ reaction was studied 0.6–5.4 MeV above threshold in the centre-of-mass frame with a deuteron beam incident on a 4He gas target. The 6He ions were detected in a ΔE − E solid-state detector telescope inserted into the CELSIUS ring. Preliminary results are presented and compared to results for the analogue reaction producing 6Li*3.56.

We report on a search for deeply bound pionic states of xenon produced in the Xenat.(d,3He)Xeπ-bound reaction at Ed = 500 MeV. In the 3He energy spectrum two peaks are observed corresponding to the production of pionic atoms in the 1s state.

We have made an exploratory experiment studying the 3He + 3He → 6Li* + π+ reaction very close to threshold. The measurement was done at the CELSIUS storage ring. The 6Li ions were detected in a zero-degree spectrometer equipped with solid state detectors.

Beta-coincidentγ-rays have been measured from implanted pure samples of12Be separated at the LISE3 spectrometer at GANIL. An intensity of 0.040(26) % can be estimated for the branching ratio of the isospin forbidden pure-Fermi transition to the 0+ excited state of12B and of 0.008(6)% of the transition to the 1−1 excited state. Both are taken to represent upper limits. The half-life has been re-measured to be 26.1(2.4) ms.