E. C. Pollacco

Evidences for resonance states in 5H

Resonance states of H-5 were investigated through the two-neutron transfer reaction t(t, P)(5) H. A triton beam at 57.5 MeV and a cryogenic liquid tritium target were used. The H-5 missing mass spectrum in triple coincidence, proton + triton + neutron, shows a resonance at 1.8 +/- 0.1 MeV above the t + 2n decay threshold. This energy is in good agreement with the result reported in Phys. Rev. Lett. 87 (2001) 092501. The resonance width, Gamma(intr) less than or equal to 0.5 MeV, is surprisingly small and difficult to reconcile with theory predictions

AGET, the GET front-end ASIC, for the readout of the Time Projection Chambers used in nuclear physic experiments

Today with the advent of intense radioactive beams, we have access to nuclear spectroscopy and reaction studies of nuclei far from stability. It has been demonstrated that Time Projection Chambers (TPC) method can be very effective as an active target for such studies yielding low thresholds, efficiency and luminosity [1]. To this end a Generic Electronic system for TPCs (GET) is in development and will cover small to medium sized instrumentation (64 to 32 k channels) with a relatively wide charge dynamic ranges for event rates of up to 1 kHz. The 64-channel AGET (ASIC for GET) front-end circuit has been developed to perform the amplification, detection and analog storage of the shaped detector signal before its digitization by an external 12-bit ADC. This design offers a large flexibility in sampling frequency (100 MHz max.), peaking time (16 values from 50 ns to 1 µs), gain (4 ranges from 120 fC to 10 pC per channel) and signal polarity (negative or positive). Fabricated using 0.35 µm CMOS technology, the AGET prototype is under test and the first results are presented.

Extension of the N = 40 Island of Inversion towards N =50: Spectroscopy of 66Cr,70,72Fe

We report on the measurement of the first 2(+) and 4(+) states of (66)Cr and (70,72)Fe via in-beam γ-ray spectroscopy. The nuclei of interest were produced by (p,2p) reactions at incident energies of 260  MeV/nucleon. The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN, using the DALI 2γ-ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2(1)(+) and 4(1)(+) energies as a function of the neutron number was observed for N≥38 and N≥40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified Lenzi-Nowacki-Poves-Sieja (LNPS) interaction in the pfg(9/2)d(5/2) valence space reproduce the observations. Interpretation within the shell model shows an extension of the island of inversion at N=40 for more neutron-rich isotopes towards N=50.

A compact gridless channel plate detector for time-of-flight measurements

Abstract A compact channel plate detector used as a start detector in heavy ion reaction studies is described. In order to avoid spurious structures in the energy spectra of the detected ions the usual electron accelerating grid is replaced by five electrodes which provide a uniform electric field. The start detector has an intrinsic resolution of 85 ps and efficiency of 90% for medium mass nuclei. The overall time resolution with a solid state detector as stop is 105 ps.

A gas secondary electron detector

Abstract A new Secondary Electron gas Detector (SED) is under development to be used in conjunction with an emissive foil to detect low energy heavy ions as an alternative to micro-channel plates. It could measure position and time of flight. Secondary electrons are accelerated to 10 keV so that they can cross through the 0.9 μm Mylar entrance window. The electrons then are multiplied in the isobutane gas of the detector at 4– 10 Torr . A time resolution of 150 ps and a spatial resolution of 3 mm have been obtained by using californium fission fragments on a prototype detector of 7×7 cm 2 . The advantage of the SED against MCP is that its size is not limited. Our final goal is to build a large size detector (15×40 cm 2 ) that will operate at the focal plane of the VAMOS magnetic spectrometer at GANIL.

Structure of unbound neutron-rich

The 8He(d,p) reaction was studied in inverse kinematics at 15.4A MeV using the MUST2 Si-CsI array in order to shed light on the level structure of 9He. The well known 16O(d,p)17O reaction, performed here in reverse kinematics, was used as a test to validate the experimental methods. The 9He missing mass spectrum was deduced from the kinetic energies and emission angles of the recoiling protons. Several structures were observed above the neutron-emission threshold and the angular distributions were used to deduce the multipolarity of the transitions. This work confirms that the ground state of 9He is located very close to the neutron threshold of 8He and supports the occurrence of parity inversion in 9He.

^{9}

In this Rapid Communication, we report on {alpha}-particle emission through the nuclear breakup in the reaction {sup 40}Ca on a {sup 40}Ca target at 50 A MeV. It is observed that, similar to nucleons, {alpha} particles can be emitted to the continuum with very specific angular distribution during the reaction. The {alpha}-particle properties seem to be compatible with an {alpha} cluster in the daughter nucleus that is perturbed and is emitted by the short-range nuclear attraction of the collision partner. A time-dependent theory that describes the {alpha}-particle wave-function evolution is able to qualitatively reproduce the observed angular distribution. This mechanism offers new possibilities for studying {alpha}-particle properties in the nuclear medium.

He studied using single-neutron transfer

Abstract The study of nuclei far from stability interacting with simple target nuclei, such as protons, deuterons, 3 He and 4 He implies the use of inverse kinematics. The very special kinematics, together with the low intensities of the beams calls for special techniques. In july 2002 we tested a new detector, in which the detector gas is the target. This allows in principle a 4π solid angle of the detection, and a big effective target thickness without loss of resolution. The detector developped, called Maya, used isobuthane C 4 H 10 as gas in present tests, and other gases are possible. The multiplexed electronics of more than 1000channels allows the reconstruction of the events occuring between the incoming particle and the detector gas atoms in 3D. Here we were interested in the elastic scattering of 8 He on protons for the study of the isobaric analogue states (IAS) of 9 He. The beam, in this case, is stopped in the detector. The resonance energy is determined by the place of interaction and the energy of the recoiling proton. The design of the detector is shown, together with some preliminary results are discussed.

Probing preformedαparticles in the ground state of nuclei

An environmentally safe liquid/solid tritium target suitable for beams of radioactive nuclei was created and used in the study of t+d and t+t transfer reactions with putting emphasis on the observation of 4H and 5H resonance states. A state of 4H with Eres=3.22±0.15 MeV and Γobs=3.33±0.25 MeV was obtained in t+d reaction from the spectra of protons leaving the target at θlab=18°–32° and detected in coincidence with tritons. A valuable fraction of protons detected in t+t reaction at θlab=18°–32° in ptn coincidence events was attributed to the states of 5H nucleus. At ∼2.5 MeV above the tnn decay threshold the 5H spectrum shows up a narrow maximum followed by a wide structure at 4–7 MeV. One can not exclude that interference effects could modify the observed 5H resonance maximum resulting in its energy shift and width reduction.

New target and detection methods: active detectors

Abstract The Fusion Utility for Secondary Ions (FUSION) has been built to measure the fusion–fission cross section induced by radioactive beams at energies around the barrier. A set of 10 parallel plate avalanche counters measures the fission fragments which are stopped therein. Twenty plastic scintillators, surrounding the inner shell of PPACs, detect the eventual residue from the projectile. This detector could work in different geometry and allow measurements of low cross section with very weak beams. The measurements at GANIL of the fission cross sections at barrier energies for 9,11 Be on U are reported.