P.J. Sellin

A double-sided silicon strip detector system for proton radioactivity studies

Abstract A new double-sided silicon strip detector has been developed to study the phenomenon of ground-state proton radioactivity. Highly proton-rich fusion-evaporation reaction products are velocity and mass separated using the Daresbury recoil separator before being implanted into the strip detector located at the separator focal plane. The double-sided strip detector has 48 strips per face with a pitch of 335 μm. Front and back strips are orthogonal providing an effective pixel area of 0.09 mm2, enabling correlations between implanted ions and subsequent decays to be clearly established. Test results obtained using the reactions 58 Ni + 92 Mo → 150 Yb ∗ and 58 Ni + 54 Fe → 112 Xe ∗ are reported. In the latter commisioning experiment the proton decay of 109I was unambiguously established using the correlated decay sequence 109I→p108Te→α104Sn.

Proton radioactivity from 146Tm. The completion of a sequence of four odd-odd proton emitters

Abstract Proton emission has been observed from the new isotope 146 Tm. Two transitions have been identified at energies of 1119±5 keV and 1189±5 keV corresponding to Q -values of 1127±5 keV and 1197±5 keV respectively. The measured half-lives of 235±27 ms and 72±23 ms are well reproduced by simple WKB calculations assuming proton emission from h 11 2 orbitals. The proton transitions are assigned to a 10 + isomeric state and a low lying 5 − or 6 − level.

Alpha decay of the new isotope172Au

The first observation of the extremely neutrondeficient nucleus172Au is reported, produced using the fusion evaporation reaction70Ge+106Cd→176Hg* (Ex≃64 MeV). Mass separated evaporation residues were implanted into a double-sided silicon strip detector, and the energy and time of subsequent decay events were recorded. The alpha decay of172Au was measured with an energy =6860±10 KeV, corresponding to =7020±10 Kev, and a half-life of 4±1 ms. No evidence was seen for a proton decay branch, implying a limit ofbp≤ 2 %.

The limit of stability of proton-rich thallium isotopes: a search for the decay of177Tl

The fusion reaction74Se+106Cd→180Pb* at a compound nucleus excitation energy Ex ≃ 40 MeV, has been used in a search for the decay of the unknown nucleus177Tl. Evaporation residues were velocity and mass analysed prior to implantation into a position sensitive silicon surface barrier detector. No evidence was discovered for the proton or alpha-decay of177Tl for a cross-section > 10−5+10 nb. This indicates either that177Tl decays too rapidly (t1/2 < 1 μs) by proton emission for the decay to be detected, or that177Tl is produced with a cross-section less than the limit established in the present experiment. The following nuclear decay half-lives were measured with improved accuracy;177Hg (t1/2 = 130±5 ms),178Hg (t1/2=250±25 ms) and177Au (t1/2=1180±70 ms). The experiment also provided the first direct confirmation of the correct mass assignment of the alpha-decay, Eα=6.26 MeV, to the decay of176Au. Two additional halflife measurements of proton-rich Osmium isotopes are also presented from a previous similar experiment;165Os (t1/2=73±8 ms) and166Os (t1/2 = 194±17 ms).

Tensor analyzing powers for Li induced transfer breakup reactions

The tensor analysing power T/sub 20/ has been measured for the /sup 120/Sn(/sup 7/Li, /sup 8/Be to 2 alpha )/sup 119/In and /sup 120/Sn(/sup 7/Li, /sup 6/Li* to alpha +d)/sup 121/Sn transfer breakup reactions at 70 MeV bombarding energy. Coupled channels and continuum discretized coupled channels calculations, incorporating a detector phase space correction, were found to give good agreement with the data.

The Ne-19(p,gamma)Na-20 astrophysical reaction rate determined from measurements with a radioactive beam

The Ne-19(p,gamma)Na-20 as well as the Ne-19(d,n)Na-20 reaction have been studied in inverse kinematics using Ne-19 radioactive beams. Upper and lower limits for the Ne-19(p,gamma)Na-20 astrophysical reaction rate have been deduced, for the first time on the basis of direct experimental data. It is concluded that the transition from the hot-CNO cycle to the rp-process in explosive hydrogen burning is most likely governed by the preceeding O-15(alpha,gamma)Ne-19 reaction.

Nuclear reactions of astrophysical interest with radioactive beams

The ARENAS(3) facility, coupling the two cyclotrons of Louvain-la-Neuve, is intensively been used over the past years for cross-section measurements of nuclear reactions of astrophysical interest. We will describe the characteristics of the available beams and the experiments going on using these beams. The Ne-19(p,gamma)Na-20 reaction, of considerable importance for the reaction now between the CNO and the NeNa mass region in high-temperature hydrogen burning conditions, will be discussed in particular. The proposed low-energy resonance at 0.447 MeV has been measured in inverse kinematics using novel activation techniques. An upper limit (90% C.L.) of 18 meV is obtained for the strength of this first particle-unbound state of Na-20 together with preliminary values for the strength of the higher excited states.

The discovery of new proton emitters at the limit of nuclear stability

Abstract A chain of four odd-odd proton emitters ( 146 Tm, 150 Lu, 156 Ta and 160 Re ) has been established from Z=69–75, representing the heaviest region in which proton radioactivity has been identified. This also clearly shows that the proton drip-line has been crossed up to Z=75. The results indicate that there is a reversal in the d 3 2 and h 11 2 proton orbitals around Z=73. Proton separation energies are compared with mass model predictions.

A new technique for spin alignment measurement and application to the study of resonances in 12C on 12C collisions

Abstract A new technique has been developed for measuring spin alignment in heavy ion collisions which excite nuclear states above breakup threshold. Breakup fragments are detected in position sensitive silicon strip detectors, in coincidence with the recoil from the collision. The alignment can be determined from the spatial distribution of the fragments. This technique has been applied to the study of resonances in 12 C on 12 C collisions. The 9.64 MeV 3 − state in 12 C is excited, which breaks up into 8 Be + α and t the 8 Be breaks up into two α-particles.

Kinematically complete study of the Li7(p, )8Be*2± reaction at Ep=25 MeV

A kinematically complete study of the {sup 7}Li({ital p},{gamma}){sup 8}Be{sup *}{r arrow}2{alpha} reaction at {ital E}{sub {ital p}}=25 MeV is described. An upper limit of {ital d}{sigma}/{ital d}{Omega}{lt}0.04 {mu}b sr{sup {minus}1} is reported for the {theta}{sub {gamma}}=90{degree} capture cross section to the 2{sup +} states at 16.62 and 16.93 MeV, which is five times smaller than published results. This new datum is shown to be more consistent with a semidirect reaction mechanism involving the giant dipole resonance built on the {sup 8}Be{sup *} 2.94-MeV state than the previously proposed direct reaction mechanism.