Centre Etudes Nucléaires de Bordeaux Gradignan

date: Juin 2000

 

Collaboration

J. Giovinazzo, B. Blank, M. Chartier, S. Czajkowski, M.S. Pravikoff, J.-C. Thomas, CENBG Bordeaux, France

C. Borcea, Institute for Atomic Physics, Bucharest, Romania

G. de France, M. Lewitowicz, F. de Oliveira Santos, GANIL, Caen, France

R. Grzywacz, Z. Janas, M. Pfützner, Institute of Experimental Physics, Warsaw, Poland

 

Motivation

 

Context

- proton drip-line reached experimentally up to A = 50 (projectile fragmentation experiments: GSI/FRS, GANIL/LISE, ...)
- full shell-model calculations in the fp-shell possible (Caurier et al.)
- mass region with good candidates for 2-proton radioactivity: correlated 2p emission from ground state (1p emission energetically forbidden) predicted since 1960’s but not observed ³⁹Ti, ⁴²Cr, ⁴⁵Fe, ⁴⁹Ni, ⁴⁸Ni are candidates

 

Experimental work

- spectroscopic studies of nuclei at the drip-line (Z = 22 to 28): decay of ^39,40Ti, ^42,43Cr, ⁴⁶Mn, ^45,46,47Fe and ⁴⁹Ni
- all isotopes are (beta-delayed) proton emitters

 

Experimental set up

GANIL cyclotrons for acceleration of primary beam:

Primary beam: ⁵⁸Ni²⁶⁺, 74.5 MeV / A, 3 mAe
SISSI device for a secondary beam

Reaction: projectile fragmentation of beam in a nickel target LISE3 spectrometer for selection of ions

Decay: particule - gamma coincidences, pixel correlation with implantation

Experimental set-up of the GANIL cyclotrons for acceleration of the beam, with the SISSI device for secondary-beam production, the LISE3 spectrometer for the selection of fragments and the detection set up.

 

 

Detection Set up:

Selected ions loose energy in the Silicon telescope (DE, E) and then are implanted in a silicon strip detector where their positions are determined. The implantation point is surrounded by 4 germanium detectors to detect the gamma-rays from decay. A third silicon detector is used as a veto detector.

 

 

Identification of implanted ions:

- energy: DE1, DE2, E3
- time of flight: with cyclotron RF and MCP (located before the Wien filter)
- veto signals: for light ions going through the telescope
- position: X-Y sensitive silicon detector (E2)

-> ion-by-ion identification almost no background !

 

 

Proton-gamma spectroscopy

need for high enough conting rates for proton-gamma coincidences

proton + gamma energy

 

 

 

 

Search for 2p emission

 

Most exotic isotopes: ⁴²Cr, ⁴⁵Fe, ⁴⁹Ni

 

Relation between proton energy and half-life: tunnel effect through Coulomb barrier

⁴²Cr: Ep = 1.9 MeV ---> expected T1/2 10-12 s

⁴⁹Ni: Ep = 3.7 MeV ---> expected T1/2 10-16 s

⁴⁵Fe: too low statistics

 

Concluding Remarks

 

First half-life measurements for ⁴²Cr, ⁴⁵Fe and ⁴⁹Ni

Improved precision for half-lives of ³⁹Ti, ⁴³Cr, ⁴⁶Mn and ^46,47Fe , deduced IAS position, proton branching ratios, ...

need for higher gamma efficiency

First decay scheme for the most produced isotopes ⁴³Cr and ⁴⁷Fe

limited by precision on mass excess of daughter nuclei need for a comparison with a theoretical approach

No evidence for a 2-proton radioactivity:

⁴²Cr and ⁴⁹Ni: indications of a beta-delayed proton emission (from half-life, proton energy) ⁴⁵Fe: need for higher statistics

other candidates to be studied: ⁴⁸Ni and ⁵⁴Zn ...