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Study of b-delayed two-proton decay of 22Al and 26P - 1998

Date: 1998


* Univ. Jyväskylä (Finland)
* CEN Bordeaux-Gradignan (France)
* GANIL Caen (France)
* IPN Lyon (France)
* IReS Strasbourg (France)
* Univ. Edinburgh (UK)



The basic purpose of this experiment was to study the β-delayed two-proton emission of 22Al and 26P at the GANIL/LISE3 facility. A setup with an array of 15 gas-silicon telescope detectors was initially planed for the measurement of proton energies. The aim of such a high granularity setup is a good discrimination of protons and b particles, and the determination of the angular correlation of the two emitted protons in order to get information on the two-proton emission mechanism.


Experimental technique

The secondary beams (22Al, 26P and 27S) are produced in the fragmentation of a 2m A and 95 MeV/A beam of 36Ar, and selected with the LISE3 spectrometer. The identification of isotopes is performed by a time-of-flight and energy loss measurement commonly used at LISE.

Two kinds of detection systems were installed at the end of the beam line :

* A stack of 4 silicon detectors: the first two ones (E1 and E2, 500 m m thick) are used for the measurement of the energy loss of selected ions; these isotopes are implanted in the third one (E3, 500 m m thick) and the last one (E4, 6 mm thick) is used for coincidences with b particles: if a proton emitter is implanted in E3, the β from its decay can escape the detector E3 while the proton loses all its energy inside. The coincidence between the two detectors E3 and E4 allows to clean the proton energy distributions.

* The detectors E3 and E4 can be removed from the beam line in order to work with the second detection setup. In this case, selected radioactive ions are implanted in a gas cell and transported (with an argon-jet technique) to a tape device. Around the collection point are installed the 15 gas-silicon telescopes for two-proton emission studies.

In both cases, a high efficiency germanium detector was added to the setup in order to locate precisely the states involved in the β or β-proton decay.


Encountered troubles

Although different experimental conditions with the gas jet were tested, the transport efficiency for isotopes of interest was about 50 times lower than expected. It was finally decided to run the experiment with only the first detection setup, that doesn’t allow to measure any correlation of two protons, but can only measure the sum energy.


Experimental data

The remaining beam time allowed us to perform measurements on the three isotopes 22Al, 26P and 27S, that will be respectively analysed at Caen, Bordeaux and Lyon. Additional runs were performed on known proton emitters in the same mass region (21Mg and 25Si), to be used for the proton energy calibration of the silicon detectors.

For the 3 isotopes under consideration, a short part of the beam time was dedicated to an improvement of life-time measurement. For this purpose, we defined the following time structure: we collected ions for about 2 decay periods of the isotope, and we stop the beam for about 5 periods in order to observe the decay. The main time was devoted to spectroscopic studies, by measuring β-γ , β-proton-γ and β-proton decays of these isotopes.


Fig. 1: Silicon stack at the end of the LISE beam line for β-γ, β-p and β-p-γ studies.


Fig. 2: setup for the argon-jet measurements


Fig. 3: offline charged particle spectrum for the decay of 26P.