ISOLDE EXPERIMENTS COMMITTEE
Minutes for the twenty-fourth meeting on
Monday 4 May 1998
 
 
 

OPEN SESSION: Monday, 4 May, at 14.00 h, Council Chamber.

After welcoming the new EP Deputy Division Leader Rüdiger Voss to the meeting, the Chairman invited the ISOLDE technical group leader to present his report. Helge Ravn discussed the work carried out during the shutdown period. Among other items this work involved beam realigment to correct for floor subsidence and installation of new HRS magnet power supplies. The cleaning procedure necessitated by the leakage of dust into the experimental hall as reported in the minutes of the twenty third meeting) had delayed the work to be carried out during the shutdown period by one month and the 1998 startup with protons by 15 days. Concerning the beam development programme, although an improvement of the negative ion source efficiency for At could not be achieved by adding Ba vapour, a rise in the BaF+ efficiency to 3% was obtained, and an efficiency of 10% was achieved with the element Mg.

On the other hand, serious troubles with the GPS front end developed during the shutdown period. While this situation has been saved by the installation of the reserve front end, the repair/replacement problem for the old front end remains and is likely to be costly and time consuming.

Thomas Nilsson, the ISOLDE coordinator then presented his proposed schedule for 1998. This has now been divided into HRS and GPS parts, the HRS programme mainly making use of days on which the GPS is unavailable or is radioactively cooling off. The HRS schedule is made up of some online experiments, a large number of target tests, and a few offline experiments. Startup of the GPS will be delayed by one week because of the front end problems noted above.

Presentations were then made of the following proposals and progress reports:

1. Lattice location of transition metals in semiconductors Leuven - Lisboa - CERN);
    ISC 98-9/P98; U. Wahl.

2. Addendum to Proposal IS342: Emission channeling studies on the lattice location of lithium,
    sodium  and rare earth isotopes in semiconductors Konstanz - Leuven - Durban- Westville);
    ISC 98-10/P62 Add.2; H. Hofsäss.

3. Study of the unbound nuclei 10Li and 7He at REX ISOLDE Göteborg - Aarhus - Madrid - Darmstadt -
    CERN - Örebro); ISC 98-11/P100; B. Jonson.

4. Acceptors in II-VI Semiconductors - Incorporation and Complex Formation;
    Saarbrücken - Konstanz - CERN); ISC 98-12/P101; T. Wichert.

5. IS345: Nuclear, electrical and optical studies of hydrogen in semiconductors Konstanz - Berlin -
    Bonn - Duisburg - Erlangen - Freiburg - Jena - Lisboa - Caen - Paris - Troitzk - CERN);
    ISC 98-14/P65 Add.2; A. Burchard.
 

CLOSED SESSION

Present: B.W. Allardyce, R. Ammerlaan, G. Bollen, J. Eades Secretary), L. Foà, W. Gelletly,
              K-L Kratz, T. Nilsson, D. Habs, R. Voss for G. Goggi), H. Ravn, A. Richter Chairman),
              F.-K.Thielemann.

Apologies: P. Van Isacker, G. Sletten
 

The Chairman once again welcomed R. Voss, who as the new EP deputy division leader, takes over from
K. Peach as the representative of the EP division leader, G. Goggi. The minutes of the twenty-third meeting were then approved without change.

Concerning the problem of repairing or replacing the damaged front end, it was remarked by  H. Ravn that the good performance of the replacement meant that this problem was not extremely pressing but that some attempt must be made to carry out the repairs or to tender for a new front end) by the end of the next shutdown. The committee took note of this situation and urged him to investigate cost-effective means of getting around the problem, contracting the work outside CERN if necessary. It furthermore asked him to report on progress in this matter at the next ISC meeting. At the same time the committee congratulated
H. Ravn and the ISOLDE technical group on the remarkably performance achieved by the laser ion source
and on the increased intensity of other beams.

Thomas Nilsson regretted the technical problems that had caused the shift of the 1998 startup date by one
week, but expressed confidence that this delay could to some extent be made up be made up. In response
to a question concerning the buildup of a large number of shifts for solid state experiments such as the ones featured in the present meeting, he remarked that time shared running made this a considerably less serious problem than the actual numbers suggest.

Asked about the status of the 1.4 GeV beam, B. Allardyce indicated that this mode of Booster operation has now been shown to work well, but that at the moment no extraction at 1.4 GeV is available. Operation at  1.4 GeV is now expected to be available after the 1999 shutdown.

The closed meeting then continued with a discussion of the proposals and progress reports presented in the open session, as follows:

P98: Lattice location of transition metals in semiconductors

The reaction of the committee was favourable to this proposal, which was considered to have been well thought out and presented, and which bears on interesting problems concerning the location and diffusion
of transition metals in semiconductors. The Chairman agreed to recommend the modest allocation of
8 shifts to the Research Board, noting that these can be easily shared with other experiments.

P62 Add.2: Emission channeling studies on the behaviour of light alkali atoms in wide-band-gap semiconductors

Among the goals of the experiment, the committee found those concerning rare earth doping into GaAs and Si particularly interesting, and looks forward to seeing results on the doping of 24Na into diamonds. However, as this is the second addendum to an existing proposal, it was felt that the experiment should definitely be considered to have come to an end after the further 10 shifts requested have been allocated.

P100: Study of the unbound nuclei 10Li and 7He at REX ISOLDE

After some discussion it was concluded that a strong case has been made by the collaboration for carrying out the proposed study of the shell structure of unbound nuclei near the n-drip line. The experiment on 10Li was thought to be a particularly suitable application of the available REX ISOLDE 2 MeV/u beams to solve problems concerning the halo nucleus 11Li.

The committee pointed out that the collaboration should also be able to complement their experiments with strong theoretical support on these reactions, which is now available at a number of institutes. The Chairman will therefore recommend that the Reseach Board allocate of the full 34-shift request.

P101: Acceptors in II-VI Semiconductors - Incorporation and Complex Formation

It was thought that the case for the planned II-VI semiconductor studies had been strongly argued but that of the many studies proposed, some were clearly more valuable than others. Most interesting is the proposed complex formation of acceptor hydrogen pairs with silver, less so are the Rb and Kr experiments. The Chairman will therefore recommend to the Research Board the allocation of 16 shifts for the 111AG and the As, Xe and Cs experiments.

P65 Add. 2: Nuclear, electrical and optical studies of hydrogen in semiconductors

The committee thought that as hydrogen is a very important impurity in semiconductors, the proposed passivation and diffusion studies were to be encouraged. However, it also noted that 65 shifts have already been allocated to this proposal and that 14 of these are outstanding. It therefore encourages the collaboration to use these remaining shifts, together with a new but final allocation of 20 shifts to solve
the above mentioned problems, after which the experiment will be considered closed.

OTHER BUSINESS

A. Richter stated that the CERN Director General has asked him to continue in his capacity as ISC Chairman for one more year, and that he has accepted. This was noted with satisfaction by the committee.

Date of the next ISC meeting is 28 September 1998.