Motivation

Guidelines for yield estimation

1. As a first step, consult the ISOLDE yield database for the extracted ISOLDE yields.

2. Runs scheduled with GPS have an expected transmission between ISOLDE and REXTRAP of at least 90%. For runs scheduled with HRS a transmission loss occurs in ISCOOL. A transmission between 50% (light elements e.g. ³⁹K) and 90% (heavy elements, e.g. ¹³³Cs) is expected.

3. The efficiency for the REX low-energy stage depends on many factors, the main being the ion mass. Fig. 1 shows the total transmission (REXTRAP+REXBIS) for different elements, mainly stable. Very light elements (A<15) has an efficiency of around 5% due to poor cooling in the REXTRAP. The moderate efficiency for ions with A>150 has several reasons, which are presently being addressed.

Fig. 1. Combined REXTRAP and REXEBIS efficiency for a selection of elements.

4. The LINAC transmission has been improved during the shutdown 2009-2010. A transmission of 80-85% is expected from the REX mass separator focus to the target station. Insertion of additional collimators near the target area will reduce the transmission.

5. The total yield at the experimental target equals yield(ISOLDE) x efficiency(ISOLDE-to-REXTRAP) x efficiency(low-energy stage) x efficiency(LINAC).

The hold-up time is comprised by the cooling time (T_cool) in REXTRAP succeeded by the breeding time (T_breed) in REXEBIS. The cooling time has to be >10 ms to be efficient. The breeding time varies between 3 and 400 ms according to the ion mass, see Fig. 2. Note that the total hold-up time will be twice the breeding time, except for very light elements when the breeding time is less than 10 ms.

Fig. 2. Breeding time as function of ion mass for a selection of elements. For the total hold-up time the time should be doubled. Closed shell breeding (*) increases the efficiency but also the breeding time.

Remarks

a. Slow extraction from the EBIS will reduce the transmission efficiency with 10-20% from the nominal value. If sufficient time (>3 h) is given for the tuning and on-line diagnostics (particle counter) is present this loss can be suppressed.

b. Note that the REX low-energy efficiency is intensity dependent. For injected average currents above a few 10 pA the efficiency drops, the exact amount depends on the cooling time. The trend is illustrated in Fig. 3 for ²³Na⁷⁺.

c. In case isobaric mass resolution is requested the low-energy efficiency is reduced with at least a factor of five. The cooling time becomes as a minimum 200 ms and the injected beam current (incl contaminants) has to be less than a few pA.

d. Radioactive beams tend to have a lower efficiency than the corresponding stable beams, even if the decay losses are taken into account. Charge exchange with ionized buffer-gas atoms in the trap can partly explain the lower efficiency.

e. The REX low-energy system is very flexible and therefore a number of different operation modes can be employed to optimize the performance for each specific beam. Already mentioned above are the isobaric mass resolution and slow extraction from the EBIS extending the beam pulse from ~40 us FWHM to around 400 us. Different buffer-gas types can be used in the REXTRAP to avoid overlapping A/q-contamination. The beam can be continuously injected into REXEBIS without making use of REXTRAP, or ISCOOL can be used instead of REXTRAP as cooler/buncher. Using in-trap decay the radioactive mother-ions are trapped in the low-energy system until they decay and the daughter-ions are accelerated to the experiment.

f. To make full use of the REX-ISOLDE post-accelerator, or for any specific yield questions, please contact the REX specialists (F. Wenander or D. Voulot).

Fig. 3. Low-energy efficiency as function of the injected current for a period time of 20 ms. The electron beam was moderate, thus a low optimal efficiency.