98Sr run 5-12 September 2011

 

Target issues
* Target number #458 UC W-surf (with F leak for molecular beam)
* Beam on target (no converter)
* Mass markers:  Na/Li, Rb
* HRS separator
* Extraction electrode 60 mm (real distance)
* Uht(HRS, CCV)=30.25 kV, Uht(ISCOOL, CCV)=30200 V, Uht(trap, CCV)=30000 V, Elec05=175 V

 

Yield measurements (preliminary conclusions):

98Sr: The sum of Sr provided by the target and produced by Rb decay on tape is (8+/-2)E5 ions/uC. The fast component of this comes with a time constant of 100ms and contributes 85% of the yield. I would expect this to be 98Rb derived. The slow component shows a time constant of 380ms which reflects the 98Sr release and its decay constant together.

 

95Sr: If a comparison to last year's measurements at REX becomes interesting, I also took some data on mass 95. In this case I derived a Rb yield in the order of 1E7 ions/uC and a Sr yield in the order of  2E6 ions/uC. Which confirms the expected Rb/Sr ratio.

 

Nothing seen on A=117 (98Sr+19F).

 

Experimental issues
*
Beam to Miniball
* IS451


Tuning
* Slow extraction from EBIS for the Sr beam
* REXTRAP triggered by protons pulses

 

ISOLDE
   A=98 transmission

HRS.FC490           355pA

CA0.FC68             230pA   -> 65%

 

Low energy
   Stable beam (normal extraction)

I_coll 182 mA

87Rb23+

T_period 100 ms, T_breed 88 ms (optimised)

TRAP.FC10       7.8 pA

BTS.FC20        4.6 pA          (59%)

RFQ.FC20        14.7 pA                                                                      ->            8.2%

 

T_period 150 ms, T_breed 88 ms (optimised)

TRAP.FC10       7.8 pA

RFQ.FC20        14.0 pA                                                                      ->            7.8%

87Rb25+

T_period 200 ms, T_breed 158 ms (optimised)                   ->            8.7%

87Rb27+ (Ne like)

T_period 200 ms, T_breed 198 ms (not optimised)           ->            14%

87Rb28+

T_period 200 ms, T_breed 198 ms (not optimised)

TRAP.FC10       7.8 pA

RFQ.FC20        7.1 pA                                                                        ->            3.6%

 

The efficiency seems to be strongly dependant on the shell configuration, unfortunately 98Sr28+ (Ne-like) falls on top of N4+. In the end 26+ is chosen with a long trapping and breeding time (160/158 ms) to enhance in-trap decay of 98Rb (T1/2_98Rb = 114 ms, T1/2_Sr = 653 ms).

 

   Linac transmission and energy: 
   No stripper foils. WITCH magnets 0.1 and 3T.

   Transmission from RFQ.FC20 to XL65.FC50

   - 84%                  for A/q = 4                          at 2.82 MeV/u

   - 84%                  for Ne6+ (3.3333)             at 2.82 MeV/u

   - 78%                  for 133Cs31+ (4.2903)     at 2.82 MeV/u

 

Total transmission (radioactive beam)
HRS.FC490_to_trap x Trap_EBIS x Linac 
0.65*0.10?*0.80=5%


Delivered beams

98Sr26+ @ 2.82 MeV/u

T_period 160 ms, T_breed 158 ms (optimised), I_coll 185 mA, T_inj 120 us

TRAP 466.2 kHz / 15 V, HT 30000V

Currents on FCs:

TRAP.FC10          2 pA in the peak

BTS.FC20              1.8-2.5 pA in the peak

RFQ.FC20 (26+) 1-3.0 pA in the peak                        4 to 12% low energy efficiency

Average currents lower as the release is fast.

 

Miniball’s estimates based on the Coulex cross section:

98Sr 2E4 pps for 0.9 uA of proton beam on HRS

Beam composition measured with the IC:

92% of Sr, 5% of 98Y and 3% of 98Rb


Setting summary
For HRS we used:

HRS_SEP_2011-09-05_30200V_98Sr.csv

RFQ_2011-09-05_30200V_98Sr.csv

REX_RFQ_2011_09_05_98Sr_30200.csv


REX low energy settings saved as:

110905_1747_TRAP_HRS_98Sr26

110905_1747_BTS_HRS_98Sr26

EBIS_110905_1745_HRS_98Sr26+

 

REX separator and linac settings saved as:
110905_1744_SEP_Aq3.7692_98Sr26+_2.82MeVu_MB

110905_1744_Aq3.7692_98Sr26+_2.82MeVu_MB

 

Technical problems

* No Sr mass marker

 

Comments / Lessons learned

* Test in-trap decay with different trapping/breeding times, not very conclusive (report by P. Delahaye)

* No time to test molecular beams. Expect lower overall efficiency.

* Investigation of beam contamination from REX

Element A/q     Residual elements
98Sr23+ 4.2609  17O     47Ti(small)     51V(?)  140Ce
98Sr24+ 4.0833  49Ti(small)     57Fe(small)     82Kr    86Kr
98Sr25+ too close to 117In30+ (A/q 0.02 away)
98Sr26+ on top of 117In31+
98Sr27+ 3.6296  40Ar    47Ti(small)     51V(?)  80Kr(small)     142Ce
98Sr28+ on top of N4+
98Sr29+ 3.3793  54Fe    84Kr    142Ce

Short report on the in trap decay tests – 7/09/2011

Exp. IS451 E. Clement

Tests concerning the beam preparation: P. Delahaye, D. Voulot, F. Wenander

Goal:

·         Coulomb excitation of 98Sr

·         Optimizing of the so-called in trap decay using REXTRAP for producing a pure beam of 98Sr  by the decay of 98Rb nuclides

·         Finding the most advantageous scheme between in EBIS and in REXtrap decay (to be continued by Fredrik on Friday)

Beam conditions:

3e13pps on UCx target #458  + surface ionization source

Injecting mass 98 into REXTRAP and REX-EBIS

Yield check not done yet on mass 98, 98Rb+98Sr, composition apriori unknown (please update when Alex would have measured the yields )

Measured on FCs in the peak following the proton pulses:

TRAP.FC10       2 pA in the peak (1/10 of this intensity during the in trap decay tests because of a wiregrid which was inserted by mistake)

Half lives:

98Sr: 653ms

98Rb: 96ms

Tests:

The optimization was attempted by looking at the time of flight spectrum after REXTRAP. This latter was found to exhibit a very broad distribution after the tests  done during last Thursday.

A systematic was done of this time of flight spectrum according to trapping time and shown on Fig. 1.

  

Figure 1 : Trapping time: 100 ms/300ms/500ms


On these pictures one can more or less see (this was clearer on- line especially using shorter trapping time of 50ms) 2 components of the time of flight distribution for mass 98: a well defined peak, certainly corresponding to non-decayed 98Rb, and a large blob, certainly corresponding to the decay products of 98Sr for which a higher energy spread is expected. This interpretation is supported by the fact that the peak disappear after a sufficiently long trapping time while the blob increases until reaching its maximum for trapping times well beyond 100ms.

From this observation one can also suppose that 98Rb is the main component of the beam, as expected from former yield measurements by the target group.

Further tests:

(Without many pictures there)

The optimization of the time – of – flight spectrum for mass 98 was attempted, as with this broad time of flight and energy distribution the injection into REX-EBIS is expected to be rather inefficient. Several ways of sharpening the time-of-flight distribution for mass 98 were tested, without success:

·         Decreasing the quadrupole excitation frequency while increasing the trapping time. In REXTRAP, the quadrupole excitation used for cooling is usually exerted with a very large RF amplitude which can yield for long trapping times alternative excitation of the magnetron and cyclotron frequency (see for instance diploma thesis of S. Sturm). So its amplitude has to be tuned according to the trapping time. Below a certain value, both magnetron and cyclotron motions are cooled together. No effect was seen on the time-of –flight distribution but comparable blob distribution could be kept with very good efficiency up to times of 4s and very small excitation voltage (down to 1V, probably better with 3V).

·         Playing with the sharpness of the axial potential. Theoretically the axial motion is decoupled from the radial motion and is only cooled by means of the buffer gas (described as a simple damped harmonic oscillator) even in practice some coupling has been seen in the past at REXTRAP, with the rotating wall cooling for example which was found to be resonant at the axial frequency . As obviously no action on the axial motion could be obtained with the radial excitation above, it was tried to apply a steeper potential well in the axis of the trap to contain more efficiently the decayed ions. Some different shape could be obtained for the blob, but the FWHM was somehow larger (some 60µs instead of 40µs, see picture). Because of this result, it was also tried to apply a more shallow potential with the same kind of deceiving result.

Figure 2 : Time – of – flight spectrum with a steeper potential well

 

Final solution (so far)

As the time-of-flight distribution could not be sharpened within the trap, it was simply decided to use the standard cycle of accumulation – cooling  and charge breeding processes, whose timing is dictated by the EBIS. With an optimum charge breeding time of 158ms for charge state 26+ (EBIS electron beam current of 185mA), the trapping time in REXTRAP was chosen to be 160ms.

The resulting intensity for 98Sr was quite good, with the following currents measured on the REX FCs:

TRAP.FC10       2 pA in the peak
BTS.FC20        1.8-2.5 pA in the peak
RFQ.FC20 (26+)  1-3.0 pA in the peak

Some 4 to 12% low energy efficiency.

After the LINAC:

L65.FC50        1-1.8 pA in the peak    

Figure 3 Pulses observed after the LINAC

The average 98Sr beam intensity observed at MINIBALL with 1uA proton was of the order of 2e4pps, which corresponds to the same order of magnitude given by L65.FC50 assuming 1 peak every 6s in average, during~500ms. The beam composition observed in the ionization chamber is consistent with a quite pure (90%) beam of 98Sr.

Figure 4: Ionisation chamber signal. The 3 peaks corresponds from left to right to 98Rb, 98Sr, 98Y

 

Preliminary conclusions

Using the in-trap+in EBIS decay, REX-ISOLDE provides with a very decent (almost nominal?) overall efficiency a pure beam to MINIBALL. It is difficult at this stage to clearly state where the in trap decay is the most efficient (either in the trap or in the EBIS). Nevertheless, the present observations rather clearly indicate that the daughter nuclides delivered from the trap are not efficiently cooled. This possibly explains why the in EBIS decay was found more effic ient during the experiment  of Jarno Van de Walle consisting in the Coulomb excitation  of 61-62Fe from the decay of 61-62Mn (IS468).

More investigations remain to be done with REXTRAP, in order to know:

·         whether the daughter nuclides can still be efficiently injected into the EBIS with such large energy and time of flight width (should be done on Friday by Fredrik)

·         and if not whether there are means to cool down the axial motion efficiently, for instance by increasing the buffer gas pressure, or changing the position of the potential well to place it within the stopping region, trying an excitation at the axial frequency (? Probably/maybe exciting more than cooling…) etc.