| HRIBF NEWS |
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| HRIBF NEWS |
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| Edition 9, No. 2 | Spring Quarter 2001 | Price: FREE |
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Feature Articles Regular Articles
Editors: C. J. Gross and W. Nazarewicz
Feature contributors: R. L. Auble, J. C. Batchelder, K. Rykaczewski
Regular contributors: M. R. Lay, M. J. Meigs, P. E. Mueller,
D. W. Stracener, B. A. Tatum
The tandem was shut down for programmed maintenance on January 29 and resumed operation on March 14. Following completion of maintenance, the tandem was conditioned for operation at terminal voltages up to 24.5 MV and the research program resumed on March 22. In parallel with programmed maintenance, measurements were made of the 18F beam intensity and total ion source output from the batch-mode source. Following completion of these measurements, the batch-mode source was removed and the RIB injector was reconfigured for installation of a UC-target/EBP ion source for the production of the n-rich RIBs.
From March 26 through April 24, several n-rich RIBs in the A=128-136 region were provided for the research program. The UC-target/EBP ion-source has thus far been very stable and shows no sign of degradation after more than 400 hours of operation. The n-rich RIB program was interrupted on April 25 due to failure of one of the generators in the tandem. Repairs have been made and the n-rich RIB research program has resumed. Approximately 200 additional hours of operation will be required to complete the approved, and ready to run, n-rich RIB experiments.
Following completion of the n-rich RIB campaign, a kinetic-ejection negative-ion source (KENIS), coupled to a hafnia target, will be installed on the RIB injector platform for production of 17F and 18F beams. Due to the unscheduled maintenance, operation with 17F/18F beams is now expected to start in late June or early July. The present operating budget will limit RIB operation to a maximum of about 1200 hours in FY 2001 and after completing the n-rich RIB campaign, the facility will have provided about 800 hours of RIBs in FY 2001. There are presently 536 hours of approved 17F and 18F experiments which are ready to run, so it is unlikely that we will be able to complete all of these experiments in FY 2001. During the time that the new target/ion source is being installed and prepared for operation, the tandem will provide approximately four weeks of stable-ion beams for the research program and facility apparatus development.
We report the observation of fine structure in proton emission from 145Tm. This HRIBF experiment at the Recoil Mass Spectrometer (RMS) [1] was performed using a new data acqusition system based on Digital Signal Processing which is described in section RA3 of this newsletter. The 92Mo(58Ni,p4n) reaction at 315 MeV and a 0.9 mg/cm2 enriched target were used to produce 145Tm with a measured cross section of approximately 0.5 ub [2]. Reaction products recoiling from the target were mass-separated with the RMS. Their time and position signals were recorded by the Position Sensitive Avalanche Counter (PSAC) after about 2.5 us time-of-flight to the final focus. Finally, the recoils were implanted in the Double-sided Silicon Strip Detector (DSSD), where both ion implantation and proton decay signals were detected.
In addition to the 1.73-MeV proton transition populating the 0+ ground state of 144Er (previously identified at the HRIBF [2]), a proton line with an energy of 1.4 MeV was observed, see Fig. 2-1. It is intepreted as the 145Tm decay populating the 2+ state at 0.33 MeV in even-even 144Er [3,4]. This is the third proton-radioactive nucleus with experimentally identified fine structure in its proton emission [5,6].
Fig. 2-1 - The energy spectrum of proton events observed within 0.5-10 us time interval after an implantation of A=145 recoils into the DSSD. The decay patterns of the 1.73-MeV and 1.4-MeV proton transitions are given in the inset. These decays of 145Tm were detected starting 0.5 us after the implantation of recoiling ions using our new digital signal processing electronics. The detection rate was reduced within first 500 ns of counting.The observed decay patterns of the 1.73-MeV and 1.4-MeV lines are given in Fig 2-1. The similarity of the half-lives, and the coincidences with the A=145 separated ions lead us to the conclusion that both lines belong to the decay of the same state, previously identified as 145Tm. The obvious explanation of experimental data points to the feeding of the 0+ ground state and 2+ first excited state in the even-even nucleus 144Er. The observed energy difference of 0.33 MeV locates the 2+ state in 144Er. Such 2+ energy in 156Er could be expected within a simple perspective of the "NpNn" systematics [7,8]. The "N=82" mirror nucleus to 144Er, i.e. 156Er, has the 2+ level at 343 keV. The value of about 0.33 MeV is also close to the known 2+ energy for three neighboring even-mass N=76 isotones, 138Sm, 140Gd, and 142Dy.
The spherical interpretation of the observed decay rates of 145Tm suggests the proton h11/2 orbital as a main component of the groundstate wave function. The observed partial half-life for the 1.73-MeV line is explained by the 70% of 145gsTm composed of the 0+ 144Er core coupled to a pure proton h11/2 configuration (decay via l=5 proton transition). The presence of the 1.4-MeV proton line (9% branch) requires about 3% admixture to the 11/2- groundstate of 145Tm, which is made of a 2+, 144Er excited core coupled to the proton f7/2 orbital. The much faster l=3 proton transition at 1.4 MeV accounts for the observed 9% branching ratio. The remaining part of the wave function could be composed mostly of the 2+, 144Er core coupled to the proton h11/2. However, the use of the spherical picture may oversimplify the interpretation. The 2+ energy of 0.33 MeV in 144Er suggests a deformed shape. A quadrupole deformation parameter beta_2 about 0.18 for 144Er could be estimated from Grodzins formula [9,10].
More advanced intepretation of observed 145Tm decay, accounting for the deformed shapes [11,12], is in progress. From the experimental side, a study of the excited levels of 145Tm is planned at the RMS using the CLARION array and Recoil Decay Tagging technique. It should help to determine the shape of this proton-emitting parent nucleus.
[1] C. J. Gross et al., Nucl. Instrum. Methods Phys. Res. A450, 12 (2000).
[2] J. C. Batchelder et al., Phys. Rev. C 57, R1042 (1998).
[3] M. Karny et al., "Fine structure in proton emission from 145Tm", to be published.
[4] K. P. Rykaczewski et al., Nucl. Phys A682, 270c (2000).
[5] A. A. Sonzogni et al., Phys. Rev. Lett. 83, 1116 (1999).
[6] T. N. Ginter et al., in Proc. of Int. Symp. on Proton-Emitting Nuclei, Oak Ridge, TN, 1999, (ed. J. C. Batchelder), AIP Conf. Proc. 518, 83 (2000).
[7] R. F. Casten, Phys. Rev. C 33, 1819 (1986).
[8] R. F. Casten and N. V. Zamfir, J. Phys. G 22, 1521 (1996).
[9] L. Grodzins, Phys. Lett. 2, 88 (1962).
[10] F. S. Stephens et al., Phys. Rev. Lett. 29, 438 (1972).
[11] A. T. Kruppa et al., Phys. Rev. Lett. 86, 4549 (2000).
[12] B. Barmore, et al., Phys. Rev. C 62, 054315 (2000).
Since its commissioning in late 1996, operation of the HRIBF has been guided by the recommendations of the March 1996 DOE Operation Budget Review Committee. Among the recommendations made by that committee was the need for a strong emphasis on target and ion-source R&D, considered to be the lifeblood of both existing and future radioactive beam facilities. The committee also determined that an operating budget of about $7M (FY96 dollars) would be required to maintain an R&D program and to provide ~1400 hours per year of RIB operation. Unfortunately, the recommended level of funding has never been achieved. As a consequence, over the past two years, several operations staff positions had to be eliminated or personnel reassigned to other duties, procurement limited to only the most essential items, and the number of cyclotron operating hours limited to ~1200 hours per year.
The FY 2002 budget will make the operation of the HRIBF even more challenging. The number of RIB operating hours in FY 2002 will likely have to be reduced, possibly to 1000 or less. The radioactive beams provided by the HRIBF, in addition to allowing the measurement of important nuclear structure and astrophysics data, are essential for development of specialized experimental apparatus and research experience needed to cope with the low beam intensities, isobaric contamination, and other problems inherent to use of radioactive beams. Therefore, it is important that the facility provide as many hours of operation as possible.
Unfortunately, reducing the number of operating hours alone will not be sufficient to accomodate the projected operating budget shortfall. Loss of additional personnel from either the facility operations staff or the R&D effort will have significant consequences. Additional reductions in the operating staff would risk the loss of expertise needed to safely operate and maintain the facility, while R&D staff reductions will slow the development of materials, hardware, and expertise essential to the success of future radioactive beam facilities such as RIA.
Difficult decisions will need to be made in the near future, and the advice and recommendations of our users will be factored into those decisions.
The ISOL'01 Conference was held March 11-14 in Oak Ridge, TN. Focusing on nuclear physics studies with radioactive ion beams at ISOL facilities, the conference was attended by over 150 scientists from Europe, Asia, and North America and had over 50 oral presentations and one poster session. The first day of the conference was dedicated to the celebration of twenty years of science at the Holifield facility and its continuing leadership role in nuclear physics research. Several Holifield "veterans" highlighted the research performed at the facility. The rest of the program presented efforts throughout the world to produce and use RIBs and the study of nuclei far from stability. Proceedings will be published electronically and will be available sometime this summer. More information may be found on our web site at http://www.phy.ornl.gov/isol01/.
The sixth Program Advisory Committee meeting has been scheduled for June 14-15, 2001. Twenty proposals for experiments have been received requesting almost 1300 hours of RIB operation and 1200 hours of SIB operation. As expected, the RIB requests are dominated by neutron-rich and fluorine beams. It is expected that some 600-800 hours of RIB experiments will be allocated along with an equal amount of SIB operations.
The measured yields were disappointingly low as shown in the following table. As mentioned above, we plan to use SiC powders having a smaller diameter, which should increase the yield by reducing the diffusion time and thus reducing the losses due to radioactive decay. Also the target holder will be changed from graphite to Ta, which, according to calculations, should reduce the sticking time and thus increase the yields. A more detailed description of the production target and the development of these beams can be found in the previous HRIBF Newsletter.
| Isotope | Half-life | Production rate (ions/s/uA) | Yield (ions/s/uA) | Efficiency (%) |
Expected beam into Tandem (ions/second)* |
|---|---|---|---|---|---|
| 25Al | 7.18 s | 1.56x1011 | 8600 | 5.5x10-6 | 1720 |
| 26mAl | 6.34 s | 3.56x1010 | 1800 | 5.1x10-6 | 360 |
| * The charge exchange efficiency for aluminum ions in a Cs-vapor cell is ~10%. This also assumes a production beam current of 2 uA of 42-MeV protons. | |||||
A period of conditioning of 134 hours was necessary to allow the machine to operate routinely at 24.5 MV. Unfortunately, when the machine was at 24 MV there was a lot of ticking and several sparks, so it was decided to reduce the voltage below 24 MV. The associated experiment was concluded at 23.28 MV with minimal sparking but still a lot of ticking. The two units that had the most ticking during conditioning are being carefully examined to find what caused the problems. The ticking continued even at the lower voltages and probably was enhanced by the deterioration of the belt on the failed Georator.
A second-stage mass meter has been implemented. This has allowed us to make more efficient measurements of actual negative ion beams (free from neutral beam contamination) with our tape system after the second-stage mass separator. During this period we have measured masses 76, 78, 80, 84, 87, 118, 126, 128, 130, 132, 134, and 136.
The table below lists the beams and their intensities on target which were delivered during this period.
| Isotope |
Intensity (ions per second) |
Contaminant Intensity (ions per second) | Energy (MeV) | Charge State |
|---|---|---|---|---|
| 126Sn | 1x107 | 1x107 | 378 | 16 |
| 128Sn | 2.5x106 | 1x107 | 384 | 16 |
| 132Te | 2x106 | <3x105 | 350 | 15 |
| 132Te | 5x106 | 2x106 | 350 | 15 |
| 132Te | 5x106 | 2x106 | 396 | 16 |
| 134Te | 2x106 | <1x106 | 396 | 16 |
| 136Te | 3x104 | 5x104 | 396 | 16 |
We have made the first use of a piglet, storing the previous uranium carbide target ion source from last fall. The dose rate increased from 17 mREM/hr on the front of the 3.5-inch steel pig to 90 mREM/hr on the front of the 2-inch steel piglet. This increase in dose rate is an acceptable tradeoff for cheaper and smaller storage space in C110.
We finally performed an autopsy on the HfO2 target-kinetic-ejection-negative-surface-ionization source that was so successful in last year's radioactive fluorine campaign. As with all other such sources, the grid was completely gone. Due to this ubiquitous result, we will increase the thickness of the grid in our next kinetic ejection ion source from 8 mil to 16 mil.
A fast digitization system has been implemented at the HRIBF for decay studies. Able to overcome the heavy-ion implantation overload observed with our previous analog electronics, its capabilities are best illustrated in the recent experiment on fine structure in 145Tm, which has the shortest half-life among identified proton radioactivities. Its value, estimated previously as 3.5(10) us [1], was remeasured in the present study to be 3.1(3) us. The very short half-life was the main limitation factor for the previous measurement of 145Tm [1], which was made with the RMS-PSAC-DSSD system equipped with analog electronics, see e.g. [2]. The shortest time observable between a recoil and its associated decay was determined by the recovery time of the amplifiers after overloading with the high-energy heavy-ion implantation signal. These large signals blocked detection of fast proton events for several microseconds. In the first experiment on 145Tm, the full efficiency for proton detection was reached in about 10 us after ions were implanated in the DSSD, thus reducing the proton rate to about 1 event/hour.
In order to improve the observation time window, a new data acqusition system has been developed [3,4]. This new system is based on the Digital Gamma Finder (DGF) modules produced by X-Ray Instrumentation Associates [5]. For the short-lived activity of 145Tm, the waveform of the preamplifier signals of the DSSD are individually analyzed. The events with two pile-up signals within 10 us are recorded (so called "proton catcher" mode). The new Digital Signal Processing electronics allows us to "trace" the implantation signal followed by the decay event by mapping the amplitude of the preamplifier signal in a 1k spectrum of 25-ns wide time bins as is shown in Fig. RA3-1. A part of the trace consisting of two hundred pretrigger points (5 us) helps to measure the baseline, i.e. to define the "zero" level for the amplitude determination. Four hundred points at the end of the trace (10 us) are used to analyze the electronic signal decay. The four hundred 25-ns samples in the center, spanning 10 us, contain the implantation and decay pulse. Events not piled up within 10 us, however, are rejected within this DGF operation mode. Effectively, it allowed us to observe decays within 0.5 to 10 us after recoil implantation during the 145Tm experiment, see insets in Fig 2-1. It is important to note that thanks to the improved observation window for recording of fast decay signals, the detection rate of 145Tm went up by an order of magnitude to about 10 events per hour. The total event rate in the data acquisition system was very low, about 1 readout per second with about 2 kHz total ion rate since the "proton catcher" mode of DGF operation selects up-front only the rare pile-up events and rejects all others. For valid 145Tm proton events, the coincidences between front and back strips are required with respect to the time and amplitude of the recorded DSSD traces, see Fig. RA3-1. The presence or absence of a recoil signal from the PSAC detector is used to confirm if a given signal in the DSSD is a recoil or decay.
Fig. RA3-1 - The traces of the DSSD preamplifier signals corresponding to the proton decay event occuring 550 ns after the 145Tm recoil implantation, recorded in the front (black curve) and back (red curve) strips.
[1] J. C. Batchelder et al., Phys. Rev. C 57, R1042 (1998).
[2] C. R. Bingham et al., Phys. Rev. C 59, R2984 (1999).
[3] R. Grzywacz et al., "Digital Signal Processing in Charged Particle Spectroscopy", to be published in Nucl. Instrum. Methods in Phys. Res.
[4] K. P. Rykaczewski et al., Nucl. Phys A682, 270c (2000).
[5] M. Momayezi et al., in Proc. of Int. Symp. on Proton-Emitting Nuclei, Oak Ridge, TN, 1999, (ed. J.C. Batchelder), AIP Conf. Proc. 518, 307 (2000).
HRIBF welcomes suggestions for future radioactive beam development. Such suggestions may take the form of a Letter of Intent or an e-mail to the Liaison Officer at liaison@mail.phy.ornl.gov. In any case, a brief description of the physics to be addressed with the proposed beam should be included. Of course, any ideas on specific target material, production rates, and/or the chemistry involved are also welcome but not necessary. In many cases, we should have some idea of the scope of the problems involved.
Beam suggestions should be within the relevant facility parameters/capabilities listed below.
| Experiment | Title | Spokesperson/Institute | Dates* |
|---|---|---|---|
| RIB-000 | Commissioning of the RMS | Batchelder/ORNL | 3/22 |
| RIB-014 | RIB Development on UNISOR | Stracener/ORNL | 4/16 |
| RIB-037 | Tandem Development | Meigs,Juras/ORNL | 3/14-16 3/19-21 3/23 |
| RIB-072 | B(E2;0+ ---> 2+) measurement in 132Te in inverse kinematics | Barton/Yale Univ | 4/5(16:00)-6(08:00) 4/9(12:00)-4/10 4/11 4/12 |
| RIB-074 | Coulomb excitation of neutron-rich Sn and Te isotopes | Radford/ORNL | 3/26-27 3/28(-12:00) 3/29(14:00)-30 4/2-4(08:00) 4/4(08:00)-4/5(08:00) 4/6(08:00-) 4/9(-12:00) 4/17(16:00)-20 4/23(12:00)-25(04:00) |
| Scheduled maintenance/shutdown | 1/29-3/13 | ||
| Unscheduled maintenance, BL14 valve leak | 4/23(-12:00) | ||
| Unscheduled maintenance, control system | 4/17(-16:00) | ||
| Unscheduled maintenance, ORIC RF servo | 4/17-18(16:00) | ||
| Unscheduled maintenance, quad power supply | 4/5(08:00-16:00) | ||
| Unscheduled maintenance, Tandem D2 Georator | 4/25(04:00)-27 4/30- | ||
| Unscheduled Maintenance-TIS accel | 3/29(-14:00) | ||
| Unscheduled Maintenance-TIS accel,controls | 3/28(12:00-24:00) | ||
| *Weekend operation is suspended due to financial considerations. | |||
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| Witek Nazarewicz | Carl J. Gross |
| Deputy Director for Science | Scientific Liaison |
| Mail Stop 6368 | Mail Stop 6371 |
| witek@mail.phy.ornl.gov | cgross@mail.phy.ornl.gov |
| +1-865-574-4580 | +1-865-576-7698 |
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| Holifield Radioactive Ion Beam Facility | |
| Oak Ridge National Laboratory | |
| Oak Ridge, Tennessee 37831 USA | |
| Telephone: +1-865-574-4113 | |
| Facsimile: +1-865-574-1268 | |