HRIBF NEWS SUPPLEMENT


Edition 9, No. 3 Spring Quarter 2001 Price: FREE

Feature Articles

Editors: C. J. Gross and W. Nazarewicz

Feature contributors: D. W. Stracener


1. HRIBF Special Limited Call for Proposals

The HRIBF is pleased to issue a limited Call for Proposals for experiments using the recently developed beams of isobarically pure neutron-rich Sn isotopes. Pure radioactive Sn beams are produced by extracting SnS+ compounds, mass separating the molecule, and breaking it up during the charge-exchange process. Sulfides of Sb and Te isotopes either do not form or are not released from the target. Pure Sn beams of significant intensity result, and measured yields are tabulated below.

Table 1-1: Summary of estimated intensities of Sn beams on target.

Estimated beam-on-target (ions/sec)
Mass of Sn isotope (amu) Beam intensity into tandem with 7 uA from ORIC (ions/sec) Single-stripping
(E/A ~ 3 MeV/amu)
Double-stripping
(E/A ~ 4 MeV/amu)

126

3.4 x 107 (est.)

3.4 x 106

6.8 x 105

127m

2.4 x 106

2.4 x 105

4.8 x 104

127g

2.8 x 107

2.8 x 106

5.6 x 105

128m

3.6 x 105

3.6 x 104

7.2 x 103

128g

1.4 x 107

1.4 x 106

2.8 x 105

129m

2.9 x 106

2.9 x 105

5.8 x 104

129g

1.2 x 106

1.2 x 105

2.4 x 104

130m

1.7 x 105

1.7 x 104

3.4 x 103

130g

1.6 x 106

1.6 x 105

3.2 x 104

131

2.3 x 105

2.3 x 104

4.6 x 103

132

2.1 x 105

2.1 x 104

4.2 x 103

133

< 900*

   

134

< 100*

   

* based on calculation of cumulative production rate in the target and the estimated ion source efficiency. Beam on target for particular energies may be estimated using a new calculator posted on the website at http://www.phy.ornl.gov/hribf/users/beams/ribs_total.html.

This special Call is of limited scope. Proposals will be reviewed by the PAC electronically and/or telephone conference call (PAC-6e), and only two or three demonstration experiments are expected to be accepted. The experiments MUST be attempted after the upcoming Fluorine campaign and tank opening and will use the present neutron-rich target-ion source that will be placed in storage. Should this source prove not useable following storage, a delay of three weeks may be expected.

Proposals are due September 4 with review occuring in the next 2-3 weeks. Proposals must be submitted electronically in portable document format (pdf) and emailed as attachments to liaison@mail.phy.ornl.gov. More detailed instructions including an equipment update may be found on our website http://www.phy.ornl.gov/hribf/users/pac6e_info.html. Experiments will be attempted around December in a single block. Should you be unable to participate in this Call, the next regularly scheduled deadline for proposals will be around December 15.

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2. Recent HRIBF Development - Isobarically Pure 132Sn Beams Now Available

Recent measurements have shown that isobarically pure beams of Sn isotopes can be delivered to experiments at the HRIBF. The Sn atoms are produced via proton-induced fission in a uranium carbide target and transported to an electron-beam-plasma (EBP) ion source. In the presence of sulfur vapors, the SnS molecule forms and is ionized and extracted from the EBP ion source as SnS+, which is then converted to Sn- in the Cs-vapor charge-exchange cell and delivered to the tandem electrostatic accelerator. The Sn beams produced in this manner are pure (>99%) with no detectable contamination from Sb and Te isotopes. When Sn isotopes are extracted from the ion source as Sn+ ions, they comprise a small fraction of the total beam delivered to the experiment (e.g. for A=132, the beam composition is Te - 87%, Sb - 12%, and Sn - 1%).

Fig. 2-1 - Positive-ion yields for Sn isotopes from a UC target at UNISOR.
Initial tests at the UNISOR facility measured the yields of positive ions of Sn and SnS for Sn isotopes from A=127 to A=134 (see Fig. 2-1). A better way to look at the SnS/Sn ratio is shown in Fig. 2-2 where the ratio is plotted as a function of half-life. The solid line is a logarithmic fit to the data, which includes some of the isomeric states of these Sn isotopes. The data show that as the half-life decreases, the SnS/Sn ratio increases, indicating that the transport of SnS through the target and ion source is faster than for elemental Sn. This would mean that the SnS molecule is less reactive and would be the preferred way to extract the very short-lived Sn isotopes. Varying the electron-beam energy between 100 eV and 210 eV had no effect on the yields or the ratio (typical operating energy is 150 eV). Below 100 eV, the yields started to drop and the SnS/Sn ratio also dropped, indicating a higher ionization potential for the SnS molecule. A measurement of yields showed no effect when the target temperature was changed from 1960 C down to 1875 C. A more detailed investigation of the dependence on a wider range of target temperatures needs to be done.

Fig. 2-2 - Ratios of positive-ion yields for Sn isotopes from a UC target at UNISOR.
The intensities of Sn- beams from a similar target and ion source on the RIB Injector were measured after the second stage mass-analyzing magnet. The measured beam intensities and the estimated beam-on-target intensities are listed in Table 2-1. Again, the Sn- beams were pure when extracted from the ion source as SnS+ ions.

Table 2-1: Summary of estimated intensities of Sn beams on target.

Estimated beam-on-target (ions/sec)
Mass of Sn isotope (amu) Beam intensity into tandem with 7 uA from ORIC (ions/sec) Single-stripping
(E/A ~ 3 MeV/amu)
Double-stripping
(E/A ~ 4 MeV/amu)

126

3.4 x 107 (est.)

3.4 x 106

6.8 x 105

127m

2.4 x 106

2.4 x 105

4.8 x 104

127g

2.8 x 107

2.8 x 106

5.6 x 105

128m

3.6 x 105

3.6 x 104

7.2 x 103

128g

1.4 x 107

1.4 x 106

2.8 x 105

129m

2.9 x 106

2.9 x 105

5.8 x 104

129g

1.2 x 106

1.2 x 105

2.4 x 104

130m

1.7 x 105

1.7 x 104

3.4 x 103

130g

1.6 x 106

1.6 x 105

3.2 x 104

131

2.3 x 105

2.3 x 104

4.6 x 103

132

2.1 x 105

2.1 x 104

4.2 x 103

133

< 900*

   

134

< 100*

   

* based on calculation of cumulative production rate in the target and the estimated ion source efficiency

This molecular beam was then passed through a Cs-vapor cell and the molecule was dissociated and the negative-Sn ions were formed. The efficiency of this process is unknown, and no attempts have yet been made to study the dependence of the Sn- yields as a function of cell thickness. The energy spread in the beam due to this molecular breakup is about 400 eV, resulting in an increased beam size (~10 mm diameter) after the second stage mass-analyzing magnet. This large beam size will probably result in reduced transmission through the tandem. The yield measurements shown in Fig. 2-3 were made with narrow slits (2 mm wide) after the second stage mass-analyzing dipole magnet, corresponding to an energy spread of 80 eV in the transmitted beam. Additional beam intensity may be available as the slit width is increased after the beam has been tuned to the experiment. The black diamonds in Fig. 2-3 are the intensities of the Sn- beams injected into the tandem and the shaded triangles are the estimated beam-on-target intensities at a beam energy of 4 MeV/amu. In this mass region, acceleration to this energy requires double-stripping in the 25-MV tandem, which explains the reduction (~1/50) in beam intensity. The solid line is the cumulative production rate of the Sn isotopes in the UC target normalized to the 127Sn data. This shows that for long-lived isotopes the overall TIS efficiency (release, transport, and ionization) is constant (2-3%) and the efficiency decreases for shorter-lived isotopes. The intensity of the 42-MeV proton beam on the uranium carbide target was 7 uA during the time that the data shown in Fig. 2-3 was taken. The proton-beam intensity has been limited to 7 uA to lengthen the lifetime of the TIS, but a previous TIS operated for 12 days with a proton current of 10 uA without failure, so some increase in the beam intensity may be possible.

Fig. 2-3 - Intensity of negative-ion Sn beams injected into the tandem after extraction from the source as SnS+.
The ratio of negative-ion yields from the RIB Injector was about a factor of ten lower than expected from the positive-ion ratios measured at UNISOR. Several factors may contribute to this difference, including lower charge-exchange efficiency, lower transmission efficiency to the tandem, or a lower sulfur concentration in the target and ion source on the RIB Injector. The charge-exchange efficiency for SnS+ to Sn- is unknown and needs to be measured. Also the transmission to the tandem will be lower for this beam due to the large energy spread introduced by the molecular breakup in the charge exchange cell. The yields of Sn+ and SnS+ from the TIS on the RIB Injector have not been measured, but the extracted S+ beam current from the target/ion source (TIS) tested at UNISOR was 900 nA while the S+ beam current from the RIB Injector TIS was less than 50 nA, which might explain the lower relative yields of SnS from the RIB Injector. An attempt to add sulfur ions to the TIS using SF6 was made with little effect on either the S+ or the Sn- (from SnS+) extracted beam currents. Further investigations will be made using H2S gas, and in future sources a refractory sulfur compound such as CeS may help to provide a higher sulfur vapor concentration.

As mentioned in the previous paragraphs, several questions still remain concerning the unexpected purity of the SnS beams, the efficiency of converting SnS+ molecular ions into Sn- ions, and the dependence on various parameters in the target, such as sulfur vapor concentration and target temperature. For example, it is not known if the TeS and SbS molecules are hindered from forming in the high temperature environment of the target (2000 C) or if they are dissociated in the ionization process. Further tests at the UNISOR facility will be done to determine the best way to introduce the optimum concentration of sulfur atoms and to measure/optimize the efficiency of the process to convert the SnS molecules to negative tin ions.

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You may contact us at the addresses below.

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

Holifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831 USA
Telephone: +1-865-574-4113
Facsimile: +1-865-574-1268