Proposal RIB-112: First g-factor measurement of the 2+ state of 132Te produced as radioactive beam


The experiment was scheduled to run at the Oak Ridge National Laboratory, Holifield Radioactive Ion Beam Facility in Jul. 2004. The target chamber used in earlier Transient Field experiments at the 88" Cyclotron Berkeley was setup with 4 Clover Ge-detectors on an angular correlation table. The experiment asked for the detection of forward scattered ions from the carbon target in coincidence with gamma rays.

Since stopping of the radioactive beam in the target and target chamber had to be avoided as much as possible, two modifications from the usual setup were introduced; the target had no or a reduced Cu backing, where usually Coulomb scattered beam recoil ions are stopped, and the particle detector had a center hole so that all projectiles could leave the chamber and stop down stream in a shielded dump.

The CLARION DGF integrated electronic was used for the data acquisition. All signals are sampled with a 25ns time resolution and fully digital analyzed. This reduces the number of electronic modules but puts the burden on computing power especially for high data rates. The final time resolution of 25ns/channel is a limitation for coincidence measurements as required for this type of experiment. All gammas or particles were acquired. Particle - gamma coincidence events are reconstructed from the data stream. The computer dead time can be limited by a "coincidence gate" set for each particle.

130Te stable beam angular correlation measurement.

The setup was tested with a stable 130Te beam at 396 MeV. The multilayer target (C4) consisted of 0.542 mg/cm² C on 10.88 mg/cm² Gd backed by 3.81 mg/cm² Cu. About 9% of the Coulomb excited Te ions stop in the target, the rest leave the target but with very low velocity (on average 0.5 %c). The knock-on carbon ions leave the target with an average energy of 83 MeV. The beam exits the target with 13.4 MeV (Output of Transi calculation).

The particle detector was an assembly (PDA) of two square (15 x 15 mm) solar cell type silicon wafers vertically arranged with 10 mm separation in the middle, left open for the beam to pass out of the target chamber. The detector face was covered with 22 mg/cm²Ta to shield from scattered beam.

The clover detectors were at a distance of 128 mm from the target.

Click on picture to enlarge view. [spectra]

The prompt time peak in the TAC spectrum is the narrow line on the large random data sample. The true to random ratio in this selection is very small. In an earlier test experiment at the YALE tandem and a 130Te of 280 MeV a similar high random count was observed. The spectra contained strong Gd excitations like here.

Nevertheless, the data can be analyzed, the angular distribution for the experimental setup yielded a slope of 2.6 for an angle 67° despite the Te recoils leaving the target (albeit with very low velocity).

After the measurement of the angular distribution the target was cooled to LN temperature and a precession run started. The resulting precession came out too small!
From the line shape of the 2+ transition it was obvious that there was a problem with the target. Warming up and inspecting the target revealed that despite cooling the beam had melted the target at the beam spot. The beam spot was very small. This was a first for this type of experiment. Reasons are the thicker than usual Gd layer, reduced Cu backing, high beam energy and too small focus.

132Te radioactive beam

The radioactive 132Te is produced in an ISOL type ion source. The intensity is orders of magnitude smaller than the stable beam. About 107 particles/s can be expected at the target. The half life of 132Te is 3.2d. The beam is not isotopically clean. It comes with an admixture of 10 - 15 % 132Sb (half life 2.78 m). The 132Sb decays into 132Te and populates mainly the 4+ and 2+ states and therefore interfers with the direct Coulomb excitation of the projectiles, 132Te.

The tune for radioactive beam was adjusted using a viewer in the target chamber. A 4 mm diaphragm was upstream in the chamber. The target (C2) 0.61 mg/cm² C on 10.41 mg/cm² Gd and 1.88mg/cm² Cu was similar to the damaged target. The average exit velocity of the C ions is again 83 MeV, the beam exits with 39 MeV and the recoil Te ions come out with about 14 MeV (Transi calculation).
Fig. 2 shows a gamma spectrum (Clover 2 at 67° forward) taken for 30 min. The 132Sb (T½ = 2.79 min) decay into 132Te (red lines denote states in Te) are dominant. The longer living components from 132Te decay are not yet visible.

Fig. 2 (Click on picture to enlarge view) [spectra]

Obviously, too much of the beam stopped in the target chamber!
After some time counting rates of up to 100 K in each clover and only 80 particles/s in the particle detector were observed. The TAC spectra showed no hint of a prompt peak.
Clearly, the target spread the beam to an extend which led to a high activity buildup. Shielding of the detectors was only a partial solution.
Data were taken for a total of 21 hours with many interruptions. There were common difficulties with the beam and data acquisition.

After opening the target chamber radioactive contamination was found especially at the particle detector center hole (Ta sleeve). Swipe test showed removable activity in the chamber (not clear how that happens).

In addition the beam did not clear the exit of the chamber presenting another source of background radiation.

Since it made no sense to proceed with a precession measurement, a set of test experiments was performed to pinpoint the origin of the unwanted radiation.

The "thinner" target was target C9: 0.54 mg/cm² C on 10.88 mg/cm² Gd, no Cu backing.
The particle detector assembly was moved in as close as possible (about 16mm, physical limit). The effective clearance hole for the beam was 17°.
This setup produced gamma spectra still dominated by the decay of 132Sb.

By then the target chamber was sufficiently activated that after a 12 hrs "cooling" time a clover was still counting 32 K/s. Without the internal assembly the rate dropped to 9.8 K. About 50% of the activity coming from the inner part stemmed from the PDA alone. Clearly, too much beam did not pass through the center hole.

The size of the actual beam after the target was then measured. The PDA was taken out and replaced by a concentric Al disk pile. After a short activation the disk pile was taken apart and the activity of the each disk measured.

Click on picture to enlarge view. [spectra]

Next, gamma spectra were taken without a target and without PDA. As expected the spectra contained only long lived activity lines, 132Sb was not seen. This confirms that the collimator in front of the target did not shave off beam.

Putting the target back resulted in gamma spectra with 132Sb decay gammas. This gammas can only come from 132Sb stopped in the target or from beam hitting the exit pipe of the chamber. The last case should result in an asymmetry in the rate between forward and backward detectors.
Assuming the 'same' efficiency for all clovers, the intensity of the 132Sb decay to the 2+ state in 132Te was nearly twice higher in the forward detectors. The question, if and how much 'background' from 132Sb comes from the target is unanswered. It was proposed to make a future test run with a target in an open beam pipe (4" cross).

As last test, a "thin" 1 mg/cm² self supporting C target was used with the particle detector assembly in place. A 4 hr measurement (about 30 particles/s) with the current electronic setup and the high residual gamma background yielded absolutely no clue to 132Te 2+ excitation and decay. Good news is, there was also no visible decay of 132Sb. All beam passed to the beam stop.
There is of course a Doppler shift and spread of the decay gammas at the high exit velocity (6.5% c) of the excited projectiles. This should lead to a difference in the gamma energies for the forward and backward detectors. Unfortunately, with the given setup no decay from projectile excitation was observed. The individual clover rate was still above 30 K and particle gamma coincidences would be of the order of one every 3 sec at best. The TAC spectra showed no discernible prompt peak.


The high activation was caused by severely underestimating the beam spread by the target. Chamber and target were aligned properly, but even a small displacement of the beam can cause a large effect down stream. The hole in the PDA has to be enlarged. Calculations revealed, the hole can be twice as large without causing too much loss of alignment. The exit pipe of the current chamber is too small.

A test was proposed to measure the amount of 132Sb stopping in various targets. For this a simple test setup; one clover next to a  4" cross in the beam line with a target ladder is all that is needed.

Even if some 132Sb stops in the target it will not impede a TF measurement.
The reason is simple but was overlooked. The Sb atoms decay at rest, the gamma transitions are sharp, no Doppler shift or broadening. Sb decays only randomly.
The probe ions,132Te, in contrast, leave the target and decay in flight.Their decay gamma rays will be shifted sufficiently to be well separated from the stopped peak. Precession angles are usually ± 67°. And in addition, they are measured in coincidence with forward scattered target ions. In Fig.4 gamma spectra of the Yale test experiment are shown where 130Te ions from a 280 MeV beam either stop in the target or exit the target with a velocity of 2% c for a clover detector at a backwards angle of 130°.

Fig. 4 (Click on picture to enlarge view) [spectra]

The clear separation of the gamma energies is proof of principle.
A new target has to be designed to assure a reasonable exit velocity for the probe ions, so that the gamma rays from the stopped contaminant 132Sb are well separated from the gamma rays coming from the Coulomb excited projectiles decaying in flight.

The experiment will continue, beam time was approved by the PAC and will be scheduled at the next available run period.
A new chamber with a 4 inch exit hole was built.

Gerfried Kumbartzki
Last modified: Wed Sep 14 14:09:34 EDT 2005