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.
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.
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.
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
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)
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.
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.
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
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)
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.