VOLUME 84, NUMBER 22 PHYSICAL REVIEW LETTERS 29 MAY 2000
Measurement of Excitation Functions in the Reactions
197
Au
11
C, xn
2082x
At
Using a Radioactive
11
C Beam
R. Joosten,
1
J. Powell,
1
F. Q. Guo,
3
P. E. Haustein,
4
R.-M. Larimer,
1
M. A. McMahan,
1
E. B. Norman,
1
J. P. O’Neil,
2
M. W. Rowe,
1
H.F. VanBrocklin,
2
D. Wutte,
1
X. J. Xu,
1
and Joseph Cerny
1,3
1
Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
2
Life Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
3
Department of Chemistry, University of California, Berkeley, California 94720
4
Brookhaven National Laboratory, Upton, New York, 11973
(Received 25 January 2000)
A light-element radioactive ion-beam capability has been developed at the LBNL 88-Inch Cyclotron.
The system is based on the coupled-cyclotrons method and utilizes short-lived species, e.g.,
11
C,
14
O,
13
N
produced by p, n and p, a reactions at the LBNL Biomedical Isotope Facility Cyclotron. In a first
experiment,
197
Au
11
C, xn
2082x
At excitation functions have been measured for energies ranging from
the Coulomb barrier up to 110 MeV using a beam of
11
C with intensities up to 1 2 3 10
8
ionssec on
target. The results of this experiment are compared to measurements of
197
Au
12
C, xn
2092x
At excitation
functions.
PACS numbers: 25.60.Dz, 27.20. + n, 29.20.Hm, 29.27. – a
In recent years, growing interest in radioactive ion
beams has led to the development of several facilities to
study reactions of nuclear physics and astrophysics inter-
est. In this framework, a light-element radioactive-beam
capability has been developed at the Lawrence Berkeley
National Laboratory (LBNL) 88-Inch Cyclotron [1,2].
Initially the focus is on light proton-rich species (e.g.,
11
C,
14
O,
17,18
F). This Letter reports on studies using the initial
beam of
11
C t
12
20 min with intensities of 1 2 3
10
8
ionssec on target. Excitation functions in the
reactions
197
Au
11
C, xn
2082x
At have been measured
for the 4n to 8n channels with bombardment energies
ranging from the Coulomb barrier of 65 MeV up to
110 MeV. Using an activation method, thin gold targets
were irradiated with the
11
C beam. After irradiation, a
decays of the produced astatine isotopes were detected.
The results are compared to similar measurements of
197
Au
12
C, xn
2092x
At reactions as well as to predictions
of the statistical fusion-evaporation code HIVAP [3].
The BEARS (Berkeley Experiments with Accelerated
Radioactive Species) system is based on the coupled-
cyclotrons method. Radioactive species are produced at
the LBNL Biomedical Isotope Facility (BIF) Cyclotron [4]
utilizing p, n and p, a reactions on gaseous targets.
The BIF cyclotron is a fixed-energy (10 MeV) 40 mA pro-
ton cyclotron located about 350 m from the LBNL 88-Inch
Cyclotron. A transfer line was built to transport the activ-
ity to the 88-Inch Cyclotron. This line consists of several
capillaries (3.2–9.5 mm diameter) enclosed in a flexible
vacuum hose serving as secondary containment. This 5 cm
diameter tube is protected against the environment by a
rigid 15 cm polyvinylchloride (PVC) tube. At the 88-Inch
Cyclotron, the transfer capillary is connected to a trapping
system in which the activity is cryogenically trapped
at liquid nitrogen temperatures prior to injection and
acceleration.
For the
11
C beam, a 20 atm
14
N
2
-gas target is irradiated
for 5 min, producing
11
C via the
14
N p, a
11
C reaction.
After irradiation, the activity is driven down the evacuated
transfer capillary using nitrogen drive gas. The transfer
time for the 350 m distance is 20 sec. At the 88-Inch
Cyclotron the activity, believed to be mainly in the form
of
11
CO
2
, is condensed in a stainless steel trap submerged
in liquid nitrogen. Because the nitrogen drive gas reaches
the end of the transfer line at a pressure well below at-
mospheric, it passes through the trap and is pumped away.
Following the transfer, the trap is valved off from the trans-
fer line and warmed to release the
11
CO
2
into a reservoir
for injection into the ion source. From the reservoir, the
activity is slowly and steadily fed into the plasma region
of the 88-Inch Cyclotron’s Advanced Electron Cyclotron
Resonance (AECR) ion-source [5,6] for ionization, using
an automatic pressure-feedback system. This ensures that
the pressure in the AECR stays constant, a crucial point
for stable ion-source operation. Ionization efficiencies of
11% have been achieved for the
11
C
41
charge state us-
ing the AECR ion source [7]. After ionization, the beam
is injected into the 88-Inch Cyclotron for acceleration with
a transmission efficiency on the order of 10%. The whole
system is automated and provides a continuous
11
C beam
with typical intensities of 1 2 3 10
8
ionssec on target.
The data presented here were taken in three runs during
both the development and the commissioning phases of the
BEARS system. During the development phase, before
completion of the transfer line, two runs were dedicated
to tests of the injection into and acceleration through the
88-Inch Cyclotron. For this purpose, a total of twelve
1 Ci batches of
11
C were produced at the BIF Cyclo-
tron and trapped in a portable Dewar. The Dewar was
transported to the 88-Inch Cyclotron and the
11
CO
2
were
injected into the AECR ion source using a prototype
BEARS system. Each batch produced a beam of typically
5066 0031-9007 00 84(22) 5066(4)$15.00 © 2000 The American Physical Society