DIPROTON
G.Cardella and G.Raciti
for ISOSPIN-FRIBS-TRASMA collaborations
Last year the PAC assigned to the C70 - Diproton proposal 17 BTU shared in two periods to test the
feasibility of a measurement of two proton decay from the 6.15(1-) level of 18Ne. In ref.[1] it was
suggested that this level could directly decay via two-proton emission because there are no available
states in the 17F+p decay scheme (see fig.1 from ref. [1]). The fusion reaction 17F+p was used in
ref.[1] to study this decay. 2p angular correlation were observed but they were not enough extended
to prove the direct 2p decay. In the C70 proposal the intention was to use a high energy 18Ne beam
impinging on a Pb target to excite this level via Coulomb excitation (E1) and to observe it
subsequent decay.
The results obtained during the tests performed are the following:
Using a primary beam of 10 enA of 20Ne7+ 45 MeV/A, colliding on a 9Be target 500 m thick, we
were able to produce about 103 18Ne ions/sec at the FRIBS point (before the switching magnet). The
transmission of such beam in the CICLOPE room was less than 10%. We were able to identify the
radioactive beams transported using the Cyclotron RF as start of our TOF measurements. The TOF
information was combined with the energy loss measured from our tagging silicon detector to get a
full identification, see fig.2.
21Na
20Na
20Na
19
21Na
F20
Ne
18
19F
19F
Ne
18
16F
18Ne
O
17F 17Ne
15O
15O
14O
19Ne
18Ne
17Ne
17F
ry
a
n
14O
e
pr
i
lim
Fig.2 E-TOF identification scatter plot obtained from one of the strips of the tagging detector
mounted near the CICLOPE room
We collected about 106 18Ne well tagged events on target (8% of the data). In such set of data we
have identified more than 1000 coincidences 1p-heavy ion, and among them we have observed up
to now two events 18Ne* →16O+2p. The obtained excitation energy of 18Ne* for the two events
analyzed is around 12 and 30 MeV.
In conclusion we proved that we can produce a beam of about 5∙104 18Ne/sec particles,
necessary to perform the real measurement, with the present configuration of the beam line, using a
primary beam current of about 500 enA (similar to the one that must be used for EXCYT). However
to have a reasonable 18Ne beam intensity on target, we need a much better transmission. After the
two tests assigned by the PAC, we studied a new transmission method. We partially tested it at the
end of July 2004 using, for few hours, an available 20Ne beam (25 MeV/A). The indications of this
last attempt, looking to an energy degraded beam, were quite encouraging.
If we will be able to have on target a beam intensity of 2∙104 particles/sec we should obtain enough
statistics for angular distribution measurements in about 10 days of measurements (1000 events
with Pb target thickness of 200 m, population cross section ≈100 mb and branching ration for the
2p channel 10-3). We therefore ask for new test beams, and beam for the final measurement under
the condition that all tests are positively performed. The main objective of these tests will be to
improve the beam transmission. However some other thinks must also be done. The first one is to
determine the best position for the tagging detector to optimize transmission, target beam spot and
contaminations from reactions on the tagging detector itself. We have also to verify the use of slits
coupled to the degrader to clean for unwanted beams. This will be necessary to reduce the number
of particles impinging on the tagging detector to a reasonable value (not more than 10 5
particles/sec). We must also determine the best trigger solution to enhance the visibility of the
studied reactions. Finally we need more events to evaluate the cross sections of the processes under
study. For all these reasons we ask for two tests. The first one, of two days, devoted mainly to
transmission measurements. This test should be possibly performed on March, with primary beam
intensity around 1 enA. ( in this test we will mount tagging detectors at the Fribs point, in the
middle of the two last dipoles of the CICLOPE beam line and at the CHIMERA target position. The
second one, of 3 days, for recovery of possible failures of the first test and for trigger test and cross
section evaluations. In this case we will use a higher current. In this test we must use all CHIMERA
rings therefore a good period should be not before May 2005.
We also ask to schedule further 10 days of measurement at the end of 2005, to be used again with
CHIMERA, if the reaction cross section and the transmission will be positively measured. To be
more confident on the results, we need also a new proton calibration campaign of the CHIMERA
CsI(Tl) detectors, as required in the CHIMERA calibration proposal.
REFERENCES
[1] J.Gomez del Campo et al; Phys.Rev.Lett 86(2001)43