EXP # :
GANIL
PROPOSAL FOR EXPERIMENT
Scheduling period :
2006 and onwards
Dead-line for submission :
October 29th, 2005
(Do not fill in)
If Be or Ni production targets are to be used, indicate the maximum beam intensity the target can handle.
Fill in the Security section if you intend to use LN2 or explosive gas (even in SPEG chambers or germanium detectors).
If you need to accelerate more than one beam or energy (stable or SPIRAL), include the beam tuning time in your requested number of beam UTs.
If not, the experiment will not be scheduled all at once.
Please contact Gilles de France if you want to use the EXOGAM germanium detectors.
Parasitic beam time is not always possible. If it is essential for your experiment, it should be included in the requested number of UTs.
Title*:
Single-Particle Energies in Neutron-Rich Oxygen Isotopes
Is it a follow up experiment? No Yes
If yes, experiment number:
Spokespersons (if several, please use capital letters to indicate the name of the contact person):
B. FERNANDEZ DOMINGUEZ (University of Liverpool) and R.C. Lemmon (Daresbury Laboratory)
Address of the contact person:
Dr Beatriz Fernandez Dominguez, Department of Physics, The University of Liverpool, Liverpool L69 7ZE, UK
Phone: +44 151 794 6971
Fax: +44 151 794 3348
E-mail: bfd@ns.ph.liv.ac.uk
Other Participants or Organisations:
W.N. Catford, C.N. Timis, J.S. Thomas, C.W. Harlin, N.P. Patterson (Surrey, UK),
M. Chartier, S. Paschalis, B. Pietras (Liverpool, UK),
R. Chapman, M. Labiche, K. Spohr, X. Liang (Paisley, UK),
N.A. Orr, L. Achouri (LPC Caen), M. Freer (Birmingham, UK)
and the TIARA collaboration*
*(likely to include the GANIL VAMOS group and also, we would hope, MUST2 members in a campaign)
Abstract :
The reaction 20O(d,p)21O is proposed to be studied using inverse kinematics with a beam of 20O provided by the SPIRAL facility. The
TIARA array with EXOGAM and VAMOS will be used as in our successful study of the (d,p) reaction induced by a 24Ne beam from
SPIRAL. Enhanced forward-angle detectors (for example MUST II) will be used so that the reactions (d,t) and (d, 3He) can be studied
simultaneously. For the first time, we will measure single-particle strengths in 21O and measure the energy of the so-far unobserved
3/2+ state that carries the 0d3/2 strength. This will test the different predictions of the USD effective interaction and recent variations
proposed for light neutron-rich nuclei. It is anticipated that information concerning negative-parity states in both 21O and 19O (and
possibly 19N) will also be obtained and may resolve uncertainties about these cross-shell excitations. The experiment is proposed to be
part of a campaign of experiments to study transfer reactions in the G1 experimental area.
Beam Line:
G1
If Lise is requested, Wien Filter? No Yes
Ion(s)
Energy (A.MeV)
Intensity (nAe)
10.9
4.8  104 pps
Primary Beam(s)
Secondary Beam(s)
if Sissi is requested
20
Spiral Beam
Will you need Sissi ?
Lise production target ?
O
No
No
If yes, Sissi Target(s) and Thickness(es):
If yes, Lise Target(s) and Thickness(es):
Requested number of beam UTs (1 UT=8h):
15 UT
Time required for setting up the apparatus: 12 UT + commissioning beam
Time required for off-beam calibration and dismounting: 3 UT
On what date would you be ready to run: middle 2006
Acquisition system: GANIL
Specific
Excluded periods:
EXOGAM + TIARA + VAMOS systems
(could also include MUST2 if in campaign)
Security, use of hazardous equipment :
EXOGAM requires liquid nitrogen; VAMOS requires detector gas
(Radioactive target, liquid nitrogen, explosive gas etc.)
Comment : We assume that the experiment would be run together with other experiments that use the same basic experimental setup,
so we add no further time for the complicated task of getting the acquisition systems of TIARA, VAMOS and EXOGAM operating
together. We note that this combined system will require some days of commissioning time, which could be with stable beam.
*
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1
Single-Particle Energies in Neutron-Rich Oxygen Isotopes
Spokespersons :
B. Fernandez Dominguez and R.C. Lemmon
and the TIARA collaboration1 :
W.N. Catford, C.N. Timis, J.S. Thomas, C.W. Harlin, N.P. Patterson (Surrey),
R. Chapman, M. Labiche, K. Spohr, X. Liang (Paisley)
M. Chartier, B. Fernandez Dominguez, S. Paschalis, B. Pietras (Liverpool)
N.A. Orr, L. Achouri (LPC Caen)
M. Freer (Birmingham)
R.C. Lemmon (Daresbury Laboratory)
Abstract
The reaction 20O(d,p)21O is proposed to be studied using inverse kinematics with a beam of 20O
provided by the SPIRAL facility. The TIARA array with EXOGAM and VAMOS will be used
as in our successful study of the (d,p) reaction induced by a 24Ne beam from SPIRAL.
Enhanced forward-angle detectors (for example MUST II) will be used so that the reactions
(d,t) and (d,3He) can be studied simultaneously. For the first time, we will measure singleparticle strengths in 21O and measure the energy of the so-far unobserved 3/2+ state that carries
the 0d3/2 strength. This will test the different predictions of the USD effective interaction and
recent variations proposed for light neutron-rich nuclei. It is anticipated that information
concerning negative-parity states in both 21O and 19O (and possibly 19N) will also be obtained
and may resolve uncertainties about these cross-shell excitations. The experiment is proposed
to be part of a campaign of experiments to study transfer reactions in the G1 experimental area.
Experimental evidence for the modification of the “magic numbers” as a function of isospin is
accumulating. Measurements of atomic masses and thus separation energies [1], intermediate
energy Coulomb excitation [2] and in-beam fragmentation gamma-ray spectroscopy [3] around
shell closures are providing convincing evidence in regions such as N=20. Theoretical work has
also pointed out that such modifications to the shell structure should appear and has suggested
certain microscopic reasons such as the modification of the monopole part of the residual
interaction with isospin [4]. To reach a deeper level of understanding, one must use reactions that
directly probe the single-particle nature of nuclei and thus measure the single-particle energies and
shell gaps. Historically, transfer reactions (and some other direct reactions) with stable nuclei have
provided this probe and established the original “magic numbers”. The region around the N=14, 16
and 20 shell gaps is a particularly pertinent area for detailed investigation. This is for two reasons:
firstly, many experimental studies have been made using different techniques and all have pointed
to the dramatic changes in nuclear structure occurring in this region close to the neutron drip-line;
secondly, the present intensities of radioactive beams are sufficient to allow reasonably detailed
experimental investigations to be made for these nuclei. We propose in this experiment to extend
our recent studies in this region by studying the reaction 20O(d,p)21O, as detailed below.
The energy levels of 21O have been studied most extensively by means of multi-nucleon
transfer [5] and fragment-gamma coincidence in secondary fragmentation [3]. A number of levels
have been identified, with tentative spin assignments, and yet the d3/2 single-particle state has not
been found. The evolution of this single-particle orbital is of great significance in light neutron-rich
1
The TIARA collaboration may include other UK and European collaborators, plus we anticipate mounting this
experiment as part of a collaborative campaign of experiments with colleagues from the MUST II collaboration and as
before from the VAMOS team at GANIL.
1
nuclei since it is the raising of this orbital in energy, as the number of protons decreases, that
causes N=16 rather than N=20 to be magic in the neutron-rich oxygen and neon isotopes. This,
inter alia, then accounts for 24O rather than 28O being the heaviest bound oxygen isotope. Shellmodel effective interactions that were developed for less exotic nuclei, notably the USD
interaction, are unable to account for this behaviour of the d3/2 orbital although modified versions
(USD-05a and USD-05b) have recently been developed. Here, we seek to locate the d3/2 single
particle strength in neutron-rich oxygen isotopes for the first time.
Since 23O is bound by only 2.74 MeV to neutron emission, the d3/2 is likely to be unbound
and very broad in 23O and there is no chance to locate it, which means that 21O is the best nucleus
in which to look. The energy of the d3/2 orbital deduced from the 3/2+ level in 21O will complement
nicely the 3/2+ energy measured in 25Ne as part of our previous experiment with SPIRAL and
TIARA, to study 24Ne(d,p)25Ne in inverse kinematics [6]. It will also provide useful
complementary data to our approved study of neutron-rich oxygen isotopes as part of experiment
E452 (M. Chartier, N. Orr et al.) to study gamma-tagged momentum distributions via neutronknockout reactions using a high-energy cocktail secondary beam.
The states that have been identified in 21O are shown in figure 1. Of these, the 5/2+ ground
state and the 1/2+ first excited state at 1.218 MeV [3] are substantially single-particle in nature, but
the 3/2+ state is different and has mostly a structure corresponding to a 0s1/2 neutron coupled to the
2+ state of 20O [5].
Figure 1 : Energy levels measured previously for 21O : (a) recent results using gamma-fragment coincidences and
secondary fragmentation [3], and (b) from multinucleon transfer 18O(18O,15O)21O and showing the interpretation of the
structure of the various states [5].
The ideal way in which to populate the single-particle states in 21O is via the reaction (d,p) onto
20
O, which can be performed using an 20O beam from SPIRAL or from SISSI. We choose to use a
SPIRAL beam because of the more appropriate energy and the superior beam optics. Following
our successful experience with 24Ne, we plan to run without beam tracking.
It is worth commenting that SPIRAL as a facility has managed to provide some highquality and pure beams of light radioactive isotopes, but the choice is not at all extensive. It is
important therefore to find niche experiments that can give interesting nuclear structure
information with the beams that can be supplied. This, we have achieved with our experiment with
24
Ne (E445S) and will extend with our approved experiment using 26Ne (E445Sa). An important
addition to the capability of SPIRAL would be the further development and exploitation of beams
other than the noble gases, and oxygen is one such opportunity. The reported intensity of 4.8  104
2
pps of 20O on target is sufficient for us to measure transfer reactions, so we propose to do this. In
the future, we would hope that the intensity of 22O might be developed further and that we could
extend our work using that beam.
Figure 2: Our previous experiment with a 24Ne beam from SPIRAL succeeded in measuring angular distributions and
spectroscopic factors [6]. Shown here are(left) the ground state in 25Ne and (right) the newly identified 3/2+ state.
19

J
5/2+
3/2+
1/2+
O
Ex (MeV)
0
0.096
1.472
21
J
5/2+
O
Ex (MeV)
0
USD
Ex (MeV)
0
S (USD)
0.345
1/2+
1.218
1.332
0.81
3/2+
7/2+
2.133
3.026
2.189
3.071
0.00
5/2+
9/2+
3/2+
3.073
3.150
4.708
4.836
0.04

S (exp)
0.57
1.00
(4 weak states omitted here)
(1/2)
3/2
3/2+
3.234
3.944
4.118
0.02*
0.11
0.33
(2 weak states omitted here)
3/2
4.584
0.15
(3 weak states omitted here)
3/2+
5.455
0.85
0.85
Table 1 : Energy levels in 19O and 21O and their neutron spectroscopic factors, S. The results for 19O are from the
compilation [8]. For 21O the experimental excitation energies are from [3], the identifications from [3,5] and the
spectroscopic factors from [7]. The 19O spectroscopic factor labelled * and the identification of this 1/2 state are
taken from [9].
The selectivity to be expected for states in 21O can be estimated using the results of shellmodel calculations for 21O (using the sd shell-model space with the USD interaction [7]) and also
by comparison with the results for nuclei having two fewer neutrons in the reaction 18O(d,p)19O
[8]. These results are summarised in Table 1. The 5/2+ ground state in these isotopes has close to
the full strength expected, based on the number of vacancies existing in the d5/2 orbital in the
respective targets. The 1/2+ state carries essentially the full single-particle strength. The 3/2+
strength in 19O is measured to be split over several levels, with the strongest of these at 5.455 MeV
excitation, but in 21O the USD prediction is that the strength is concentrated in one rather pure state
and that this is substantially lower, at around 4.8 MeV. The very recent USD-05b interaction [7]
predicts 5.2 MeV and it is important to know which interaction is right.
Thus, we expect to see a very clear signature in 21O of three strong states, being three levels
carrying the full strength of, respectively, the d5/2, the s1/2 and the d3/2 single-particle states. It may
also be that there will be population, as in 19O, of the 3/2 negative-parity state through 2p-2h
excitation of p-shell neutrons to the sd shell in 20O, although this is less likely than for 19O
according to the predicted USD spectroscopic factor. For the 1/2+ state in 21O, with the TIARA
3
setup, we will record gamma rays in coincidence with the protons from (d,p). For the 3/2+ state,
this is predicted to be above the neutron threshold and no gamma rays are anticipated. The state is
predicted to lie above the decay threshold by an amount similar to the 3/2+ state in 19O at 5.455
MeV, and since the latter state has a width of 320 keV we take this as an indicative width of the
state that we wish to discover in 21O.
Figure 3:
Schematic cross section view of the TIARA
experimental arrangement.The target is in the
centre of a barrel of two layers of silicon
detectors, and further silicon detectors cover
the angles exposed through the ends of the
barrel. Four EXOGAM gamma-ray detectors
are placed 50mm from the beam spot, around
the barrel. At zero degrees, VAMOS records
the beam-like particles.
The kinematics for the (d,p) to 21O are included in figure 4. The energies and angles are in
the range detectable by TIARA. However, to ensure that the lowest energy protons escape from the
target with minimal energy loss and hence minimal deterioration of the resolution, it will be
necessary to use a thin target of CD2, of thickness no more than 0.5 mg/cm2. The complete angular
coverage of TIARA (cf. figure 3) will also allow data to be accumulated for the (d,t) reaction and
to some extent for (d,3He). For these channels we will enhance the particle measurement by using
either our improved forward angle array (with a Si-CsI sectored annular detector) or, if it is
available, the MUST II array.
Figure 4: Kinematics calculations for the
(d,p) reaction in inverse kinematics using a
CD2 target and a beam of 21O at 10.9
MeV/nucleon from SPIRAL. The (d,p) is
shown backward of 90 degrees (<45
degrees c.m.) for the ground state (heavier
line) and 5 MeV. The loci for tritons from
(d,t) start at around 2 MeV at 0 and (d,3He)
is also shown ; both for 0 and 3 MeV
excitation.
For the transfer measurements, coincidences between TIARA and VAMOS will be
recorded, along with any coincident gamma-rays. In addition to these transfer channels, our plan is
to set up our electronics so that coincidences between just TIARA and EXOGAM will be recorded,
and from this we aim to assess the quality of information gained for the inelastic scattering
12 20 20
C( O, O)12C from the carbon in the CD2 target.
The expected cross sections are shown in figure 5. Assuming 2 mb/sr, to be prudent, and
for bins in the angular distribution of 2 degrees (equal to the strip widths, for our backward-angle
4
array), then the cross section for each angular bin is approximately 2  2 / 180    2  sin()
when allowing for the full cylindrical coverage of TIARA. This gives 0.44  sin() mb per bin.
Figure
5:
Calculations
for
d(20O,p)21O using the adiabatic
distorted
wave
approximation,
showing the differential cross
sections expected for S=1 for the
strong states included in Table 1.
(Except the 3/2 state which is shown
for
S=0.1
to
reflect
weak
population).
We assume, as stated above, a target thickness of 0.5 mg/cm2 CD2, which implies 0.125
mg/cm2 of 2H. For a beam intensity of 20O of 4.8  104 pps, as given on the SPIRAL web page,
this gives a counting rate per bin of 0.602/2  0.125  103  0.44  103  sin()  4.8  104 
3600 = 2.9  sin() counts/hr. This ranges from 0.5 to 3.0 counts/hr, between our most backward
angle of 170 and 90. Using 0.5 counts/hr/bin, then in 4 days (96 hours) we would get 48 counts
per bin for the smallest bins (angles closest to the beam direction). This implies about 15%
accuracy in counting statistics or, if we were to amalgamate adjacent bins in this angle range, we
could get 10% accuracy in 4 days. This counting time would also allow reasonable measurements
of the negative parity states, if one of these were populated with a spectroscopic factor of order 0.1
as in 19O.
The tuning of the 20O beam and the focussing onto the TIARA target, together with the
necessary setting-up of the VAMOS spectrometer is estimated to require another 3 UT. We assume
that the experiment would be run together with other experiments that use the same basic
experimental setup, so we add no further time for the complicated task of getting the acquisition
systems of TIARA, VAMOS and EXOGAM operating together. We note that this combined
system will require some days of commissioning time, which could be with stable beam.
To summarise, we request a total of 15 UT:
- 3 UT of 20O for tuning and setting up ;
- 12 UT of 20O from SPIRAL for the transfer reactions measurements ;
plus some commissioning time (actual number of UT depending on the scheduling of campaigns of
experiments requiring the same setup).
[1] H. Savajols et al., Eur. Phys. J. A 25, 23 (2005)
[6] W.N. Catford et al., J. Phys. G 31, S1 655 (2005).
[2] T. Glasmacher, Annu.Rev.Nucl.Part.Sci. 48, 1 (1998)
[7] http://www.nscl.msu.edu/brown/resources/SDE.HTM
[3] M. Stanoiu et al., Phys. Rev. C 69, 034312 (2004).
and http://www.nscl.msu.edu/brown/resources.html
[4] T. Otsuka et al., Phys. Rev. Lett 87, 082502 (2001).
[8] D.R. Tilley et al., Nucl. Phys. A 595, 1 (1995).
[5] W.N. Catford et al., Nucl. Phys. A 503, 263 (1989).
[9] E.K. Warburton, Phys. Rev. C 38, 935 (1988).
5