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{\Large\bf Results from Charissa runs at ANU during 1998}
(SPGC)

\section{ANU running Jan/Feb '98}
A new strip detector array was installed in the Charissa MEGHA vacuum
chamber and commissioned with a series of experiments during January
and February 1998. In total 19 days of beamtime were used during this
period including 1 day for the installation and commissioning of the
new array and associated electronics. The measurements used a common
set-up of eight 16-strip detectors which provided angular coverage
from 5$^{\circ}$ to 60$^{\circ}$ in the laboratory. A summary of
results to date from this period are given in this section following a
brief description of the new array.

\subsection{The Strip detector array}

The array [1] consists of a mounting plate which can accommodate
either position sensitive strip detectors (PSSD's) or two-dimensional
position sensitive detectors (PSD's) in a selection of thirty fixed
positions at a typical distance of 150mm from the target. With all
positions filled the array has almost complete angular coverage from
5$^{\circ}$ to 85$^{\circ}$ degrees in the laboratory. At present, the
MEGHA data acquisition system provides for up to 320 signal
channels. This allows for up to ten PSSD's to be independently
instrumented in the array.  The mount is designed such that removal of
the detectors at forward angles allows PSSD's to be used in
conjunction with the MEGHA module Gas-Si-CsI telescopes. Twisted-pair
36way ribbon cables connect the detectors to 42-pin hermetically
sealed ``Amphenol'' connectors fixed in one of the MEGHA chamber side
flanges. Custom designed preamplifiers are located in racks
immediately adjacent to the vacuum vessel.

\subsection{High spin resonances in $^{12}$C+$^{12}$C scattering}



\begin{figure}[t]
\vspace{10cm}
\special{psfile=exfncc.ps vscale=100 hscale=100 hoffset=-40 voffset=-80}
\caption{Excitation function for the $^{12}$C[$^{12}$C, $^{12}$C($3_1^-$)]
$^{12}$C($3_1^-$) reaction (Diamonds from the 4 PSSD measurement of
ref.~[2]. Open circles from 8 PSSD measurement at ANU)}
\label{fig:exfncc}
\end{figure}

We have previously observed [2--4] large resonant like structures in
the cross section of the 3$^-$+3$^-$ channel in $^{12}$C+$^{12}$C
scattering at $E_{c.m.}=32.5,37.5$ and 43
MeV~(figure~\ref{fig:exfncc}). Unfortunately, the angular coverage of
the detection system used in the measurements (four position sensitive
strip detectors (PSSD's)) was not sufficient to determine the dominant
spins of these structures. Following the installation of the new strip
detector array, we performed detailed angular correlation measurements
at the energies corresponding to the peaks of each resonance. These
measurements used 6 days of beamtime. The new strip detector array
gave a large efficiency for the detection of high particle
multiplicities. This enabled us to use a thin target
(10$\mu$gcm$^{-2}$). As a result the resolution obtained in these
measurements is significantly improved.
\begin{figure}
\vspace{16.5cm} % Or \vskip
\special{psfile=corr.ps vscale=70 hscale=70 voffset=-30}
\caption{\rm Comparisons of measured and simulated angular correlations
projected onto a single axis using the optimum projection angle
$\theta_P$ indicated at each of the three centre of mass
energies. (a--c) Simulation for three consecutive $L'$ values and no
incoherent background. (d--f) Simulation including a coherent
contribution of two partial waves and an incoherent background. (The
background is shown as the dotted curves in the figures scaled by a
factor of 0.5 for clarity). (d) $E_{c.m.}=32.5$ MeV with
$a_{12}=0.92$, $a_{14}=-0.39$ (e) $E_{c.m.}=37.5$ MeV with
$a_{14}=0.95$, $a_{16}=0.32$ (f) $E_{c.m.}=43.0$ MeV with
$a_{16}=0.98$, $a_{18}=-0.22$. The projection angles
$\theta_P=103^{\circ}, 101.5^{\circ}$ and $100.5^{\circ}$ yield
$L'=12.5, 14.2$ and 15.7 respectively which is consistent with our
assignments.}
\label{fig:corr}
\end{figure}

The details of the data analysis and angular correlation method are
well described elsewhere~[4]. Briefly, the angular correlation for the
present case depends on five angles, namely the primary scattering
angle $\phi^*_B$ and the two decay angles $\chi_B$ and $\psi_B$ of the
$^{8}$Be-$\alpha$ relative velocity with respect to the beam for each
$^{12}$C nucleus. In ref.~[4] we demonstrated that the semiclassical
angular correlation function for mutual decay could be approximated
over the measured angular range by
\begin{eqnarray}\label{eq1}
W(\phi^*_B,\chi_B,\chi'_B,\psi_B,\psi'_B) & \propto & 
\frac{1}{\sin{\phi^*_B}}\left|
\sum_{L'} a_{L'} 
\sin\left[\left(L'+\frac{1}{2}\right)\phi^*_B + k\chi_B + k'\chi'_B
\right. \right. \nonumber \\
& & \left. \left. + \frac{\pi}{4}\right]\right| ^2 
d^I_{k0}(\psi_B)^2 d^{I'}_{k'0}(\psi'_B)^2  + \frac{A^2}{\sin{\phi^*_B}}
\end{eqnarray}
where $\psi_B$,$\psi'_B\simeq\pi/2$. Here $a_{L'}$ is the amplitude
for a single value of the partial wave in the final channel $L'$ and
$A^2$ is an incoherent background. The projections of the $^{12}$C
angular momentum on the $z$-axis, which is perpendicular to the
reaction plane, are $k$ and $k'$. In ref.~[3] we showed that by making
assumptions about the quantum numbers $k$ and $k'$ we could define a
two-dimensional correlation function $W(\phi^*_B,X)$ where
$X=\chi_B\pm\chi_B'$ and hence determine the dominant $L'$ and total
angular momentum $J=L'+k+k'$ from the projections of the ``ridge''
pattern onto a single axis labelled by $\phi^*_P$. A slightly
different and more rigorous approach is presented in
figure~\ref{fig:corr}. In the figure, we compare the projected
correlation data with full Monte-Carlo simulations of the reaction for
three different consecutive partial waves at each centre of mass
energy (figure
\ref{fig:corr}a to
\ref{fig:corr}c). Note that the different partial waves are out of
phase away from the centre of the $\phi^*_P$ range for each energy. In
figures \ref{fig:corr}d to \ref{fig:corr}f we compare the data with
simulations for a coherent sum of two $L'$ values and an incoherent
background. The suggested dominant $L'$ values indicated in the figure
are consistent with the optimum projection angle $\theta_P$ at each
energy. We suggest that the dominant $L'$ values for the structures at
$E_{c.m.}=32.5,37.5$ and 43 MeV are 12, 14 and 16 respectively.

Assuming that these resonances dominate the angular correlations at
the centre of each peak in the excitation function they have angular
momenta up to $J=L'+6=22$ and are consistent with a rotational
band. We believe that these resonances extend the sequence of broad
resonances first observed by Cormier {\it et al.}~[5]. A comparison of
resonance energies and spins with theoretical calculations support
this view~[6]. These results have been submitted for publication~[6].
We also have results for a number of other channels from this
experiment that are being prepared for publication.

\subsubsection[8Be decay]{$^{8}$Be decay of $^{16}$O and $^{12}$C}

The $^{12}$C+$^{12}$C reaction at $E_{lab}=93$ MeV was used to study
the $^{8}$Be decay of highly excited states in $^{16}$O and
$^{12}$C. Approximately 2 days of beamtime were used for these
measurements.

\begin{figure}
\vspace{15cm}
\special{psfile=oproj.ps vscale=70 hscale=70 hoffset=50 voffset=-50}
\special{psfile=o.eps vscale=80 hscale=80 hoffset=-20 voffset=-80}
\caption{\rm (a) Modified Dalitz plot for the
$^{8}$Be$_{\rm g.s.}$+$^{8}$Be$_{\rm g.s.}$+$^{8}$Be$_{\rm g.s.}$
final state. (b) Projection of the region abcd (defined in frame a)
onto an axis perpendicular to $E_{12}$.}
\label{fig:o}
\end{figure}
The $^{12}$C($^{12}$C,$^{8}$Be$_{\rm g.s.}$,$^{8}$Be$_{\rm
g.s.}$)$^{8}$Be$_{\rm g.s.}$ channel was isolated by detecting
4$\alpha$ particles in the final state and then gating on the
$^{8}$Be$_{\rm g.s.}$ peak in each $\alpha-\alpha$ relative energy
spectrum. Undetected particles were reconstructed from the missing
momentum. The parenthood of the three $^{8}$Be$_{\rm g.s.}$ nuclei in
these events was investigated using a ``modified Dalitz plot''~[7], as
presented in figure~\ref{fig:o}a. In the geometry of the Dalitz plot,
an excited state of the parent nucleus (in this case $^{16}$O*) is
identified as a stripe at a fixed relative energy $E_{ij}$ between the
daughter particles parallel to the sides of the triangle shown. The
data are confined to a circle of diameter equal to the available
energy in the triple $^{8}$Be$_{\rm g.s.}$ final state. The lack of
symmetry in the plot is a result of non random particle order in the
data sorting routine. Three states in $^{16}$O* have been identified
(fig.~\ref{fig:o}b) at $E_{x}\simeq 19$, 23 and 30 MeV. The spins of
the states have been determined using angular correlation
techniques. The 6$^+$ states at 19 and 23 MeV are consistent with
those found in previous work~[8]. Interestingly, the 8$^+$ state at 30
MeV is close to the energy predicted for the $^{16}$O 4$\alpha$ chain
in a coupled-channels cluster model~[9]. This model predicts a 10$^+$
state some 4 MeV higher in energy than the 8$^+$. Such a state would
lie at the edge of the Dalitz plot in figure~\ref{fig:o}a and is out
of the range of the present experiment. We propose to search for the
10$^+$ state using the technique demonstrated in this measurement~[10].

\begin{figure}[t]
\vspace{9cm}
\special{psfile=c.eps vscale=80 hscale=80 hoffset=-20 voffset=-150}
\caption{\rm Excitation in $^{12}$C* versus excitation in $^8$Be* for
$^{12}$C*$\rightarrow$ $^{8}$Be*+$\alpha$ decays.}
\label{fig:c}
\end{figure}
The analysis of $^{12}$C($^{12}$C,3$\alpha$)$^{12}$C$_{\rm g.s.}$
events is slightly more complicated than for the case described above,
since the three particles under examination have their energies fixed
by the excitation in $^{12}$C$^*$, which may take several values. In
particular, the decays to $^{8}$Be($2^+$) are not easily identified
due to the large width of the state. In order to examine these events
the spectrum in figure~\ref{fig:c} was constructed. Only one of the
three possible $\alpha-\alpha$ relative energies is plotted in this
spectrum. If a pair of $\alpha$ particles was found to be consistent
with $^{8}$Be$_{\rm g.s.}$ decay then that relative energy was
plotted, otherwise there was no selection. Hence the $^8$Be* spectrum
contains a background of events where the alpha particle daughters in
the sequential decay
$^{12}$C*$\rightarrow$$^8$Be*+$\alpha$$\rightarrow$3$\alpha$ are
incorrectly assigned. These data confirm that the 14.08 MeV $4^+$
state in $^{12}$C decays predominantly via the 2.9 MeV $2^+$ state in
$^8$Be~[11]. We measure a branching fraction of 0.71$\pm$0.01 for this
decay. 

We propose to investigate the inelastic scattering of
$^{12}$C+$^{12}$C to the single and mutual 4$_1^+$ channels using
these analysis techniques~[12].


\subsection{$^{28}$Si+$^{12}$C orbiting}

A pilot study was made into the suitability of the new strip detector
array for investigating the phenomenon of dinuclear orbiting [3]. We
used approximately 2 days of beamtime to study the $^{28}$Si+$^{12}$C
reaction in a kinematic coincidence measurement at E$_{c.m.} = 20.1,
26.0$ and 33.6 MeV. All events in which at least two particles were
detected in the strip array were recorded. Following a detailed
offline analysis of the data we have discovered that the strip
detector array is not well suited to such measurements without more
specific event selection. The desired 2-fold elastic and inelastic
scattering events in the data were swamped by high fold events. The
cross section for scattering to multi-particle final states coupled
with the large efficiency for detection of these events in the new
array has been found to give an unacceptably large deadtime for the
recording of 2-fold coincidences.


\subsection{Investigation of $^{16}$O$^{*}$$\rightarrow$
$^{12}$C$_{g.s.}$+$\alpha$ breakup}

Reactions between an $^{16}$O beam and a LiO target (with a natural
carbon backing) were used to investigate the energy dependence of the
breakup $^{16}$O$^{*}$$\rightarrow$ $^{12}$C$_{g.s.}$+$\alpha$ via the
single excitation of $^{16}$O to the 11.09 MeV 4$^{+}$ excited
state. Eight days of beamtime were used to measure excitation
functions between 26 and 47 MeV in the centre-of-mass. Final state
determination and resonant nucleus reconstruction was achieved using a
new technique, which relies on a variety of kinematic gates to
identify and isolate the breakup channels of interest~[17].

\begin{figure} 
\vspace{15cm} 
\special{psfile=exfnoo.ps vscale=70 hscale=70 hoffset=0 voffset=-50}
\caption{\rm Energy dependence of the
$^{16}$O($^{16}$O,$^{16}$O$^{*}$$\rightarrow$
$^{12}$C$_{g.s.}$+$\alpha$)$^{16}$O$_{g.s.}$ reaction for the
$^{16}$O$^{*}$ states indicated. Dominant partial waves are given in
the figure.} 
\label{fig:exfnoo}
\end{figure}
Excitation functions for the excitation of $^{16}$O$^{*}$ to the
2$^{+}$ (9.84 MeV), 4$^{+}$ (10.35 MeV) and 4$^{+}$ (11.09 MeV) states
are given in figure~\ref{fig:exfnoo}. Angular correlation
techniques~[4] were used to determine the dominant angular momentum
associated with specific energies in the centre-of-mass where
sufficient data were available (See figure \ref{fig:exfnoo}). It is
interesting to note that the enhancements at 31 MeV and 36 MeV (in the
centre-of-mass) in the excitation function for the 11.09 MeV 4$^{+}$
excited state are correlated with enhancements observed by Wells {\it
et al}~[16] for the single and mutual excitation of $^{16}$O to the
3$^{-}$ (6.13MeV) state. It has been proposed~[13] that the ground
state of $^{16}$O has tetrahedral symmetry. The states usually
associated with the ground state rotational band are 0$^{+}$ (the
ground state), 3$^{-}$ (6.13 MeV), 4$^{+}$ (10.35 MeV), 6$^{+}$ (16.29
MeV), 7$^{-}$ (21.19 MeV) and 8$^{+}$ (29.18 MeV). However the 4$^{+}$
(10.35 MeV) and 6$^{+}$ (16.29 MeV) states also fit into the 4p-4h
K=0$^{+}$ ``kite'' rotational band~[14]. Better candidates for the
tetrahedral band may be the states at 4$^{+}$ (11.09 MeV) and 6$^{+}$
(14.82 MeV)~[15]. If the 4$^{+}$ (11.09 MeV) state is a member of the
tetrahedral band, it is expected that strong correlations should exist
between the excitation function for $^{16}$O$^{*}$$\rightarrow$
$^{12}$C$_{g.s.}$+$\alpha$ breakup from the 4$^{+}$ (11.09 MeV) state,
and the measured excitation function for single and mutual excitation
to the 3$^{-}$ (6.13MeV) state.  The present results suggest that the
4$^{+}$ (11.09 MeV) excited state may indeed be associated with the
tetrahedral band in $^{16}$O.  These results and those on a number of
other final state channels are being prepared for publication~[17].


\section*{References}
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\item{[2]} S.P.G.~Chappell {\it et al.}, ANU Annual Report 1995,
section C2.3; 1996, section C2.8; 1997,section C.
\item{[3]} W.D.M.~Rae, S.P.G.~Chappell and P.E.~Fry, Nuovo Cimento
110A (1997) 1001.
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\item{[5]} T.M. Cormier {\it et al.}, Phys. Rev. Lett. 40 (1978) 924 and 38 (1977) 940.
\item{[6]} S.P.G.~Chappell {\it et al.}, to be published in
Phys. Lett. B.
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Phys. Rev. C {\bf 13} (1976) 967.
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775.
\item{[10]} S.P.G. Chappell, Charissa proposal CH/PROP/ANU/99/02.
\item{[11]} D.D. Caussyn {\it et al}, Phys. Rev. C {\bf 43} (1991)
205.
\item{[12]} S.P.G. Chappell, Charissa proposal CH/PROP/ANU/99/01.
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A437} (1985) 208.
\item{[14]} R.A. Boldock, B. Buck and J.A. Rubio, Nucl. Phys. {\bf
A426} (1984) 222.
\item{[15]} D.R. Tilley, H.R. Weller and C.M. Cheves, Nucl. Phys. {\bf
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\item{[16]} W.K. Wells, D.P. Bybell and D.P. Balamuth, Phys. Rev. {\bf
C25} (1982) 2512. 
\item{[17]} G.K. Dillon, DPhil. thesis, University of York, U.K. (to be
submitted).
\end{description}



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