# MuSIC@Indiana: an effective tool for accurate measurement of fusion with
low-intensity radioactive beams

J. E. Johnstone  Rohit Kumar  S. Hudan  Varinderjit Singh  R. T. deSouza  J.
Allen  D. W. Bardayan  D. Blankstein  C. Boomershine  S. Carmichael  A. M.
Clark  S. Coil  S. L. Henderson  P. D. O’Malley  Department of Chemistry,
Indiana University, 800 E. Kirkwood Ave., Bloomington, Indiana 47405, USA
Center for Exploration of Energy and Matter, Indiana University, 2401 Milo B.
Sampson Lane, Bloomington, Indiana 47408, USA  Department of Physics,
University of Notre Dame, Notre Dame, Indiana 46556, USA

###### Abstract

The design, construction, and characterization of the Multi-Sampling
Ionization Chamber, MuSIC@Indiana, are described. This detector provides
efficient and accurate measurement of the fusion cross-section at near-barrier
energies. The response of the detector to low-intensity beams of 17,18O, 19F,
23Na, 24,26Mg, 27Al, and 28Si at Elab = 50-60 MeV was examined. MuSIC@Indiana
was commissioned by measuring the 18O+12C fusion excitation function for 11
$<$ Ecm $<$ 20 MeV using CH4 gas. A simple, effective analysis cleanly
distinguishes proton capture and two-body scattering events from fusion on
carbon. With MuSIC@Indiana, measurement of 15 points on the excitation
function for a single incident beam energy is achieved. The resulting
excitation function is shown to be in good agreement with literature data.

###### keywords:

MuSIC , Multi-sampling ionization chamber , Active target , Fusion

††journal: Journal of LaTeX Templates

##  1 Introduction

The structure and reactions of neutron-rich isotopes is presently a topic of
significant interest [1]. As nuclei become more neutron-rich their properties
are expected to change and new collective modes may emerge. The availability
of neutron-rich beams at radioactive beam facilities now allows the systematic
exploration of fusion for an isotopic chain of neutron-rich nuclei [2, 3, 4,
5, 6]. While the next generation of radioactive beam facilities, such as the
Facility for Rare Isotope Beams (FRIB), will provide radioactive beams closer
to the neutron drip-line than ever before [7], it also presents experimental
challenges. Due to their short half-lives, these exotic beams will only be
available at low intensity mandating use of an effective and efficient means
for accurately measuring fusion probability.

The low intensity of exotic radioactive beams suggests that a thick target
approach should be used. Thick target approaches have previously been used in
the measurement of fusion by identifying the fusion products via their
characteristic $\gamma$-radiation as they de-excite [8]. However, utilizing
this approach requires accurate knowledge of the $\gamma$ detection efficiency
– which is often low – as well as knowledge of the decay properties of the
neutron-rich fusion products – which may not exist.

An alternative approach is to use an active target in which direct detection
of the primary charged fusion products provides the signal that fusion has
occurred. A Multi-Sampling Ionization Chamber (MuSIC) detector [2] provides an
effective means of measuring the fusion cross-section by identifying the heavy
fusion product. While MuSIC detectors were originally developed for use in
high-energy heavy-ion experiments [9, 10, 11], more recently their use has
been extended to low energy nuclear reactions namely the measurement of the
fusion excitation function for 10-15C+12C [2, 12] and 17F+12C [13], or studies
of ($\alpha$,n)/($\alpha$,p) reactions [14, 15].

The MuSIC approach provides a couple of intrinsic advantages over the typical
thin-target measurement. Traditional thin-target measurements were performed
with limited angular coverage, identifying the fusion products by either
$\Delta$E-E [16, 17] or ETOF [18] techniques. Extraction of the fusion cross-
section thus required integration of the angle and energy distributions for
the individual heavy product introducing an uncertainty into the total
extracted fusion cross-section. Use of a MuSIC detector provides a direct
integrated measure of the fusion cross-section. In contrast to the thin-target
approach where the incident beam energy must be changed, MuSIC detectors allow
measurement of multiple points on the excitation function simultaneously [12].
In addition, MuSIC detectors are self-normalizing since the incident beam is
detected by the same detector as the reaction products. These advantages make
MuSIC detectors an efficient means for measuring fusion excitation functions
for neutron-rich nuclei when available beam intensities are limited.

This paper describes the design and construction of a MuSIC detector at
Indiana University designated MuSIC@Indiana, along with its characterization
both with an $\alpha$-source and 18O beam. To commission the detector, the
fusion excitation function for 18O + 12C was measured. A simple analysis of
the data is described which allows one to isolate fusion by distinguishing it
from events corresponding to proton capture and two-body scattering. The
measured excitation function is compared with previously reported fusion
excitation functions for this reaction in the literature.

##  2 Detector design and characterization

A MuSIC detector consists of a transverse-field, Frisch-gridded ionization
chamber with the anode subdivided into strips along the beam direction. The
signal from each anode segment is readout independently allowing the energy
deposit of an ionizing particle to be sampled. As it traverses the detector,
the beam loses energy in the detector gas at a rate characterized by its
specific ionization. If a fusion event occurs in the detector the compound
nucleus formed is higher in atomic and mass number than the incoming beam. At
energies near and below the fusion barrier, excitation of the compound nucleus
is modest, E∗= 30-50 MeV, and consequently light-particle de-excitation of the
compound nucleus results in an evaporation residue (ER) with atomic and mass
number that are also higher than those of the beam. The ERs can thus be
identified by a marked increase in energy deposit ($\Delta$E) due to their
increased atomic and mass number relative to the beam. The segmentation of the
anode means fusion events are associated with discrete locations (and
therefore discrete energies) inside the detector.

The overall design of MuSIC@Indiana is similar to other MuSIC detectors
presently in use [2, 15, 13]. The active volume is formed by six printed
circuit boards which together constitute a rectangular box. The top and bottom
of the box serve as the anode and cathode respectively. Between the anode and
cathode is a wire plane (50 $\mu$m diameter Au-W wires on a 1 mm pitch) that
acts as a Frisch grid. A side view of MuSIC@Indiana indicating the anode-to-
Frisch grid and Frisch grid-to-cathode spacings is presented in Fig. 1. To
provide a short collection time of the primary ionization produced by an
incident ion, the detector was operated at a reduced electric field of
$\sim$0.7 kV/cm/atm between the cathode and the Frisch grid. This field yields
an electron drift velocity of $\sim$10 cm/$\mu$s in both CH4 and CF4 [19, 20].
A significantly higher reduced electric field between the Frisch grid and the
anode ($\sim$1.4 kV/cm/atm) minimizes termination of electrons on the Frisch
grid. Field shaping at the edges of the detector is accomplished using printed
circuit boards with 1.613 mm strips and a center-to-center pitch of 3.226 mm.
A 30 mm diameter hole in the upstream and downstream PCB boards allows the
beam to enter and exit the active volume of the detector. The hole in the
downstream PCB also enables the precise insertion of a small silicon surface
barrier detector (SBD) using a linear-motion vacuum feedthrough (Huntington
L-2211-6). This ability to insert a SBD precisely into the active volume is
critical in the calibration and operation of MuSIC@Indiana.

Figure 1:  Schematic side view of MuSIC@Indiana. Insertion of the SBD from
downstream into the active volume is also indicated.

The dimensions of the active area of MuSIC@Indiana are indicated in Fig. 2.
The relatively large width of MuSIC@Indiana means the measurement of ER energy
loss will have high efficiency even for reactions where ERs reach angles as
large as 45∘. The anode in MuSIC@Indiana is subdivided into 20 distinct
segments along the beam direction. Further segmentation transverse to the beam
direction provides the left (L0-L19) and right (R0-R19) geometry depicted in
Fig. 2. Each anode segment is 1.219 cm wide with a 0.031 cm inter-strip
separation between anodes. This width for an anode segment along the beam
direction was chosen to provide a sufficiently large $\Delta$E signal to yield
a good signal-to-noise ratio. When the detector is operated at P = 150 Torr of
CH4 gas, an incident 18O ion with Elab = 50 MeV deposits a $\Delta$E of
$\sim$1.5 MeV for an anode.

Anode 0 is used as a ”control anode” to reject fusion or scattering events
from the beam on nuclei in the entrance window or gas prior to entering the
detector active volume. For adjacent anode strips, left and right anode strips
alternately overlap the 0∘ beam path by 1 cm as indicated in Fig. 2. This
left-right geometry has been successfully used in other MuSIC detectors [2] to
distinguish fusion events from two-body scattering (discussed further in
Section 4).

Figure 2:  Schematic layout of the MuSIC@Indiana anode (top) and cathode
(bottom). The dimensions given are for the active areas of the detector.

The cathode is divided into 5 strips which run parallel to the beam direction.
These strips are labeled C0, CL1-2, and CR1-2 as illustrated in the schematic
shown in Fig. 2. The labels ”CL” and ”CR” on these strips correspond to the
beam-left and beam-right cathode strips respectively. Segmentation of the
cathode reduces its capacitance making the capacitance of each cathode strip
comparable to an anode segment enabling a fast response for the sensing of the
electron motion away from the cathode.

Figure 3:  CAD of the chamber that houses the active region of MuSIC@Indiana.
The arrow indicates the direction of the incident beam.

The active detector is housed inside of a 18” (W) x 18” (L) x 15.5” (H)
chamber which was machined from a solid block of aluminum by Indiana
University Mechanical Instrument Services resulting in a cube as shown in Fig.
3. This fabrication approach ensures a clean machined interior surface which
is free of welds. The six sides of the cube are sealed by six large flanges
with ’O’ rings. SMA electrical feedthroughs transport the 40 anode signals
through two flanges situated on the top flange. Connected to each of these two
flanges is a motherboard housing 20 high-quality charge sensitive amplifiers
(CSAs) [21]. Coaxial cables transmit the CSA output to analog electronics
which process the signals before being recorded by the data acquisition.
Signals from the cathode, biasing of the anode and cathode, along with pumping
and gas inlet and outlet are provided on the bottom flange. The upstream
flange provides a re-entrant window while the downstream flange provides the
means to insert the SBD detector previously described using a linear
positioner. The re-entrant window provides separation of the gas volume from
the vacuum upstream. During an experiment, this window consists of a 2.6
$\mu$m doubly-aluminized mylar window sealed with an ’O’ ring. This window was
successfully tested to a pressure of 200 Torr. A thinner window of 1.5 $\mu$m
doubly-aluminized mylar exhibited leakage at a pressure of 100 Torr.

As it functions as an active target, maintaining contaminant free gas at a
stable pressure is critical to the proper operation of MuSIC@Indiana. This was
accomplished by using an oil-free gas handling system (GHS). During operation,
gas was continuously flowed through the detector via the GHS with the flow
controlled by an electronic valve/controller (MKS 0248D-00500RV). The gas flow
rate was chosen so that the gas volume of the detector ($\approx$55 L) was
replenished in approximately one hour. Feedback for the solenoid valve was
provided by monitoring the pressure inside the detector using a MKS Model 226
Differential Pressure Transducer. With this GHS it was possible to maintain a
stable pressure in the detector to within 0.1 Torr of the set pressure. The
pressure inside MuSIC@Indiana was independently measured using a piezovaccum
transducer (Newport 902B) with an accuracy of 0.1 Torr.

First tests on MuSIC@Indiana were carried out using a spectroscopy-grade 105
nCi 148Gd disk source which emits a 3.183 MeV $\alpha$ particle. To ensure
that the entire $\alpha$ energy was deposited over a single anode, the
detector was operated at a pressure of 400 Torr of CF4 gas. The source was
then positioned over each anode segment and the energy deposited by the
$\alpha$ particle over that segment was measured. Under these conditions, the
adjacent segments showed no appreciable energy deposit from the $\alpha$
particle. The results of these bench tests revealed that the inherent
resolution of each anode is $\sim$100 keV FWHM.

##  3 Characterization of MuSIC@Indiana with beam

The fusion excitation function for 18O+12C has been well measured [17, 22, 16,
23] and therefore provides a useful reference measurement for the
commissioning of MuSIC@Indiana. To measure this excitation function, a beam of
18O6+ ions was accelerated to an energy of Elab = 55 MeV by the Notre Dame
Nuclear Science Laboratory’s 10MV Tandem Accelerator. The beam intensity was
reduced to an intensity of $\sim$104 particles/s in a contrtolled manner by
passing it through slits and a 1/1000 sieve well upstream of the setup. The
resulting low-intensity beam was focused onto MuSIC@Indiana filled with CH4
gas at a pressure of 150 Torr. The cathode and anode were biased to voltages
of -1500 V and 400 V respectively with the Frisch grid held at ground. The
fusion excitation function measured was acquired in just 10 hours.

Figure 4:  Representative CSA signals from single MuSIC@Indiana anode and
cathode strips. The inset focuses on the rising edge of the same signals.

Representative anode and cathode signals from a putative fusion event,
processed by the high quality charge-sensitive amplifiers [21], are presented
in Fig. 4. These CSAs yield $\approx$9 mV amplitude signal for a 3.183 MeV
$\alpha$ particle in 400 Torr of CF4. Collection of electrons by the anode
together with inversion by the CSA determines the polarity of the anode
signal. The risetime of this signal is approximately 100 ns as evident in the
inset of Fig. 4, consistent with the drift velocity of $\sim$10 cm/$\mu$s and
the Frisch grid to anode spacing of 1 cm. Examination of the cathode signal
reveals a much larger amplitude which can be understood by noting that the
cathode integrates the entire energy along the beam direction while the anode
only collects a small portion of the particle’s total ionization. The risetime
of the cathode signal ($\sim$500 ns) is observed to be slower than the anode.
This difference is due to the larger cathode-to-Frisch grid spacing as
compared to the one for the anode-to-Frisch grid. It is also observed that the
cathode signal precedes the anode signal by 400 ns. The delay of the anode
relative to the cathode is due to the shielding of the anode from electron
motion until they have passed the Frisch grid. It should be noted that the
signal observed for the cathode is not due to the motion of the cations but
due to the motion of the electrons away from the cathode. The fall time of the
CSA signals is only $\sim$8 $\mu$s which allows successful operation of
MuSIC@Indiana at a rate up to 1 x 105 particles/s.

The CSA signals from the detector are processed through standard shaping
amplifiers and peak sensing digitizers (CAEN V785 ADC) before being acquired
by the VME data acquisition system (DAQ) and recorded on the computer. The DAQ
was triggered using signals from the segmented cathode. Each cathode’s CSA
signal was processed by a timing filter amplifier (TFA) and shaping amplifier.
The TFA signals were summed, discriminated and the resulting logical signal
was used to gate the ADCs as well as trigger the data acquisition system.

Measuring the fusion excitation function requires knowledge of the incident
energy across each anode. To measure this energy, a surface barrier detector
was inserted from downstream into the active volume of the detector. Use of a
precision linear positioner allowed the SBD to be positioned at the front and
back of each anode with an accuracy of 0.5 mm. At these positions the energy
of the beam was recorded at low beam intensity. The result of this measurement
is shown in Fig. 5. In addition, the energy loss curves for several heavier
products was also measured by impinging low-intensity beams of 19F, 23Na,
24Mg, 26Mg, 27Al, and 28Si with Elab = 50-60 MeV on the detector. The
resulting energy loss curves are presented in Fig. 5. These measurements
characterized the response of MuSIC@Indiana making the use of energy loss
programs such as SRIM [24] unnecessary. It has been established that energy
loss programs have uncertainties of approximately 10% [12, 15].

Figure 5:  Measured energy loss for several ions including potential residues
and the beam. Listed along each isotope is the incident ion’s energy in MeV.

##  4 Simple data analysis for extraction of the 18O+12C fusion excitation
function

Figure 6:  MuSIC@Indiana analysis logic flowchart.

Schematically illustrated in Fig. 6 is the sequence for analyzing data from
MuSIC@Indiana. The initial step in the analysis of MuSIC data involves the
calibration of the left and right anode segments using the energy loss of the
beam as measured by the SBD. Once the left and right segments have been
calibrated, the two sides can be summed to calculate the total energy loss in
an anode. For all subsequent steps in the analysis only the summed anode
energy loss is used.

The second step in the analysis is to require the incident ion have the
$\Delta$E of the beam ($\pm$ 300 keV) in anode 0. This step is critical to
accurately measuring the excitation function as it eliminates events where
fusion occurs in either the window or gas upstream of the active area of
MuSIC@Indiana. It also removes any beam pileup events.

The next step in the MuSIC@Indiana analysis requires that the anode of maximum
energy loss is not anode 19. Similar to anode 0, anode 19 is used as a control
anode. Requiring $\Delta$EA(19) ${\neq}$ $\Delta$EMax removes events (both
fusion and scattering) which occur in anode 19.

After these first two requirements have been implemented, the correlation
between the deposited energy in an anode and the energy deposit in the
subsequent anode is examined. A representative correlation is shown in Fig. 7a
for anodes 12 and 13. Several features appear in the correlation each of which
was identified by examining plots of $\Delta$E vs anode number (called traces)
associated with each feature.

Figure 7:  ${\Delta}$EA(12) vs ${\Delta}$EA(13) correlations. The upper plot
shows the correlation after the first three steps of the analysis. The bottom
plot shows the correlation only for events which were identified as ERs.
Features of the correlations are identified by numerals and are explained in
the text.

### Feature I

The most prominent feature in the spectrum is the bright spot at
$\Delta$EA(12) = 1.8 MeV and $\Delta$EA(13) = 1.8 MeV which corresponds to
events which are beam in both anode 12 and anode 13.

### Feature II

The second feature is a horizontal band which extends from the beam peak out
to $\Delta$EA(13)$\approx$6 MeV. This feature corresponds to events where
fusion occurs in anode 13. The fusion product with its larger atomic and mass
number than the beam has a larger specific ionization and consequently a
larger energy deposit.

### Feature III

Extending vertically from $\Delta$EA(12) = 1.8 MeV and $\Delta$EA(13) = 5.2
MeV is a faint line which corresponds to events where fusion occurred in anode
12.

### Feature IV

Starting from $\Delta$EA(12) = 5.2 MeV and $\Delta$EA(13) = 5.2 MeV and
extending as a tail to lower $\Delta$E are events where fusion occurred in
anodes prior to anode 12. This locus terminates at a distinct peak at
$\Delta$EA(12) = 0 MeV and $\Delta$EA(13) = 0 MeV. This peak is associated
with events in which fusion occurred much earlier in the detector and the ER
has already ranged out in the detector gas prior to anode 12.

### Feature V

The near-vertical band extending from the beam peak corresponds to two-body
events which are subsequently eliminated in the analysis.

### Feature VI

Extending diagonally from the beam peak up to $\Delta$EA(12) = 2.8 MeV and
$\Delta$EA(13) = 2.8 MeV, and then turning with a tail back down to
$\Delta$EA(12) = 0 MeV and $\Delta$EA(13) = 0 MeV are proton capture events
resulting from the fusion of beam on hydrogen in the CH4 detector gas. Proton
capture of the 18O beam results in 19F which exhibits a larger specific
ionization than the beam but less than that of the ERs. These proton capture
events are characterized by $\Delta$E values that are higher than the beam for
several consecutive anodes before dropping to $\Delta$E = 0 MeV at the end of
the detector. A representative proton capture event is presented in Fig. 8a

Correlations like the one shown in Fig. 7a are used in the analysis to
establish the quantity ${\Delta}$EHigh(I) for each anode. For example, Fig. 7a
was used to set ${\Delta}$EHigh(12) = 3.2 MeV, a measure of the maximum energy
deposit associated with proton capture for that anode. Requirement that at
least one anode has ${\Delta}$EA(I) ${>}$ ${\Delta}$EHigh(I) eliminates proton
capture events from the data leaving putative fusion events.

Figure 8:  Experimental MuSIC@Indiana traces. Panel a) shows the trace for a
proton capture event. Panel b) shows the trace for a two-body scattering
event. Panel c) shows the trace for a residue from fusion occurring in anode
3. For all panels, the average beam trace is shown as the black line. The
error bars on the average beam trace represent the FWHM of the beam $\Delta$E
distribution in that anode.

After removing the proton capture events, the analysis requires that if
multiple anodes have ${\Delta}$EA(I) ${>}$ ${\Delta}$EHigh(I), those anodes
must be adjacent. This requirement rejects the majority of two-body scattering
events. A representative two-body scattering event is shown in Fig. 8b. Two-
body scattering events are characterized by two particles with different
specific ionization and consequently two ranges. This behavior is clearly
evident in Fig. 8b where one particle has a range of 1-2 anodes and the other
has a range of approximately 11 anodes. Observation of two Bragg peaks in the
trace is a clear indication of two particles in a single event.

Not all two-body scattering events are eliminated through the previous
analysis step. To remove the remaining scattering events, the analysis
requires that the ${\Delta}$E drops monotonically after the anode of maximum
${\Delta}$E. This requirement specifically eliminates scattering events where
the low specific ionization (beam-like) particle has a long track and does not
pass the high threshold later in the detector. This analysis is distinct from
previous MuSIC analyses which required the use of the detector’s left/right
anode structure to distinguish two-body events from fusion events [12]. While
events containing two particles can be eliminated by using the left/right
information, the analysis steps described in this paper provide a simple and
equally effective way of removing two-body scattering events from the data.

All remaining events are assigned as fusion events. A representative trace of
a fusion event is presented in Fig. 8c. Prior to anode 2 the $\Delta$E
observed is consistent with that of beam. At anode 2 the $\Delta$E increases
markedly reaching a maximum at anode 5 whereupon it decreases monotonically
until anode 10. No additional energy is observed at subsequent anodes. Using
the fusion events selected in this manner, correlations like the one shown in
Fig. 7b are made. After all of the analysis steps have been followed, Features
V and VI as well as the scattering events in Feature II have been removed. The
residues appearing in Features II, III, and IV are now clear. The remaining
events in Feature I of Fig. 7b correspond to fusion events which happen in
anodes after anode 13. Using this correlation a low threshold,
${\Delta}$ELow(I), is established for each anode. This threshold is set just
above Feature I and is used to assign the anode of fusion. For example, Fig.
7b was used to set ${\Delta}$ELow(12) = 2.2 MeV. The anode of fusion is
assigned to the first anode with ${\Delta}$EA(I) ${>}$ ${\Delta}$ELow(I).

Figure 9:  Fusion excitation function of 18O+12C. The literature datasets are
Kovar [17], Steinbach [22], Eyal [16], and Heusch [23].

Once the occurrence of fusion has been identified and assigned to the
appropriate anode, the cross-section can be calculated using:
$\sigma=N_{ER}/(\epsilon*I*t)$, where NER is the number of ERs in an anode,
$\epsilon$ is the detector efficiency of an anode, I is the number of incident
beam particles, and t is the target thickness as defined by the anode width
and the gas pressure.

Use of anodes 0 and 19 as control anodes prohibits using them in the
measurement of the excitation function. Moreover, the most downstream anodes
are less than 100% efficient. An anode can be considered 100% efficient if
there are a sufficient number of anodes to observe the peak in the
corresponding trace, which requires 4-5 anodes. Consequently, the fusion
cross-section is measurable for anodes 1 - 15 with 100% efficiency. Given this
intrinsic efficiency, no efficiency correction is necessary to extract the
fusion cross-section. In the present measurement this enabled us to measure
the fusion cross-section for 11 MeV $<$ Ecm $<$ 20 MeV.

In order to assign an energy to the cross-section associated with a particular
anode, the SBD measurement of the beam at the front and back of each anode was
used. Each datapoint in the excitation function is initially assigned the
energy in the middle of the associated anode (calculated as the average of the
energies at the front and the back of the anode). In reality the energy
average of the anode is weighted toward higher energy where the cross-section
is higher. To correct for this, the initial excitation function is
parameterized using a Wong formalism [25]. Each datapoint is then segmented
into 20 equally-spaced slices in energy. The new energy is calculated as the
weighted average of the energy of the slices using the Wong-calculated cross-
section:

$E^{\prime}=\frac{\sum[{\sigma}_{Wong}(slice)*E(slice)]}{\sum{\sigma}_{Wong}(slice)}$

This process is repeated until the energy converges. Horizontal error bars
represent the difference between the assigned anode energy and the energy at
the front/back of the anode. The size of the vertical error bars are
calculated from the experimental statistics.

The measured 18O+12C excitation function from this work is displayed in Fig.
9. It is observed to be in good agreement with the previously reported cross-
section in the literature. Below Ecm = 14 MeV the present work matches the
Eyal [16] and Steinbach [22] measurements even to the extent of interpolating
between the published points in those datasets. In this same region there are
two datapoints from Kovar [17] and one datapoint from Heusch [23] which are
high relative to Eyal, Steinbach, and the present work. This result suggests
that Kovar and Heusch may provide a systematically high measurement of the
cross-section. This trend continues above Ecm = 14 MeV with the present work’s
cross-sections below all Kovar datapoints. The present work’s excitation
function overlaps two points from Heusch but these points are also low
relative to all other Heusch points. It should be appreciated that the prior
measurements of the fusion cross-section were thin-target measurements with
limited angular coverage. Extraction of the fusion cross-section required
integration of the angle and energy distributions for the individual ERs which
introduces uncertainties into the total extracted fusion cross-section. Use of
a MuSIC detector provides a direct integrated measure of the fusion cross-
section.

##  5 Conclusions

MuSIC detectors with their direct measurement of the angle-integrated fusion
cross-section and ability to simultaneously measure multiple points on an
excitation function are a powerful tool for radioactive beam experiments,
particularly for intensities below 104-105 particles/s. MuSIC@Indiana is
differentiated from other MuSIC detectors in its ability to precisely insert
an SBD into the detector active volume. The SBD enables the accurate
measurement of the beam energy at each anode allowing calibration of
MuSIC@Indiana and eliminates the uncertainties associated with energy loss
programs in constructing the fusion excitation function. To characterize the
response of the detector to evaporation residues the energy loss of heavier
ions in the CH4 gas was also measured. An analysis procedure was developed
that provided a simple means of discriminating fusion events from proton
capture and two-body scattering events. The effectiveness of this analysis was
demonstrated by the good agreement between the extracted excitation function
and previously reported cross-sections. The quality of the 18O+12C measurement
over a short time interval demonstrates that MuSIC@Indiana is an effective
tool for accurate measurement of fusion with low-intensity radioactive beams.

##  6 Acknowledgements

The authors would like to thank Indiana University Mechanical Instrument
Services and Electronic Instrument Services in the Department of Chemistry for
their help in constructing the MuSIC@Indiana detector. This work was supported
by US Department of Energy under Grant No. DE-FG02-88ER-40404. Research
sponsored by NSF Grant No. PHY-2011890 and by the University of Notre Dame.

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