Beta-decay studies at the NSCL using a double-sided silicon
strip detector
A.C. Morton
National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824-1321 USA
Abstract. A β counting system has been developed at the NSCL to study the β-decay properties of nuclei produced by
fast fragmentation. This system employs a double-sided silicon strip detector (DSSD) to observe both the implantation
of these fragments and their subsequent β decay. By correlating implant and decay events within the DSSD, we identify
on an event-by-event basis the nuclides observed to decay and obtain a direct measurement of the decay time. A β
calorimeter is being developed to augment the counting system; this will allow for the measurement of total β-decay
energies for decays with low Q values and the determination of Gamow-Teller strengths for the decays of exotic nuclei.
This system will first be used to study the decay of 100Sn. Individual detector components are on site and being tested,
and Geant4 simulations of the system are underway.
with beams of mixed nuclei while retaining the
unambiguous identification of the β-emitting source.
INTRODUCTION
The β-decay properties of nuclei far from the
valley of stability can serve as sensitive tests of
nuclear structure models in these regions.
Furthermore, β-decay half-lives and endpoint energies
often have astrophysical importance, serving as input
parameters in network calculations of nucleosynthetic
processes. Recent progress in the study of nuclear β
decay has been driven by the development of
improved particle-detection techniques using fast
fragment beams.
At the National Superconducting Cyclotron
Laboratory (NSCL) at Michigan State University, a
new β counting system has been developed to carry
out such experiments. Fast fragments from the
laboratory’s new Coupled Cyclotron Facility (CCF)
are implanted in a double-sided silicon strip detector
(DSSD). Implant and decay events are directly
correlated within each pixel of this detector, allowing
for both the measurement of decay times and the
unique identification of the parent nucleus. This
system does not, however, allow the determination of
total β-decay energies, as the detectors used are not
sufficiently thick to stop the emitted β particles. A β
calorimeter is currently under development which,
when complete, will allow such measurements.
Traditional β-decay experiments have relied on
bulk measurements of activity to deduce decay
properties. As the β-decay energy spectrum is
continuous, identification of the β-particle-emitting
nucleus is difficult with such a technique; as a result,
such measurements require isotopically-pure sources
of β activity. By fragmenting heavier primary nuclei
in fixed targets, many radioactive species can be
produced. With fast beams, the atomic number and
mass of each fragment can then be determined from
energy loss, time of flight, and magnetic rigidity on an
event-by-event basis. By correlating the implantation
of these fragments in a target with their subsequent β
decays, this information can be applied to the decay
events themselves, allowing measurements to be made
BETA COUNTING SYSTEM
The β counting system is built around a Micron
Semiconductor Ltd. Type BB1 DSSD. This device is
a 4 cm × 4 cm × 985 µm silicon wafer segmented in
40 1-mm wide strips in each of the x and y directions.
A 5 cm × 5 cm × 500 µm Si PIN diode is located
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every master trigger, providing each event with a time
stamp with 30.5 µs resolution. Decay times are
determined by subtracting the time of a fast fragment
implant from that of its correlated β decay.
upstream of the DSSD. Two 5 cm × 5 cm PIN diodes,
one 500 and one 300 µm thick, are placed downstream
of the DSSD. An additional 5 cm × 5 cm × 500 µm
PIN diode is located approximately 1 m upstream of
this array to provide energy-loss and time-of-flight
information for particle identification. A parallel-plate
avalanche counter (PPAC) located just upstream of
this detector is used for beam diagnostics. The
arrangement of detectors in the β counting system is
shown in Figure 1.
ADC
β
PIN
PIN
MCS CPA16
pre-amp
(non-inv.)
High gain
PIN
Coin. Reg.
PPAC
Low gain
MCS CPA16
Grounding
pre-amp
board
(inverting)
Back
strips
ADC
DSSD
ADC
DSSD
Low gain
PIN
High gain
Pico Sys.
shaper/
disc.
Pico Sys.
shaper/
disc.
Delay
Delay
Scaler
FIGURE 2.
electronics.
Front
strips
Scaler
ADC
Coin. Reg.
Schematic representation of the DSSD
Additional details of the β counting system and its
performance characteristics are discussed in Ref. [1].
FIGURE 1. Schematic representation of the β counting
system (not to scale).
Because implanted fragments deposit more than 1
GeV of energy in the DSSD while the β particles
subsequently emitted deposit less than 1 MeV, dualoutput preamplifier modules are used to process the
DSSD signals. The present system yields reliable
energy information for both high-energy implant and
low-energy decay events with a majority of events
having a multiplicity of one in both the front and back
strips of the DSSD.
BETA CALORIMETER
In nuclear β decay, experimentally-determined
Gamow-Teller (G-T) strengths tend to be significantly
smaller than those predicted by theory. In many
nuclei, G-T strength is distributed over many levels,
complicating both experimental measurements and
theoretical calculations. The case of 100Sn is different.
Most of its G-T strength has been calculated to lie
below the β-decay Q-value [2]; furthermore, it is
believed that the β decay of 100Sn is only allowed to a
single 1+ in 100In. As a result, virtually all of the G-T
strength is expected to go to this state. An experiment
has been approved at the NSCL to study this decay
and determine its G-T strength.
The preamplifier modules used are 16-channel
devices manufactured by MultiChannel Systems,
model MCS-16, and provide separate high-gain (2
V/pC) and low-gain (0.1 V/pC) outputs. The low-gain
signals, carrying energy information from high-energy
fragment implantations, are sent directly to analog-todigital converters (ADC’s) with no further processing.
The high-gain signals, which provide energy
information for low-energy decay events, are
processed further with Pico Systems 16-channel
shaper/discriminator modules in CAMAC.
The
shaped signals are sent directly to ADC’s, in either
CAMAC or VME, as before. The discriminator
outputs are combined in a logical OR and serve as the
master trigger. Individual discriminator signals are
also sent to scalers and coincidence registers.
Simplified DSSD electronics are shown in Figure 2.
To differentiate between different theoretical
models of 100Sn decay, it is necessary to determine its
G-T strength to better than ~15% precision [2, 3]. To
achieve this, the Q-value must be measured to better
than ~2.5% precision, or 95 keV at the observed β
endpoint energy of 3.8 +−0.7
0.3 MeV [4]. Furthermore, the
β-decay half-life must be measured to better than ~7%
precision; the current value is 1.00 +−0.54
0.26 s [4]. While
the latter measurement is within the capabilities of the
β counting system, the former is not; the counting
system does not contain a sufficient thickness of
silicon to observe β-particle energies of more than a
few hundred keV. For this reason, it was decided to
Two EG&G RC014 CAMAC-based realtime
clocks are used in a master-slave configuration to
provide a time reference. The clocks are read with
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augment the counting system with a β calorimeter
capable of measuring total β-decay energies up to 3.8
MeV with better than 2.5% resolution. This system is
currently in development.
with the DSSD. As the calorimeter will be used only
to observe low-energy β decays, only the high-gain
outputs of the preamplifier modules will be used. As
with the DSSD, the high-gain signals will be sent to
Pico Systems CAMAC-based shaper/discriminators
for further processing. The shaped SSSD signals will
be sent directly to CAEN V785 VME ADC’s, while
individual discriminator outputs will be sent to scalers
and bit registers as before. The discriminator outputs
will not contribute to the master trigger. Simplified
SSSD electronics are shown in Figure 4.
Technical aspects
As currently planned, the β calorimeter will consist
of eight large-area silicon detectors from Micron
Semiconductor Ltd., six Type W single-sided silicon
strip detectors (SSSD’s) and two Type MSX PIN
diodes. All eight detectors have active areas of 5 cm ×
5 cm and thicknesses of ~1000 µm. The SSSD’s are
segmented into 16 strips of equal width, while the PIN
diodes are non-segmented. This array will replace the
two downstream PIN diodes, providing an 8 mm
thickness of Si for the observation of total β-particle
energies of up to ~4 MeV. Furthermore, the use of
position-sensitive detectors will allow the tracking of β
particles through the system and improved
discrimination between β particles and light fragments
from the Coupled Cyclotron Facility. Because few of
the emitted β particles will carry energies near the
decay Q-value, the β endpoint energy will be
determined by fitting the total energy spectrum
obtained, taking into account the calculated response
of the system. This obviates the need to observe the
endpoint directly; however, a direct observation of the
endpoint would improve the accuracy of the
determination.
PIN
SSSD SSSD SSSD
PINPIN
β
DSSD
SSSD SSSD SSSD
FIGURE 3. Schematic representation of the β calorimeter
(not to scale).
SSSD
MCS CPA16
pre-amp
High gain
Pico Sys.
shaper/
disc.
ADC
Delay
Scaler
To maximize the geometric efficiency of the
system, the eight detectors will be stacked as closely
as possible along the beam axis. The SSSD’s will be
located immediately downstream of the DSSD and
placed so that the strips in each detector are at a right
angle to those in the next. As a result, position
information in both the x and y dimensions will be
available from any adjacent pair of SSSD’s. The two
PIN diodes will be placed at the downstream end of
the calorimeter stack to provide additional energy
information. Since future experiments may require the
determination of endpoint energies greater than 3.8
MeV, the use of additional detectors to increase the
stopping power of the calorimeter is being
investigated. Si, Ge, and CdZnTe charged particle
detectors are being considered; these detectors would
be placed in the calorimeter stack behind the second
PIN diode, and would permit the measurement of total
β-decay energies to ~10 MeV. The arrangement of
detectors in the calorimeter is shown in Figure 3.
FIGURE 4.
electronics.
Coin. Reg.
Schematic representation of the SSSD
The signals from the PIN diodes will be processed
using a Tennelec TC178 quad preamplifier. The
output signals will be sent to Tennelec TC241S
spectroscopy amplifier/timing filter amplifer (TFA)
modules. The unipolar spectroscopy outputs will be
sent to ADC’s, while the TFA outputs will be sent to a
quad constant fraction discriminator (CFD). As with
the strip detectors, the CFD outputs will be sent to
scalers and coincidence registers.
Performance expectations
Development of the β calorimeter is continuing on
two fronts. Single-detector tests are being carried out
to determine the performance characteristics of
individual detectors, while the system as a whole is
being modeled using the detector simulation code
Geant4 [5].
Signals from the SSSD’s will be processed using
MCS-16 preamplifier modules identical to those used
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Using α particles from a 228Th source, the energy
resolution of individual DSSD strips has been
measured to be better than 80 keV at 8.78 MeV.
Comparable resolutions are expected for the SSSD’s
and PIN detectors. The intrinsic efficiency of the
DSSD has been measured, using β particles from a
90
Sr source, to be 50±10%; this value is thresholddependent, and is expected to improve as sources of
electronic noise are identified and eliminated. Similar
measurements will be carried out for the calorimeter
detectors. The results of these tests will be used both
to ensure that the detectors meet the manufacturer’s
specifications and to provide a basis for comparison
with Geant4 simulations.
β decays. Using this technique, particle identification
data from an implanted fragment can be applied to its
correlated decay, providing a unique identification of
the parent nucleus. A comparison of time stamps from
correlated implant and decay events provides a direct
measurement of the decay time.
A β calorimeter is being developed to augment the
β counting system. This multi-detector device will
first be used to determine Gamow-Teller strengths in
the decay of 100Sn. This requires a determination of
the β-decay half-life to better than 7% and of the
endpoint energy (~3.8MeV) to better than 2.5%. The
former requirement is within the capabilities of the
existing NSCL β counting system. Component testing
and Geant4 simulations of the calorimeter are
currently being carried out.
Detector simulations are necessary to predict their
performance in the 100Sn decay experiment for which
the calorimeter is being developed. In this experiment,
energy losses will be observed in several successive
detectors; total β-decay energies will be obtained by
summing these energy losses. The endpoint energy
will be determined from a fit to the resulting β-decay
spectrum. The complete Geant4 model currently
includes the detectors shown in Figure 3. The eight
detectors of the calorimeter are equally-spaced, with 1
mm separations, and are located 5 mm downstream of
the DSSD in a cylindrical aluminum chamber. A 4 cm
× 4 cm source of positrons with an endpoint energy of
4.5 MeV has been located at the midpoint of the DSSD
to simulate the decay of implanted 100Sn fragments,
taking into account the error in the measured value [4].
A determination of the response function for the
detector array is underway; this function will first be
used to investigate the relationship between individual
detector resolutions and the precision to which
endpoint energies can be measured. Once this is
completed, the total efficiency of the system will be
calculated.
ACKNOWLEDGMENTS
The author wishes to thank P.F. Mantica, J.I.
Prisciandaro, S.N. Liddick, B.E. Tomlin, and A. Stolz
for their contributions to the development of the β
counting system and calorimeter, and the technical
staff of the NSCL for their assistance. This work has
been supported in part by National Science Foundation
grant PHY-01-10253.
REFERENCES
1. Prisciandaro, J. I., Morton, A. C., and Mantica, P. F.,
Nucl. Instrum. Meth. in Phys. Res. A, in press.
2. Brown, B. A., and Rykaczewski, K., Phys. Rev. C 50,
R2270-2273 (1994).
A simplified model of the β-detection system has
been used to determine the range of β particles in the
set-up. This model uses a point-like source of
positrons with an endpoint energy of 3.8 MeV. The
available thickness of Si was sufficient to stop 98% of
such particles.
3. Bobyk, A., Kaminski, W., and Borzov, I.N., Acta Phys.
Pol. B 31, 953-963 (2000).
4. Stolz, A., Faestermann, T., Schneider, R., et al., “Decay
studies of N≈Z nuclei from 78Y to 102Sn” in International
Nuclear Physics Conference 2001, edited by E. Norman
et al., AIP Conference Proceedings 610, New York:
American Institute of Physics, 2002, pp. 728-732.
5. The Geant4 Collaboration, http://wwwinfo.cern.ch/asd
/geant4/geant4.html.
SUMMARY
Recent developments in particle detection with fast
fragment beams have helped further the study of
nuclear β decay. At the NSCL, a new β counting
system has been developed, allowing the direct
correlation of fragment implants with their subsequent
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