Decay Detector
Development for Giant
Resonance Studies
By: Gus Olson
Mentor: Dr. D.H.Youngblood
Giant resonance has been thoroughly studied in
stable nuclei over a wide range of A (12-208).
Future research will focus on giant resonance in
unstable nuclei. Several changes must be
made to the current experimental set-up,
however, to make measurements with unstable
nuclei possible. As part of this we need to
construct a decay detector to be placed in the
target chamber to measure light decay products
as the Giant Resonance excited states decay.
Giant Resonances
__________electric____________
isoscalar
isovector
spectrum into equally sized groups.
An angular distribution is obtained for each
energy range. Each angular distribution is fit by
weighting theoretical angular distribution
(DWBA calculation) for each of the resonance
modes to match the experimental angular
distribution for each energy range. The weights
on each resonance mode for each energy give
the strength distribution of each resonance.
Giant monopole energy is then extracted from
the strength distribution of the giant monopole
resonance.
Data shown at right is for inelastic scattering of
α particles off of 28Si.
____________magnetic_________
isoscalar
isovector
Si*  Al  p
28
24
Si*  Mg  
28
We cannot use the radioactive nuclei as
targets because they will decay in the target
chamber. Some α’s would scatter off of the
original nuclei and some off of the decay
products contaminating the data. We will need
to use inverse reactions: instead of
accelerating α particles into heavier targets we
will need to accelerate the heavier nucleus (the
one in which Giant Resonance will be excited)
into a lighter target. Using He gas as a target is
troublesome because of its low density and
difficulty in containing it in the scattering
chamber so it is likely that a solid 6Li target will
be used instead.
Normal Reaction:
The giant resonances are collective nuclear
excitations. This means that all of the
nucleons are oscillating at the same time.
Giant resonances occur in several modes:
Monopole,
Dipole,
Quadrapole
etc.
Resonances where protons and neutrons
oscillate in phase (isoscalar) and resonances
where they are out of phase (isovector) exist
for each resonance mode. There exist both
electric and magnetic giant resonances, but
we are concerned only with the electric giant
resonances.
28
28
28Si(α,α’)
Excitation Energy Spectrum: Large peak is the
Giant resonances, low energy peaks are
single nucleon excitations. Continuum shown
as thick line.
Si ( ,  ')
6
6
Si ( Li, Li ' )
The decay detector will be constructed out of
plastic scintillators. Two layers of 1mm thick
scintillator strips will be oriented vertically and
horizontally to measure the scattering angle.
This allows us to determine the particle’s
energy. 3” scintillator blocks behind these
layers will measure the energy loss of the
particles. This allows us to distinguish between
protons and α particles (based on the
scintillator’s light output at a known energy).
Each of the scintillators will be coupled via
optical fibers to a Photomultiplier tube outside
the scattering chamber where the scintillation
light is converted into an electrical signal.
 ( Si, Si*)
6
28
28
Strength Distributions
for several modes
Peak and continuum data fit with
theoretical angular distributions
The two main decay modes of 28Si*
Inverse Reaction:
28
27
28
Li( Si, Si*)
Light Output
In order to make particle determinations (deciding
whether an event was a proton or an α) with the decay
detector we need to know what light output to expect
from the 3” scintillators at different particle energies.
Using a computer program called SRIM we obtain
estimates for the stopping power (dE/dx), and the
range (x) of each type of particle dependant on the
particles energy. Using the relation
dL
dE
[1]
2
 L og(1  a )
dx
a  25(mg / cm ) / MeV
dx
and integrating we can get the light output as a
function of the range or the energy.
Light Output for Protons and Alphas
Macroscopic diagrams of the giant resonances
Measuring Giant Resonances
L
After excited into the energy range of the Giant
resonances, 28Si* will decay rapidly by emitting
a lighter particle: either a proton or an α. We
need to be able to keep track of all fragments
of this decay in order to determine the
excitation energy of the original nucleus. We
can keep track of the larger fragments with the
MDM spectrometer but we need to construct
an additional detector to be placed in the target
chamber to detect the lighter particles.
A drawing of the decay detector
Proton
L =0.0008E1.2972
50
100
150
200
250
300
Internal reflection
Scintillators
Incoming charged particles excite the
Scint.
molecules of the scintillator. The excited
↑
molecules decay by visible light emission External reflection
Al
(peak at ~420nm for our scintillator). The Internal and external
energy deposited in the scintillator, and hence reflection in scintillator
the light output, depends on the kinetic energy
of the particle, its charge, and the thickness of
the scintillator. Some light is totally internally
reflected in the scintillator, but we use Al foil as
an external reflector as well.
Results
Schematic drawing of the MDM spectrometer
Alpha
[1] T.J. Gooding and H.G. Pugh, Nuclear
Instruments And Methods 7, 189-192
Test set-up: scintillator is on the left wrapped in Al foil. Fiber
bundle is in the middle. PMT is on the right.
Most of the tests we conducted were to
determine the best configuration to transmit
light from the scintillator to the PMT. We tested
the fiber length to make sure we were not
getting too much attenuation in the fiber; we
tested wrapping the fibers in aluminum foil to
get more light that would not otherwise have
been transmitted via total internal reflection,
and we tested the coupling between the optical
fiber and the scintilllator.
Lp =0.0033E1.1415
E(MeV)
Scintillator Testing
Plastic Scintillators are ideal for our needs
because they have a very fast response (~2ns
decay time) and can be easily machined into
the variety of shapes that we need. We use an
optical fiber bundle to connect the scintillator
(BC408) to a photomultiplier tube (PMT) which
converts the scintillation light into a current
pulse.
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
There was no great benefit in wrapping the
fibers with aluminum foil. Using a small (3cm)
scintillator segment there was no significant
attenuation with different lengths of optical
fibers. However for the long (7.3”) scintillator
there was significant position dependence:
with the source placed at the end of the
scintillator near the fibers the output was about
-150mV, but at the end far from the fibers the
output dropped significantly to only -40mV.
20
0
-20
voltage(mV)
The experimental procedure for 28Si excited
into giant resonance by α scattering is looked
at here as a typical example, but we are
interested in this reaction, or rather its inverse,
to test the decay detector. A beam of 240 MeV
α particles, accelerated by the K500 cyclotron,
is directed into the target chamber. The beam
is incident an a target foil of 28Si atoms. The
beam particles are inelastically scattered off of
the target and enter the dipole magnet of the
MDM spectrometer. The dipole magnet
separates the particles based on their
momentum and charge. The scattered
particles then pass through the focal plane
detector. In 0° measurements the primary
beam is passed to the side of the detector into
a Faraday cup. At larger angles the primary
beam is stopped by a second Faraday cup in,
or just after, the target chamber. The focal
plane detector measures the position of the
particles using resistive wires at four points in
a gas ionization chamber. The ionized particles
hitting the wires generate current pulses which
are measured at both ends of the wire allowing
the position to be determined. Using a raytracing program with the position from the four
wires we can determine the initial scattering
angle and energy of each scattered particle.
We use the measured α-scattering energy
spectrum to extract information about giant
resonances. First we separate the giant
resonance peak from the “continuum” which
consists of other nuclear reactions: the knockout reactions where the α simply knocks a
nucleon out of the target nucleus and the pickup→break-up reactions where the α particle
absorbs a nucleon from the target nucleus and
subsequently decays. These reactions cannot
be separated from the giant resonance
reactions during the experiment because the
resulting particle is still an α particle and is at
similar energy. The spectrum is then separated
into energy “bins” by dividing the energy
E=240 MeV
Decay Detector
Motivation
The Nuclear Giant Resonances are important
in understanding the structure of the nucleus.
The incompressibility of nuclear matter (Knm)
can be determined by measuring the energy of
the Giant Monopole Resonance. Knm is directly
related to the curvature of the equation of state
of nuclear matter. This makes it a good test for
theoretical effective interactions between the
nucleons. It is also of interest in nuclear
astrophysics in studying super-nova collapse
and neutron stars.
Data Analysis
-40
-60
-80
-100
-120
-140
-20
-10
0
10
20
30
tim e(ns)
Sample phototube output: 7.3” scintillator, 18” fibers using a β-source
Optical fibers
Optical fibers operate on the principle of total
internal reflection. The fiber core is surrounded
by a thin “cladding”. The core index (nf) is
greater than the cladding index (nc) so light
incident greater than the critical angle
(θc=sin-1(nc/nf)) is all reflected internally.
Fiber-bundle ends
Photomultiplier tubes
Photomultiplier tubes are designed to convert
light into electrical signals. Light incident on the
photocathode causes it to emit electrons via
the photoelectric effect. These electrons are
accelerated and multiplied by the dynode
configuration, and they are converted to a
current pulse at the anode.
A diagram of a PMT