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Decay Detector Development for Giant Resonance Studies By: Gus OlsonMentor: Dr. D.H.Youngblood

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.

Motivation

Measuring Giant Resonances

Schematic drawing of the MDM spectrometer

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 ray-tracing program with the position from the four wires we can determine the initial scattering angle and energy of each scattered particle.

E=240 MeV 28Si(α,α’)

Excitation Energy Spectrum: Large peak is the Giant resonances, low energy peaks are single nucleon excitations. Continuum shown as thick line.

Peak and continuum data fit with theoretical angular distributions

Strength Distributions for several modes

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 [1]

and integrating we can get the light output as a function of the range or the energy.

og(1 )dL dE

L adx dx

225( / ) /a mg cm MeV

Light Output

Light Output for Protons and Alphas

0

0.20.4

0.6

0.81

1.2

1.4

1.61.8

2

0 50 100 150 200 250 300

E(MeV)

L

Alpha

Proton

1.1415pL =0.0033E

1.2972L =0.0008E

Incoming charged particles excite the molecules of the scintillator. The excited molecules decay by visible light emission (peak at ~420nm for our scintillator). The energy deposited in the scintillator, and hence 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.

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.

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.

Scintillators

Optical fibers

Photomultiplier tubes

isoscalar isoscalarisovector isovector____________magnetic___________________electric____________

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.

Macroscopic diagrams of the giant resonances

Giant Resonances

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.

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.

Data AnalysisWe 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 knock-out reactions where the α simply knocks a nucleon out of the target nucleus and the pick-up→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

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.

Decay Detector

28 28( , *)Si Si

28 27*Si Al p 28 24*Si Mg

28 ( , ')Si Inverse Reaction:Normal Reaction:

The two main decay modes of 28Si* 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.

A drawing of the decay detector

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.

-140

-120

-100

-80

-60

-40

-20

0

20

-20 -10 0 10 20 30

time(ns)

vo

ltag

e(m

V)

[1] T.J. Gooding and H.G. Pugh, Nuclear Instruments And Methods 7, 189-192

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.

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.

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.

Test set-up: scintillator is on the left wrapped in Al foil. Fiber bundle is in the middle. PMT is on the right.

Results

Fiber-bundle ends

)',( 6628 LiLiSi *),( 28286 SiSiLi

Internal and external reflection in scintillator

Sample phototube output: 7.3” scintillator, 18” fibers using a β-source

A diagram of a PMT