Decay Detector Development for Giant Resonance Studies By: Gus Olson Mentor: Dr. Youngblood

Motivation The energy of the Isoscalar Giant Monopole Resonance (EGMR) can be used to deduce Knm, the incompressibility of nuclear matter. Knm is an important parameter in several fields. Directly related to the curvature of the equation of state of nuclear matter. Helps in understanding nuclear structure and heavy ion collisions Important value in nuclear astrophysics: supernova collapse and neutron stars. Provides a test for theoretical nuclear models, and nucleon-nucleon effective interactions. The giant resonance has been thoroughly studied in stable nuclei over a wide range of A (12C-208Pb). Future research directed towards the study of giant resonances in unstable nuclei.

Giant Resonances Collective nuclear excitations Several oscillation modes: Monopole, Dipole, Quadrapole etc. Isoscaler and Isovector resonances, as well as electric and magnetic resonances exist for each resonance mode __________electric____________ ____________magnetic_________ isoscalar isovector isoscalar isovector Macroscopic diagrams of the giant resonances

Measuring Giant Resonances Procedure for 28Si(α, α’): MDM Spectrometer Beam of 240MeV α’s from the K500 cyclotron is inelastically scattered by target nuclei Momentum of scattered particles is analyzed by Dipole magnet Focal plane detector Gas (isobutane) is ionized by incoming particles High voltage causes liberated electrons to drift upwards 4 resistive wires measure position Plate at top of detector measures ΔE for particle identification Plastic Scintillator measures total energy and gives a fast signal to trigger the electronics to acquire data. Scattering angle and energy for each particle are obtained by using position signals from each wire. To clearly identify the monopole resonance small angle (including 0°) measurements are necessary Focal Plane Detector Dipole Magnet Target Chamber

The break-up processes Data Analysis E=240 MeV 28Si(α,α’) Giant Resonances exist at about 10-40 MeV excitation energy Lower energy peaks are single particle excitations Large peak consists of all Giant Resonance collective excitations Energy spectrum is separated into peak and continuum contributions. Continuum due primarily to knock-out and pick-up→break-up reactions. The break-up processes

Data Analysis (cont.) 28Si Spectrum is separated into energy “bins” (equal width energy intervals) Angular distribution for each energy bin Each energy bin is fit by a weighted sum of the theoretical cross-sections for each of the resonance modes (from DWBA calculations) . The weights give the strength distribution of each resonance mode. Using the strength functions of the resonance modes we can obtain the energy of the resonance 28Si

Giant Resonance in Radioactive Nuclei Problem: Can’t use a radioactive target: decay products contaminate the target Use the inverse reaction, with a radioactive beam. Low density of gaseous helium target means fewer interactions. Also, it is difficult to contain the gas in the target chamber. Beam intensity for a radioactive beam will be much lower so having a solid target is essential. Using solid 6Li target allows us to avoid difficulties involved with a gas target. We will use 28Si (which is, of course, not radioactive) as a test case to be sure the new detector gives us results consistent with previous methods. Inverse Reaction: Normal Reaction:

Giant Resonance in Radioactive Nuclei Problem: The GR excited state has a very short lifetime Excitation energy of 28Si* can only be determined if the scattering angle and energy of both fragments are known. Large fragments can be detected in the Focal plane detector as before. Small fragments require a new detector placed in the target chamber. Two main decay channels

Decay Detector Two 1mm thick layers of scintillating plastic strips oriented vertically and horizontally measure the scattering angle of α’s and p’s. 3’’ thick scintillator blocks measure the total energy of the particles. Together these scintillators allow us to make particle determinations Scintillators will be connected to photomultiplier tubes (located outside the target chamber) via optical fibers Will be able to measure particles at ±35° vertically and horizontally. (each strip measures 5°)

Plastic Scintillators Incoming charged particles lose energy in the scintillator by exciting the molecules of the scintillator. Excited molecules decay by photon emission (peak output at ~420 nm for our scintillators (BC408)). Energy loss in the scintillator, and hence the light output, depends on the kinetic energy of the particle, its charge, and the thickness of the scintillator. Plastic scintillators are ideal for our needs Very fast response (~2ns decay time) Can be easily machined into the shapes we need for our detector

Where x=range and t=thickness of scintillator. Light Output Calculating relative light output Energy loss per unit length (dE/dx, the stopping power) and range (x) estimates are obtained using a computer program (SRIM). Light output is related to energy loss by dL/dx is integrated to obtain L(x). total light output of a particle which stops completely in the scintillator at a range x. This can be used for particle determinations with the 3” scintillators. Light output for particles not totally stopped (as in the case of the thin scintillator strips) is obtained using the relation [1] Where x=range and t=thickness of scintillator. [1] T.J. Gooding and H.G. Pugh, Nuclear Instruments And Methods 7, 189-192

Optical Fibers Operate on the principle of total internal reflection Cladding nc=1.49 Core nf=1.6 Operate on the principle of total internal reflection Most of fiber is core, surrounded by a thin “cladding” with a lower index of refraction. At incident angles greater than the critical angle (θc=sin-1(nc/nf)) all light is reflected internally. Plastic optical fibers are flexible and can transmit light even when bent. We used fibers 1mm in diameter arranged in bundles to connect the scintillator to the PMT. θ

Photomultiplier Tube Scintillation photons incident on photocathode. Photocathode emits electrons via the photoelectric effect High voltage accelerates electron towards dynodes On impacting each dynode secondary electrons are emitted Avalanche of electrons is converted to an electrical pulse at the anode

Test Case Plastic scintillator Fiber-bundle ends Photomultiplier tube One scintillator strip connected via optical fibers to a photomultiplier tube with a beta source (90Sr) to test light output.

Testing Scint. Al Internal reflection External reflection ↑ We must collect as much of the light as we can to PMT to get reliable particle detection. Scintillation light is emitted in all directions some travels directly to the fibers but most must be reflected at the surface of the Scintillator Total internal reflection External reflection by aluminum foil Must have good optical coupling between each of the components Surfaces need to be very flat and very clear Optical cement, and optical grease are used to make connections Light Tight We must make sure that we can reliably seal off each component from any outside light leaking in or we will get false detects. Prevents cross-Talk between different scintillator strips. Sample PMT output: 7.3” long scintillator, 18” long fibers using a β-source (90Sr).

Testing (cont.) We were concerned that we might not get enough light reflected in the fibers due to the acceptance angle so we tested wrapping the fibers in Al: We tested using 2” long fibers that had been wrapped in Al foil but this showed no change in output amplitude. Light attenuation in optical fibers: Tested with fiber lengths of 2”, 12”, and 18” with no appreciable amplitude difference. Light attenuation and reflection losses in scintillator: Output shows great dependence on the position of the test-source: ~150-200mV with source close to the coupling with the fibers compared with ~40-60mV at the far end of the scintillator. The manufacturer’s rating indicates that light attenuation should not be a great problem at such short lengths (1/e of the original amplitude at 210cm), thus it seems that we are losing too much of the light on the multiple reflections down the scintillator.

Acknowledgments Department of Energy, National Science Foundation, Texas A&M University, Cyclotron Institute. DHY group: Dr. Dave H. Youngblood, Dr. Y.-W. Lui, Dr. Yoshiaki Tokimoto, Xinfeng Chen.