Decay Detector Development for Giant Resonance Studies

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

2. 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.

3. 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

4. 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

5. 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

6. Data Analysis (cont.) • 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 28Si

7. Normal Reaction: Inverse Reaction: 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.

8. 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

9. 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°)

10. 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

11. 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

12. Cladding nc=1.49 Core nf=1.6 Optical Fibers • 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

13. 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

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

15. Internal reflection ↑ Scint. ↑ ↑ External reflection Al Testing • 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).

16. 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.

17. 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.