Giant Monopole Resonance Kadmiel Beauvais, Texas A&M Advisors: Dr. Youngblood, Dr. Lui, Jonathan Button Funded by NSF grant number: PHY-1263281

Giant Resonances Broad resonances in the excitation energy range, 10-30MeV Giant Monopole Resonance (GMR) can be related to incompressibility KA and then to Knm, the incompressibility of nuclear matter Generally, Giant Resonances are broad resonances in the excitation energy range of 10 to 30 MeV. These collective excitations of the nucleus can be characterized by the quantum numbers L, spin S, and isospin T; where L describes the shape, S = 0 is the electric oscillations, S=1 is the magnetic oscillations, and T=0 is isoscalar and T=1 is isovector. Of primary concern is the so-called compression mode, the giant monopole resonance, because of the relationship between its centroid energy and the incompressibility K_A. Measurements of K_A in many nuclei make it possible to extrapolate to the incompressibility of nuclear matter, K_nm, an important part of the nuclear matter equation of state used to study the physics of supernova and neutron stars, for example. It has been shown that improvements to K_nm can be made by measuring the GMR in unstable nuclei. Giant Resonances are described by quantum numbers L (angular momentum), S (spin), and T (isospin)

Measuring GMR Procedure for 28Si(α, α’)28Si* 240 MeV α MDM Spectrometer 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

Data Analysis Giant Resonance ~10-40 MeV Large peak is the sum of all giant resonance contributions, smaller straight line is continuum Must be separated into peak and continuum contributions P.R.C 69, 054312 D.H. Youngblood et al

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 P.R.C 69, 054312 D.H. Youngblood et al

Continuum Extraction Shape of continuum not well understood Need a way to keep continuum choice consistent I improved a program to extract continuum shape given a spectrum and assumptions of strength functions

How it works Assume strength distribution for L=0-4 Use DWBA calculations for cross sections as a function of angle Convert angular distributions to counts Subtract these counts from the spectrum and obtain the shape of the continuum

Sample continuum

GMR in unstable nuclei Measure GMR in unstable nuclei using inverse reaction (40MeV/u beam on 6Li target) Nuclei excited to GR are particle unstable and will decay by p, α, or n emission shortly after excitation In sd-shell nuclei, direct decay to p or α accounts for 40-80% of total strength Array with 3 layers of plastic scintillator to measure ΔE-ΔE-E and angle of decay particle built and tested Plan to first measure GMR in 28Si using this technique (Ex = 21.5±0.3 MeV; RMS width 5.9 MeV) Our strategy to measure the gmr in unstable nuclei is to use the inverse reaction. (alpha,alpha’) has been the primary method for investigating GMR because it excites isoscalar state preferentially over isovector. Because of the difficulty inherent in using a gas target, we would like to use a target of 6Li. In his phd thesis, xinfeng chen showed that 6Li is a suitable probe of giant monopole resonance.

Giant Resonance in unstable nuclei Problem: Can’t use a unstable target: decays before you can use it. Use the inverse reaction, with a unstable beam. Gaseous helium target impractical Beam intensity for a unstable 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 stable 28Si as a test case to be sure the new detector gives us results consistent with previous methods. Inverse Reaction: Normal Reaction:

Giant Resonance in unstable nuclei Problem: The GR excited state decays by particle emission 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

Must measure at least 3 of the 4 following quantities : Decay particle energy Residual nucleus energy Decay particle angle Residual nucleus angle Experimental Layout: The decay detector in the target chamber has an angular range of 4° to 45°. The gas detector in the focal plane of the MDM spectrometer has an angular range of 0° to 2° M.S. Thesis Jonathon Button (2013) .

Decay Detector Description 14 vertical and 12 horizontal scintillator strips give the ΔE1 and ΔE2 signals & angles Signals taken in coincidence from the two strip layers give angle information (overlapping strips form 1 cm2 pixels) Transport light response from strips to PMTs 5 scintillator blocks give the E signals Angular range of 5° to 45° Recently ran calibration testing of the decay detector using 15, 30 and 45 MeV protons First GMR run next week M.S thesis Jonathon Button (2013)