Detecting Giant Monopole Resonances

1. Energy Loss Giant Resonances Detecting Giant Monopole Resonances • Using SRIM, a program that computes the energy associated with scintillator thickness, the energy loss after striking the scintillator is calculated and subtracted from the initial energy • Discovered in the late 1940s by bombarding nuclei with gamma rays • Giant resonances is a collective motion of nucleons that occurs when the nucleus becomes excited • Each mode has an associated multipole integer value L to represent the angular momentum transfer • Classification • Isoscalar means the protons and neutrons move in phase and is denoted as ∆T = 0 • Isovector means the protons and neutrons do not move out of phase and is denoted by ∆T = 1 Peter Nguyen Advisors: Dr. Youngblood, Dr. Lui Texas A&M University Stable Nuclei Isoscalar Giant Monopole Resonances (ISGMR) • Excessive studies have been made on the stable nuclei by using alpha particles scattering • Through inelastic scattering, information of ISGMR and ISGDR have been obtain from the stable nuclei (12C - 208Pb) • Researcher are focusing more on unstable nuclei • ISGMR is the “breathing” mode where the nucleons compress and expand causing the nucleus’ radius to fluctuate • ISGMR can be related to the nucleus, denoted as Knm Unstable Nuclei • Unstable nuclei cannot be placed in the target chamber because of its decaying nature. The nuclei will immediately decay into another element • To study the unstable nuclei, an inverse reaction is needed, the unstable nuclei becomes the projectile • Detector on the back of spectrometer combined with decay detector inside target chamber to measure the resonance of unstable nucleus • Reaction - 28Si(6Li, 6Li) 28Si* • Inverse Reaction - 6Li(28Si, 28Si*) 6Li Motivation Behind Knm • It is a fundamental quantity describing the ground state properties of nuclear matter • Uses • Supernova collapses • Neutron stars • Heavy-ion collisions • Determine the Nuclear Equation of State • Measuring it • Deduce information from the frequency of the compression mode of the nucleus during ISGMR and ISGDR • Relate the compressibility to the centroid energy of the ISGMR Decay Detector in Target Chamber • The detector is compose of a thick scintillator block, and vertical and horizontal thin strips that are 1 mm thick • The particles will go through the vertical strip first and then the horizontal strip. This will determine the position of the outgoing particles • The scintillator block measures the energy of the particles Light Output Detection of ISGMR • Difficult to detect because Giant Quadrupole Resonance GQR hid the GMR except at small scattering angles • Beam analysis system provides a very clean beam which can be used in the measurement • Using a beam of specific MeV, the beam will collide target nucleus Scintillator • A scintillator is a device that absorbs energy and emits light • Several kinds of scintillating material exists including: organic, inorganic and plastic • The particle hits the scintillator which excites the molecules in the scintillating material to emit light • The photons released is then capture by a photomultiplier that is coupled to the scintillator via a light guide or directly attached • Sensitive to Energy • Represented as a linear function • Fast Time Response • Recovery time is short • Pulse Shape Discrimination • Determining different particles MDM Spectrometer Identifying The Particles Photomultiplier • The target nuclei in the target will excite to a higher energy level • α particles with different energy will separate by MDM spectrometer and focus on different position of the detector • The photomultiplier absorbs the emitted light and electrons are release via photoelectric effect at the photocathode • The cathode, dynodes, and the anodes create a potential “ladder” that directs the electrons • The electrons travel from the photocathode to the first dynode and excite more electrons in the dynode • The excited electrons leave the dynode and travel to the next dynode to repeat the process • At the anode all the electrons are collected and then amplify to create a readable current