Lecture 9 - Decay Rates and Interactions - Experimental Nuclear Physics PHYS 741 Text heeger@wisc.edu References and Figures from: - Basdevant et al., Fundamentals in Nuclear Physics - Henley et al., Subatomic Physics 1
Lecture Schedule room 5280 2
Measurement of Double Beta Decay Measurement of Double Beta Decay Lifetime 100 Mo -> 100 Ru 2e - 2νe scintillators 40μm thick foil scintillators 1433 observed events in 6140 hrs T( 100 Mo)=0.95 x 10 19 yr enriched 100 Mo (98.4%) natural abundance 9.6% 3
Isotopic Enrichment A process by which the relative abundance of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one particular isotope and depleted in its other isotopic forms. Gas Diffusion 4
Isotopic Enrichment Gas Centrifuge - heavier molecules will segregate to the bottom of the centrifuge while the lighter molecules will segregate to the top of the centrifuge - output lines take these separations to other centrifuges to continue to the centrifugation process 5
Isotopic Enrichment Electromagnetic Separation (Calutron) Laser Enrichment laser is tuned to a wavelength which excites only one isotope of the material and ionizes those atoms preferentially 6
Lifetime of First Excited State of 170 Yb Measurement of Excited State Lifetime production of 170 Tm 169 Tm(n,γ) 170 Tm 170 Tm -> 170 Yb e - νe electrons are momentum selected and focused on scintillator counts in both scintillators = beta electron + conversion electron double armed magnetic spectrometer 7
Doppler Shift Attenuation Method Measurement of Radiative Decay Lifetime of Excited States of 74-Br thick target 19 F 58 Ni beam produced nuclei stop in target state may decay before stopping ( in-flight ) or at rest in-flight decays are Doppler shifted (positive or negative) 8
Euroball Array 9
Gamma Sphere 10
Determining the Lifetime from the Doppler Method Example: consider the following photon spectra 1. From maximum Doppler shift of forward going photons estimate velocity and energy of decaying 74-Br nuclei. 2. Estimate energy loss rate of Br ions in Ni and time for nuclei to come to rest 3. Estimate lifetime of decaying state. 11
Moessbauer Effect Recoilless Nuclear Resonance Absorption of Gamma Radiation atoms move due to random thermal motions - when the atoms are within a solid matrix the effective mass of the nucleus is very much greater - recoiling mass is now effectively the mass of the whole system, making ER and ED very small. Simple Mössbauer spectrum from identical source and absorbe 12
Moessbauer Effect 13
Width of State Measurement of Width of Excited State Through Moessbauer Spectroscopy beta-decay scanning in energy => scanning in velocity with ΔEγ/Eγ=v/c de-excitation inverse process resonant absorption is possible only with v=0 if nucleus is in lattice => photon then takes all the energy Moessbauer effect not present for photons with E > 200 kev because nuclear recoil is sufficient, can excite phonon modes in crystal 14
Radiative Decays: Lifetime of Excited States Various Electric and Magnetic Multiples 15
Selection Rules for Radiative Transitions type symbol ang. momentum change ΔJ <=1 parity change el dipole E1 1 yes mag dipole M1 1 no el quadrupole E2 2 no mag quadrupole M2 2 yes el octopole E3 3 yes mag octopole M3 3 no el 16-pole E4 4 no mag 16-pole M4 5 yes 16
Internal Conversion ejection of electron looks like 2-step process classically -> single quantum process, amplitude can be calculated with pertubation theory 17
Beta-Spectrum and Internal Conversion Lines Internal Conversion Process: - information from the internal conversion electrons about the binding energies of the electrons in the daughter atom - relative intensities of these internal conversion electron peaks can give information about the electric multipole character of the nucleus. 18
Beta-Spectrum and Internal Conversion - 203 Hg, which decays to 203 Tl by beta emission, leaving the 203 Tl in an electromagnetically excited state. - can proceed to the ground state by emitting a 279.190 kev gamma ray, or by internal conversion. In this case the internal conversion is more probable. - internal conversion process can interact with any of the orbital electrons, the result is a spectrum of internal conversion electrons which will be seen as superimposed upon the electron energy spectrum of the beta emission. - Energy yield of this electromagnetic transition: 279.190 kev => ejected electrons will have that energy minus their binding energy in the 203 Tl daughter atom. Electron emissions from the Hg-203 to Tl-203 decay, measured by A. H. Wapstra, et al., Physica 20, 169 (1954) 19