Research Assistant
1989 to 1994
Exclusive Muon Capture
The weak interaction at low energies can be
pictured as an interaction between two weak currents. Consider the weak interaction between a lepton,
such as a muon, and a nucleon. The
weak current of the lepton is characterized by vector current and axial
vector current components, with coupling gv and ga,
respectively. A nucleon, unlike a
lepton, is a strongly interacting particle.
The presence of the strong interaction induces additional structure
into the weak current, including an induced magnetic component into the
vector current and an induced pseudoscalar component into the axial vector
current, with couplings gm and gp, respectively. Because the weak axial vector current is not
conserved (PCAC hypothesis), the couplings ga and gp
may change inside the nucleus with respect to the free nucleon values. Nuclear muon capture offers an ideal means
for exploring the changes in ga and gp inside the
nucleus. A negative muon incident on
matter loses energy through collisions with atomic electrons and subsequently
is captured into an outer atomic orbit.
It then quickly (~ 10-13 s) cascades down to the 1s atomic
ground state, emitting muonic x-rays as it descends. For non-zero spin nuclei, the 1s state of
the muonic atom is split into two hyperfine states. If the muon is in the upper hyperfine
state, it may subsequently fall to the lower hyperfine state through a
hyperfine transition, a process similar to internal conversion. From either of the two hyperfine states,
the muon may either decay or capture.
The capture rates from the upper and lower hyperfine states to specific
daughter states can be strong functions of ga and gp,
and thus provide a means for measuring ga and gp inside
the nucleus. In addition, some muon
capture transitions to specific excited daughter states result in the
emission of gamma rays that are doppler broadened and whose energy
distribution is sensitive to ga and gp. These transitions also provide another
means for measuring ga and gp inside the nucleus. Research work required bombarding desired
targets with low momentum muons and collecting data on the subsequently
observed nuclear processes, determining the
g-ray yields following observed muon capture transitions,
determining the muon capture rates, determining the energy distribution for
observed doppler broadened
g-rays
following muon capture transitions, and then extracting values for ga
and gp inside the nucleus. Research Work
Developed the DISPLAY workstation-based data acquisition and analysis system which allowed scientist to control nuclear physics experiments, acquire and process experimental data, and perform various statistical and numerical analyses. The data acquisition system was composed of a network of mini-computers, CAMAC communications interface, particle tracking system, gamma‑ray detection system, and applications software. The CAMAC communications interface was used to transfer CAMAC protocol control commands and real time experimental data between the computer and the electronics of the particle tracking system and the gamma‑ray detection system. The particle tracking system followed the trajectory of each charged particle through drifts, dipole (bending) magnetic fields, and quadrupole (focusing) magnetic fields—generating a transport matrix—and then determined each particle’s initial momentum and position by inversion of the transport matrix. The gamma‑ray detection system detected gamma‑rays following nuclear reactions and then calculated each gamma-ray’s energy, time, intensity, etc. Developed the applications software to allow scientists to control the experimental setup, acquire particle tracking information, acquire gamma‑ray data, process the data into meaningful spectra, display the spectra on several graphic interfaces, and perform various statistical and numerical analyses.
Performed nuclear
muon capture experiments at the Tri-Universities Meson Facility (TRIUMF).
Studied theoretical and experimental publications on nuclear muon capture.
Developed monte carlo computer simulations of the muon capture process and the
predicted gamma ray emission spectra. Performed two large scale nuclear muon
capture experiments at TRIUMF to examine the nuclear structure of five light
nuclei - 19F, 23Na, 27Al, 31P, and
35Cl. Utilized a novel experimental technique to measure the
hyperfine capture rates following muon capture to each of the five nuclei, in
order to test the Partially Conserved Axial Current (PCAC) hypothesis.
Acquired and analyzed muon capture time and energy data, performed advanced
gamma ray spectroscopic analyses of the observed nuclear reactions, and
utilized various statistical and numerical analysis techniques to extract the
hyperfine capture rates. These capture rates were compared with the 1s 0d
shell model predictions showing good agreement with the PCAC hypothesis. The
results of these experiments have been published – see references below. B.L. Johnson, T.P. Gorringe, D.S. Armstrong, J. Bauer, M.D. Hasinoff, M.A. Kovash, D.F. Measday, B.A. Moftah, R. Porter and D.H. Wright, Observables in muon capture on 23Na and the effective weak couplings ga and gp, Physics Review C 54, 2714-2731 (1996)
T.P. Gorringe, B.L. Johnson, D.S. Armstrong, J. Bauer, M.A. Kovash, M.D. Hasinoff, D.F. Measday, B.A. Moftah, R. Porter and D.H. Wright, The Hyperfine Effect in m- Capture on 23Na and gp/ga, Physics Review Letters 72, 3472-3475 (1994)
T.P. Gorringe, B.L. Johnson, J. Bauer, M.A. Kovash, R. Porter, P. Gumplinger, M.D. Hasinoff, D.F. Measday, B.A. Moftah, D.S. Armstrong and D.H. Wright, Measurement of Hyperfine Transition Rates in Muonic 19F, 23Na, 31P, and natCl, Physics Letters B 309, 241-245 (1993)
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johnsonb@kwc.edu
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