High resolution gamma-ray spectroscopy and the fascinating angular momentum realm of the atomic nucleus

Abstract

In 1974 Aage Bohr and Ben Mottelson predicted the different 'phases' that may be expected in deformed nuclei as a function of increasing angular momentum and excitation energy all the way up to the fission limit. While admitting their picture was highly conjectural they confidently stated '...with the ingenious experimental approaches that are being developed, we may look forward with excitement to the detailed spectroscopic studies that will illuminate the behaviour of the spinning quantised nucleus'. High resolution gamma-ray spectroscopy has indeed been a major tool in studying the structure of atomic nuclei and has witnessed numerous significant advances over the last four decades. This article will select highlights from investigations at the Niels Bohr Institute, Denmark, and Daresbury Laboratory, UK, in the late 1970s and early 1980s, some of which have continued at other national laboratories in Europe and the USA to the present day. These studies illustrate the remarkable diversity of phenomena and symmetries exhibited by nuclei in the angular momentum-excitation energy plane that continue to surprise and fascinate scientists.

Beyond the Unified Model

“…Choosing the rotational frequency as the band parameter facilitates the comparison with the cranking mean field theory because the latter is formulated for a given frequency. The relation (60) between the energies and the frequency, which holds exactly for the selfconsistent cranking model, ensures that the frequency used in the calculations can be identified with the experimental frequency.…”

“…Indeed, the moment of inertia J (2) of the lowest rotational band in superdeformed 152 Dy changes only by 10% over the observed angular momentum range of ∆J = 38 (see e. g. picked fence spectrum in Fig. 5 in the contribution to this Focus Issue by M. A. Riley, J. Simpson and E. S. Paul [60]). These features are in line with the large number n ph = 96 of terms in the sums (145) and (152).…”

“…which results in the the decrease of ω when the yrast levels change from the g-to the s-band. More about the discovery of back bending can be found in the contribution by M. A. Riley, J. Simpson and E. S. Paul to this Focus Issue [60]. A beautiful mechanical simulation of the effect is available online [73].…”

“…The rotational bands are identified by measuring multi coincidence events in large arrays of γ ray detectors. The method and important results are discussed in the contribution to this Speccial Edition by M. A. Riley, J. Simpson and E. S. Paul [60].…”

Digital Data Acquisition for Gamma-Ray Spectroscopy

“…The DPP system can count the real event rate from the detector using the fast channel since this has a very short shaping time and is not rate limited. If the fast threshold is correctly set the ADMCA software displays the number of true event as Input Counts [11], [12]. The slow channel records the actual measured number of events and displays it as Counts.…”