Interpreting and predicting structure Useful interpretative models; p-n interaction Second Lecture

We will now discuss some simple collective models to interpret structure Two classes: Macroscopic or phenomenological and microscopic (femtoscopic, really)

We will mostly discuss macroscopic models. Such models, generally, are NOT predictive. They need to be “fed” some data in a given nucleus. However, once fed, they can predict (correlate )huge quantities of data. They also give an intuitive grasp of the kinds of structure observed, whereas complex microscopic models, while more (but not completely) predictive are often less transparent physically (note: this is my opinion, others might disagree)

Macroscopic Models Examples: Bohr-Mottelson model, Interacting Boson Approximation (IBA) Model, Geometric Collective Model (GCM), and some simplified versions of these. We have already encountered some examples of these models when we briefly discussed the vibrator and rotor models Probably the most successful, phenomenalogically, of the collective models has been the IBA, for the wide range of structures it can accommodate and for its extreme economy of parameters. This will be the subject of three full morning lectures in mid-June. For now, some very simple approaches.

Development of collective behavior in nuclei Results primarily from correlations among valence nucleons. Instead of pure “shell model” configurations, the wave functions are mixed – linear combinations of many components. Leads to a lowering of the collective states and to enhanced transition rates as characteristic signatures. How does this happen? Consider mixing of states. The following will be a little mathematical and uses some quantum mechanics. If you don’t follow it, don’t worry.

A illustrative special case of fundamental importance T Lowering of one state. Note that the components of its wave function are all equal and in phase Consequences of this: Lower energies for collective states, and enhanced transition rates. Lets look at the latter in a simple model.

W The more configurations that mix, the stronger the B(E2) value and the lower the energy of the collective state. Fundamental property of collective states.

Transition rates (half lives of excited levels) also tell us a lot about structure B(E2:  )    E2   2 Collective enahancement

Two very useful techniques: Anharmonic Vibrator model (AHV) (works even for nuclei very far from vibrator) E-GOS Plots (Paddy Regan, inventor)

AN harmonic Vibrator – Very general tool 2 + level is 1-phonon state, 4 + is two phonon, 6 + is 3-phonon … BUT R 4/2 not exactly 2.0 in vibrational nuclei. Typ. ~ Write: E(4) = 2 E(2) +  where  is a phonon - phonon interaction. Can fit any even-even nucleus by fixing  from E(4). Allows one to predict higher lying states. How many interactions in the 3 – phonon 6 + state? Answer: 3: E(6) = 3 E(2) + 3  How many in the n – phonon J = 2n state? Answer: n(n-1)/2

AHV Hence we can write, in general, for any “band”: E(J) = n E(2) + [n(n – 1)/2]  n = J/2 Very general approach, even far from vibrator structures. E.g., pure rotor, E(4) = 3.33E(2), hence  = 1.33 E(2)  /E(2) varies from 0 for harmonic vibrator to 1.33 for rotor (and 0.5 for gamma-soft rotor) !!!

What to do with very sparse data??? Example: 190 W. How do we figure out its structure? P. Regan et al, private communication

W-Isotopes N = 116

E-GOS Plots E Gamma Over Spin Plot gamma ray energies going up a “band” divided by the spin of the initial state E-Gos plots are, for spin, what AHV plots are for N or Z Evolution of structure with J, N, or Z

E-GOS plots in limiting cases Vibrator: E(I) = h  Hence E  (I) = h  Rotor: E(I) = h 2 /2J [I (I + 1)] Hence E  (I) = h 2 /2J [4I - 2] AHV (R 4/2 = 3.0): E(4) = 2E(2)       Hence E  (I) = 2(I + 2) R = 2 = constant

R 4/2 = 3.0 (AHV)

Comparisons of AHV and E-GOS with data J = Ru Egamma AHV (  /E(2) = 0.14 E-GOS Dy Egamma AHV (  /E(2) = 1.29 E-GOS

102 Ru

R 4/2 = 3.0 (AHV) Vibr  rotor

Estimating the properties of nuclei We know that 134 Te (52, 82) is spherical and non- collective. We know that 170 Dy (66, 104) is doubly mid-shell and very collective. What about: 156 Te (52, 104) 156 Gd (64, 92) 184 Pt (78, 106) ??? All have 24 valence nucleons. What are their relative structures ???

Thus far, we have mostly discussed WHAT nuclei are doing. Now a key question is WHY does structure vary the way it does and ways in which we can exploit such understanding. There are many aspects of this but a dominant driver of structural evolution is the competition between two interactions – pairing and the proton-neutron interaction

Valence Proton-Neutron Interaction Development of configuration mixing, collectivity and deformation – competition with pairing Changes in single particle energies and magic numbers Partial history: Goldhaber and de Shalit (1953); Talmi (1962); Federman and Pittel ( late 1970 ’ s); Casten et al (1981); Heyde et al (1980 ’ s); Nazarewicz, Dobacewski et al (1980 ’ s); Otsuka et al( 2000 ’ s); Cakirli et al (2000 ’ s); and many others.

Sn – Magic: no valence p-n interactions Both valence protons and neutrons The idea of “both” types of nucleons – the p-n interaction

If p-n interactions drive configuration mixing, collectivity and deformation, perhaps they can be exploited to understand the evolution of structure. Lets assume, just to play with an idea, that all p-n interactions have the same strength. This is not realistic, as we shall see later, since the interaction strength depends on the orbits the particles occupy, but, maybe, on average, it might be OK. How many valence p-n interactions are there? N p x N n If all are equal then the integrated p-n strength should scale with N p N n The N p N n Scheme

NpNn scheme

Valence Proton-Neutron Interactions Correlations, collectivity, deformation. Sensitive to magic numbers. N p N n Scheme Highlight deviant nuclei P = N p N n / (N p +N n ) p-n interactions per pairing interaction

The idea that p-n interactions produce collectivity and a lowering of collective states is seen everywhere, as the Sn-Xe figure shows. It has, in particular, one spectacular phenomenon – Intruder states Consider the following 2p-2h excitation across a magic number Clearly, this requires considerable energy. However, if the upper orbit, j’, has high overlap with the neutrons, then, as the number of valence neutrons increases, this state can decrease sharply in energy. Consider what happens when the shell gap is a) large and b) small

Consider the large Z = 50 gap. Towards mid- neutron shell this excited state drops in energy and becomes a low lying excitation. Since it involves orbits from another major shell which come into the spectra of levels from the main shell, it is called an “intruder state” Consider the small, subshell, gaps at Z = 40 and 64. Here the “intruder state” can drop enough that is drops BELOW the “ground state” and becomes the new, deformed, ground state. That, in fact, is the mechanism and reason for the sudden onset of deformation that we have seen in the Sm region.

Disappearance of Z = 64 proton shell gap as a function of neutron number

The NpNn scheme: Interpolation vs Extrapolation

Predicting new nuclei with the N p N n Scheme All the nuclei marked with x’s can be predicted by INTERpolation