Kent
Gifford
Kent
Gifford
Xavier University (Cincinnati)
Dr. Jollivette
Supported by
NSF-REU
If one examines the characteristics of the neutron and proton, they find
many similarities. For instance, their masses differing by one-tenth of a
percent. More importantly, they both possess spin one-half. The main distinction
between the two comes about in their electromagnetic properties. Protons
are positively charged, while neutrons are electrically neutral. One may
envision instances where it is useful to view protons and neutrons as different
states of a single particle, the nucleon. By not distinguishing between proton
and neutron, the nuclear force allows us to do this very thing. In order
to view them as manifestations of a single particle, one introduces the concept
of isospin. Recall that all nucleons have spin one-half. The convention is
to assign the proton isospin up and the neutron spin down. Many physicists
claim that isospin is always conserved in nuclear reactions. The genesis
of our research is to establish that isospin is only an approximate quantum
number and to discover states seen in isospin reactions.
In the case of 24Mg(d,alpha_2)22Na, isospin is not conserved. Basically, the coulomb interaction brings about this isospin breaking. To produce the aforementioned reaction, one accelerates deuterons via a Van de Graaf accelerator. The beam of deuterons is then brought into a scattering chamber and hits the natural magnesium target. The target is made by evaporation of natural magnesium onto a carbon foil. The scattered particles are collected in silicon surface barrier detectors. A typical spectrum shows the ground states and first through third excited states of protons and alphas. To reveal resonances in the cross section an excitation function is taken. This involves fixing the detector angle and varying the energies of the deuteron beam. If one fixes the energy at a supposed resonance and changes angle an angular distribution can be taken. Angular distributions reveal the spins and the parities of the compound nucleus. In this case 26Al is the com pound nucleus. For this particular reaction, the spin and parity combination(0+ + 1+ - 0+ + 0+) allows only natural parity states to contribute. This permits simpler calculations. Xplot, a multipurpose nuclear diagnostic software is used to statistically show the presence of the isospin forbidden state since the cross section is fairly low. After seeing that the isospin forbidden state does indeed exist, the goal is to identify the spins and parities of the compound nucleus. This is done by fitting the resonances to a Breit-Wigner shape. Outputs of the analyses reveal the strengths, width of the resonance, and the energies of the compound nucleus states.
In sum, the goal of uncovering the isospin forbidden state in the 24Mg(d,alpha_2)22Na was reached. Further, two new states of the compound nucleus 26Al were discovered. In addition to this, the spins and parities of these states were also found. From the research that was performed, it appears that isospin is only an approximate quantum number in the aforementioned reaction.
The states seen are:
| Ed(MeV) | Ex(MeV) | Width(kev) | Strength |
| 2.42 | 13.64 | 47.2 | .3531 |
| 2.54 | 13/76 | 42.2 | .3374 |
Here Ed is the deuteron beam energy and Ex is the energy of the compound nucleus state in 26Al.