A. Neutrino Physics

 

a) Neutrino oscillations: In the first half of 2007 the KamLAND Collaboration continued its $20M liquid scintillator purification task to convert the KamLAND anti-neutrino detector into a low-threshold solar neutrino detector. At the end of July the purification effort had to be halted due to blasting in the Kamioka mine (for a new cave for a xenon based dark matter experiment). In the Spring of 2007 I spent 10 days in Japan to work on the KamLAND  scintillator purification task. In December I performed eight days of KamLAND remote (off-site) shift service.

A draft for a PRL with the title “Precision Measurement of Neutrino Oscillation Parameters with KamLAND” is currently being circulated amongst the KamLAND collaboration members.

 

b)  Double-beta decay: In 2007 we leased from DOE about 50 grams of Nd-150 (95% enriched) at a cost of $30,000.  After 100 days of double-beta decay studies to excited states of the daughter nucleus we concluded that the purity of the Nd-150 sample was not sufficient for a successful measurement. We returned the sample to Oak-Ridge National Laboratory where efforts are now underway to remove or reduce the thorium impurities. Gamma-ray lines from decay products of the thorium chain interfere with our transitions of interest. It is not clear at the present time by how much Ra-228 can be removed which is responsible for the Ac-228 lines which almost coincide with the Nd-150 lines of interest. We expect to have the purified Nd-150 sample back at TUNL in early 2008. Our 8’ x 20’ mobile laboratory will then be moved to the Kimballton mine (VA) where we will continue the Nd-150 measurements at the 1700 ft level. These measurements are part of the thesis project of Mary Kidd.

       While we were waiting for a purified Nd-150 sample we started a feasibility study of a promising alternative to neutrinoless double-beta decay: neutrinoless double electron capture. There seem to exist one or two nuclei for which this process can proceed through an excited state of the daughter nucleus which is degenerate with the parent nucleus, resulting in zero phase space for the two-neutrino double-electron capture, and providing in principle a severable order of magnitude enhancement of the capture rate. We acquired 4 g of Sn-112 enriched to 90% (natural abundance ~1%) from Livermore National Laboratory and during 100 days of measuring time at TUNL we obtained not a single count in the energy region of interest (not even a background count!). This result gives very useful information on how to design a larger scale experiment based on our coincidence approach.

       In support of the Majorana Project we continued our measurement program at TUNL to determine the neutron induced background in a typical Ge-76 based double-beta decay experiment. This background becomes crucial once the other known background sources have been reduced to the level envisioned for the Majorana Project. Here, gamma rays from neutron induced reactions on lead and copper, and of course 76Ge, are of special interest. After studying lead and copper at two incident neutron energies in 2006, we took data on Ge-76 in 2007. These data are currently being analyzed .

 

 

 

 

 

B. Few-Nucleon Physics

 

a)    Two-body system:  In 2007 we submitted our paper on the analyzing power in neutron-proton scattering for publication. The associated data are the most accurate data ever obtained in polarized neutron scattering off any hadron or nucleus. The data disagree significantly from nucleon-nucleon potential model predictions and global nucleon-nucleon phase-shift analyses. The concluding sentence of our manuscript reads: “Perhaps the impass comes because we are at the point where the precision of our data and the development of our low-energy theoretical models has pushed the paradigm of meson-exchange based nucleon-nucleon potential models beyond its limit.”

       Our Russian colleagues from the DIANNA Collaboration made quite some progress in 2007 toward our goal of measuring the neutron-neutron scattering length in a direct neutron-neutron collision experiment using the pulsed high-flux reactor JAGUAR in Snezhinsk, Russia. Using a new neutron detector, they observed neutron-He-4 scattering events when the reaction channel was filled with helium.

 

b)    Three-body system: We continued our neutron-deuteron elastic analyzing power measurements at 21.9 MeV, where the three-nucleon analyzing power puzzle is expected to be less pronounced than at lower energies. However, our new data support the two proton-deuteron data points at 22.7 MeV, which indicate an increased discrepancy and resonance-like behaviour. Again, this observation is not understood at all, and I have to admit that I didn’t trust the two proton-deuteron data points either, but nevertheless decided to check on these data with neutrons as a probe due to the lack of a polarized proton beam in the energy range of interest. This new spin to the three-nucleon analyzing power puzzle will keep us busy for the foreseeable future. We plan to submit a proposal

to RCNP (Osaka, Japan) to obtain beam time in the 20 to 30 MeV energy range. RCNP is the only facility worldwide where polarized proton beams are available in the energy range of interest. With polarized neutron beams it’s just too hard to do these experiments to the necessary accuracy within the “lifetime” of a graduate student

 

c)    Four-body system:  In 2007 we completed our measurements of the analyzing power in elastic neutron-He-3 scattering at low energies. This experimental program is part of the thesis project of James Esterline. Here the goal is to obtain information needed to resolve the so-called analyzing power puzzle, the largest discrepancy (25-50%) known to exist in low-energy nuclear physics between data and rigorous theoretical calculations. To the great surprise of the few-body community our data are in much better agreement with theoretical predictions than those previously obtained for proton-He-3 scattering in the same energy region. This result, although still unpublished, triggered quite some activity in the theoretical few-body community. With our help it was found that proton-triton analyzing power data are also in much better agreement with theory than the proton-He-3 data. These observations may point to a completely unexpected isospin dependence of three-nucleon forces in the four-body system.

       

        Our recent observations have earned me invitations to present talks at international conferences in Europe and Asia. I am also scheduled to present an invited talk about our findings at the APS April 2008 Meeting.

 

C. Nuclear Physics for Society

 

a) (n,2n) measurements: We completed our (n,2n) measurements on U-238 and U-235 which constitute the thesis project of Tony Hutcheson. We also completed our Am-241(n,2n)Am-240 cross-section measurements. This project is headed by Anton Tonchev. In 2008 we will pursue measurements of the Pu-239(n,2n) Pu-238 cross section These data will be obtained with collaborators from LANL and LLNL who also will provide the 239Pu targets. Very stringent safety procedures are in place to handle this special nuclear material on a university campus. We are working with the Duke University Radiation Safety Office and the State of North Carolina to obtain a license for handling the small amounts of Pu-239 needed for this NNSA funded project.

 

b) Transmutation of radioactive waste: We have not done any experimental work during 2007.

 

D. Nuclear Physics for Fun

 

a)   Neutron production with a pyroelectric crystal:  In 2005 Naranjo, Gimzewski and Putterman (Nature 434, 115, 2005) and in 2006 Geuther, Danon and Saglime (Phy. Rev. Lett. 96, 054803, 2006) showed that a pyroelectric crystal operated in a dilute gas of deuterium and augmented with a nanotip and a deuterated target foil can be used to produce neutrons via the fusion reaction 2H(d,n)3He. In contrast to the "cold fusion" days of Pons and Fleischman back in 1986, the findings of these two research groups regarding "hot fusion" are generally being considered as plausible. Here, the pyroelectric crystal acts as a miniature particle accelerator. Although I managed to produce neutrons with a LiTaO3 crystal operated in 3 mTorr of deuterium gas, I could not reproduce the high neutron flux reported in the references given above. I think these experiments are flawed. A manuscript about my experimental work is almost completed.

 

 

b)  Manipulating the decay rate of radioactive nuclei: Ever since E. Rutherford and F. Soddy published their theory of radioactive decay in 1902 the common conjecture has been that the decay rate of radioactive nuclei cannot be manipulated by external macroscopic processes. During the last 100 years only very tiny changes of the decay rate has been observed in a very specific radioactive decay, the electron capture of nuclei. Here large pressures were applied which resulted in an increase of the electron density near the nucleus and therefore, provided an enhanced capture probability. However, the situation has changed dramatically during the last year. B. Wang et al. (Eur. Phy. J. A28, 375 (2006)) reported that the half-life time for the electron capture of Be-7 in the metallic environment of Pd and In at a temperature of 10 K was found to be increased by (0.9 +- 0.2)% and (0.7 +- 0.2)%, respectively, while in the insulator Li₂O it was unchanged within experimental uncertainties. Furthermore, B. Limata et al. (Eur. Phy. J. A28, 251 (2006)) observed a decrease of (1.2 + -0.2)% in the β+ decay rate of Na-22 in the metallic environment of Pd at T=12 K. Even more impressive, B. Limala et al. (private communication, C. Rolfs, 2006) very recently reported a (10 +- 1)% decrease in the α-decay rate of Po-210 embedded in a Cu environment at T=14 K.

       We are trying to reproduce these effects, but so far, have not seen a decay-time change of Cu-64 ( emitter) and Zn-65 ( emitter) between samples at room temperature and samples cooled to12 – 13 K. Our radioactive samples were produced at TUNL by bombarding natural copper with proton and deuteron beams, respectively. There was no need to implant the radioactive nuclei in a metallic environment, because they were produced already in a metallic environment.