REU Projects for 2007
Manipulating the Decay Rate of Radioactive Nuclei
Advisor: Dr. Werner TornowEver 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's 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 halflife for the electron capture of 7Be 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 22Na 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 of 210,Po embedded in a Cu environment at T=14 K.
The REU student will design and build an apparatus and initialize a program at TUNL to measure α-decay and β+-decay rates of suitable radioactive nuclei in a metallic environment at low temperatures.
Productions of Neutrons by means of the Pyroelectric Effect
Advisors: Dr. Werner Tornow
The REU student will join a small research group that is following up on work published by B. Naranjo et al. (Nature 434, 1115 (2005)) and J. Geuther et al. (Phys. Rev. Lett. 96, 054803 (2006)) who used pyroelectric crystals to initiate the fusion reaction 2H(d,n)3He in a dilute gas of deuterium. Our initial work has concentrated on neutron production without attaching a tip to the LiTaO3 crystal and without using a deuterated target foil. In this case the deuterium gas provides the projectile deuterons as well as the deuterium target nuclei. However, as expected, the neutron yield is very low compared to the more sophisticated published work, where a nano-size tip and a deuterated target were used . We are now in the process of investigating neutron production using a tungsten tip and various kinds of deuterated targets.
The REU student will participate in the associated measurements. Our final goal is to initiate the fusion reaction 3H(d,n)4He with a pyroelectric crystal. If successful, this reaction will provide 14.5 MeV neutrons compared to the 2.7 MeV neutrons produced via the 2H(d,n)3He reaction.
Nuclear Data Review and Evaluation of beta-decay reactions
Advisor: Dr. John Kelley
The Nuclear Data Evaluation Group is responsible for
maintaining a database of nuclear structure properties in
the atomic mass range of A=2-20. Our group surveys
published literature and provides summary reviews to the
nuclear data user community. In this project, publications
with relevant information from beta-decay reactions will
be evaluated using standardized techniques in order to
obtain the best values for decay properties and
associated structure parameters. Activities will include
searching databases of published articles for beta-decay
reaction data, familiarizing oneself with the information
that can be obtained from these measurements, and
evaluating the published data sets to deduce a set of
"best values". The student will develop a web
interface for presenting the information. As time
permits, the student may choose to develop discussion
about specialized topics. In addition the student will become
familiar with useful experimental procedures that are
relevant to measurements with High-Purity Germanium
gamma-ray detectors.
Measurements of 187Re(n,2nγ)186mRe cross-section
Advisors: Drs. John Kelley and Anton Tonchev
The aim of this project is to measure partial (n,2nγ)
cross sections that lead to population of levels above a very
long-lived state in 186mRe (T1/2 ≈ 2.0105 y). These data are
needed in order to evaluate the production (and destruction)
cross sections of 187Re, and hence, to reduce the nuclear
physics uncertainties for the 187Re/187Os cosmochronometer
that is used to date the age of the r-process
nucleosynthesis. In addition, rhenium has one of the highest
melting points of all elements (exceeded only by tungsten and
carbon) and it is used to make parts of jet engines, and in
space reactor technologies. Therefore, the accurate knowledge
of neutron induced reaction cross-sections for this element is
of vital importance to these applied technologies. We will
study the 187Re(n,2nγ)186mRe reaction using
monoenergetic neutron beams in the 5-15 MeV energy range at
the TUNL accelerator facility. A suite of high-resolution Ge
detectors will be used to detect prompt gamma-rays above the
186Re isomer in both singles and coincidence modes. The
nuclear model codes TALYS and EMPIRE will be utilized in
reconstructing the total 187Re(n,2n)186mRe reaction
cross-section.
Monte Carlo studies of backgrounds in low radioactive background underground experiments.
Advisor: Dr. Reyco Henning
Certain sensitive physics experiments have to escape the ubiquitous
cosmic-ray background at the surface of the earth and can only be performed
deep underground. These experiments also require extremely low levels of
radioactivity in their components and excellent shielding to protect them
from activity in the environment. Our group is specifically interested in
an experiment (called CLEAN/DEAP) that will search for the interactions of
WIMP dark matter with atomic nuclei, as well as an experiment (called
Majorana) that will look for the neutrinoless double-beta decay of Ge-76.
The discovery of this decay would imply that the neutrino is its own
antiparticle and provide a key component of a theory that could explain why
the universe is matter dominated. These next generation underground
detectors will require a detailed understanding of all their radioactive
backgrounds. The key tool for quantifying this in terms of the purity
requirements for these detectors' components, how deep they have to be
underground, their shielding requirements, etc. is Monte Carlo computer
simulation programs. These programs simulate a real detector to determine,
among other things, the effect of background on the potential signals.
There are multiple background and simulation studies that need to be
performed for these experiments that are suitable for an undergraduate
research project. These projects would involve coding in C++, learning
existing simulation programs, writing simulation programs, analyzing the
simulated data and comparing to existing data. Familiarity with C++ or ROOT
is useful, but not required.
Characterization of HPGe detectors with application to the Majorana experiment.
Advisor: Dr. Reyco Henning
The Majorana experiment will field an array of 57 enriched, High-Purity
Germanium (HPGe) detectors at a deep underground site to search for the
neutrinoless double-beta decay of Ge-76 (see preceding project description). An important
requirement for Majorana is that the response of these HPGe crystals to
ionizing radiation be well-characterized. Our group is interested in
developing a detector-characterization technique that will be fast,
accurate, safe, and be able to process several crystals in parallel. We
will build a prototype characterization system this summer that will
consist of a collimated source, HPGe detector, DAQ, and computer. This
project is open and students can pick topics to focus on, such recording
data from a highly-segmented HPGe using sources, working with the
data-acquisition electronics, performing analysis of the data and/or
simulating the detector response to sources.
Precision Neutron Induced Cross Section Measurements for Neutrino and Homeland Security Experiments
Advisors: Drs. Anton Tonchev and Werner Tornow
Neutrinoless double beta decay (0νββ) is a
second-order weak process, which is predicted to occur, if the
neutrino is a massive Majorana particle. The half-life of this process
is a function of the neutrino masses, their mixing angles, and CP
phases and the nuclear matrix elements involved. An observation of the
process would therefore give information about the absolute neutrino
mass scale. The search for the 0νββ-process has
become especially important after the discovery of the non-zero
neutrino mass from flavor oscillations and the recent claim of an
observation of neutrinoless double-beta decay based on data of the
Heidelberg-Moscow experiment. A research program at TUNL is
focused on the muon-induced fast neutron background for the next
generation double-beta decay and dark matter searches. Experiments
which aim at the observation of rare decays are limited in sensitivity
by the number of background events. A low background surrounding is
therefore essential for the success of all 0νββ
experiments. High-purity Germanium crystals can be used
simultaneously as source of 0νββ-decays and
detectors. High precision neutron induced cross section measurements
on natural copper, lead, and on enriched 76Ge are essential for future
neutrino activities.
Another aspect of these nuclear data measurements are their application in many disciplines. The most important one relates to future nuclear energy production and homeland security. Neutron data for structural materials are needed to estimate the gas-production by (n,p) and (n,α) reactions, since gas inclusions negatively influence the mechnical stability and thus the usability time of these materials. The development of low activation materials as well as decay heat calculations requires the knowledge of such data.
The measurements will be performed at TUNL using state-of-the-art segmented high-purity germanium clover detectors. Key instruments for this research include the TUNL ion source and beam-pulsing system, and the associated accelerator for generating pulsed and monoenergetic neutron beams via the 2H(d,n)3He reaction.
3He Injection Test for the Neutron Electric Dipole Moment (nEDM) Experiment
Advisor: Dr. Haiyan Gao
Precision measurements of the properties of the neutron present an opportunity to
search for violations of fundamental symmetries and to make critical tests of the validity
of the standard model (SM) of electro-weak (EW) interactions. Recently, a new
experiment is underway on a search of the neutron electric dipole moment (EDM) aiming at
an unprecedented sensitivity. The new search will have a two orders of magnitude
improvement over the current experimental limit on this quantity. A search for a non-zero
value of the neutron EDM is a direct search of the time reversal symmetry (T) violation.
A direct search for T symmetry violation such as the search for the neutron EDM is a
unique way of search for CP violation because of CPT invariance.
The new nEDM experiment relies on the spin dependence of the nuclear absorption cross section: n + 3He → p + t + 764keV . Therefore, polarized 3He is important to the success of the nEDM experiment. The purpose of the 3He Injection test is to make sure that the number of 3He atoms collected should be big enough in the collection volume, while the polarization loss should be small enough. During the test, 3He atoms with nuclear polarization of nearly 100% from Atomic Beam Source will pass through a transfer tube of 1.5 meters of length first and then be injected into collection volume which is a pyrex cell partially filled with superfluid of 4He. The collection volume with superfluid 4He will be operated at 300 mk. Both a Cesium ring and a passive film burner will be implemented along the transfer tube to reduce effectively the evaporation of superfluid helium film along the wall. A superconducting solenoid coil will provide magnetic field along the 3He injection path. A superconducting tricoil system will provide a vertical magnetic holding field of 1 KGauss for the collection cell, which is 45 degrees to the injection path. The 3He spin direction will rotate by 45 degrees, aligning with the magnetic field. Due to low concentration of 3He in the collection volume, the NMR signal is expected to be small. A magnetic holding of 1.2 Kgauss will be used to enhance the NMR signal and we plan to use pulsed NMR for the measurement.
An REU student will have the opportunities to test various components of the injection test apparatus at Duke this summer and perhaps make one trip to Los Alamos National Lab. For detailed information, please contact Prof. Gao at gao@tunl.duke.edu.
Nuclear & Nucleon Compton Scattering at the High Intensity Gamma-Ray Source (HIGS) and Commissioning of the HIGS NaI Detector Array (HINDA)
Advisors: Drs. Mohammad W. Ahmed, Sean Stave, Henry R. Weller & the Capture Group at TUNL
In a simple, non-relativistic picture when an external electric and magnetic field is applied to a collection of charged particles, a charge-mass separation occurs in response to these fields. This process, commonly referred to as polarization, also occurs in nuclei (where protons and mesons act as charged particles) and nucleons (where quarks are massive charged objects). Different wavelengths of the applied electromagnetic field (or a photon) are required to study polarization in nuclei or nucleons.
The first-order study of polarization phenomenon consists of measuring electric and magnetic polarizabilities. The low-energy theories, such as Chiral Perturbation Theory (CPT), predicts them via energy (ω ) expansion of the Compton scattering amplitude.
Scattering Amplitude = A 0 + α f( ω ) + β G( ω ),
Where A 0 is a constant (depends on parameters such as charge and mass of the particle), alpha is the electric polarizability, and beta is the magnetic polarizability. The expansion is in terms of the energy dependent functions f and G.
At HIGS, we plan to perform precision measurements of alpha and beta. The experiment requires a polarized photon beam, a target (a nucleus or a nucleon), and a set of detectors to measure the scattered photon and the recoil nucleon(s). The beam will be provided by HIGS; various targets such as protons (liquid hydrogen), deuterons (liquid deuterium), and He-3 are in production; a new spectrometer array called HI GS N aI D etector A rray ( HINDA ) is being constructed to detect the scattered photons.
This REU project will focus on understanding the basics of Compton Scattering on nuclei and nucleons (including calculations of count rates, and limits on experimental measurements), commissioning of the HINDA, and Monte Carlo calculations of the HINDA. The project will aid the student in developing strong skills in Compton Scattering theory, methods of photon detection, and hands-on experience in collecting, analyzing and organizing data from scintillating detectors.
The Study of the 16 O( γ,α ) 12 C Reaction and Commissioning of the Optical Time Projection Chamber (O-TPC)
Advisors: Drs. Mohammad W. Ahmed, Sean Stave, Henry R. Weller & the Capture Group at TUNL
The inverse of the capture reaction 12 C( α,γ ) 16 O , termed the holy grail of nuclear astrophysics by Willie Fowler will be measured . The cross section for the capture reaction can be deduced from this by means of the Principle of Detailed Balance . The ratio of carbon to oxygen at the end of helium burning is crucial for understanding the fate of Type II Supernovae and the nucleosynthesis of heavy elements.An oxygen rich star is predicted to end up as a black hole, while a carbon rich star leads to a neutron star. And a minor change in the astrophysical S-factor of the 12 C( α,γ ) 16 O capture reaction at 300 keV (from 170 to 200 keV-b) can make all the difference. At present, the experimental uncertainty in this factor is about a factor of 10 or more.
We plan to start a program to perform these measurements at HIGS using a novel detector called an Optical Time Projection Chamber (O-TPC). The REU project will focus on understanding the astrophysics of this critical reaction, count rate calculations, and developing various components of the O-TPC such as a high resolution CCD camera, and Flash Analog-to-Digital convertors (FADCs).
Measurement of 11 B and Proton Capture Reaction for Nuclear Fusion Power Reactors
Advisors: Drs. Sean Stave, Mohammad Ahmed, Ralph France, Henry Weller & the Capture Group
There is a renewed interest in practical power generation using nuclear fusion. In the past, these methods have explored standard fusion fuels such as tritium. This method is not ideal due to the useof long half-life radioactive materials (tritium) and the high fluxes of neutrons produced in this reaction. A new possibility is being proposed and evaluated which uses Boron-11 isotope (stable) which undergoes neutron-free fusion via 11 B(p, α )2 α . In order to estimate the power generation, accurate values of the cross section of this reaction at various angles and energies are needed. TUNL is involved in measuring the analyzing power and cross section using our polarized ion source and multiple silicon detectors. This past summer, we performed a test run and plan to take more data this summer. If a polarized beam can enhance the cross section, there is a possibility of using this to enhance the energy produced in this reaction.
The REU student involved in this project will take part in setting up the experiment, doing calibrations of the silicon detectors using alpha emitting radioactive sources, and acquiring and analyzing parts of the data.
Measurement of 13 C(d,n) Reaction for Astrophysical Studies
Advisors: Drs. Sean Stave, Mohammad Ahmed, Henry Weller & the Capture Group
The standard model of big bang nucleosynthesis assumes a homogeneous baryon density. However, this may not be how the early Universe actually appeared. Neutrons are able to diffuse through matter more easily than protons due to their lack of charge. Therefore, low density regions will have relatively more neutrons than high density regions. Neutron rich isotopes are better for producing heavy elements so, these low density, neutron rich regions will then produce more heavy elements adding to the inhomogeneity.
The proposed primary path to elements with A > 12 uses neutrons on Carbon-13 to create Carbon-14 (plus a gamma ray). A reaction which competes with neutrons on Carbon-13 is deuterons on Carbon-13 to create Nitrogen-14 (plus a neutron) and so this reaction may be important in inhomogeneous nucleosynthesis. This project proposes to use the TUNL mini-tandem accelerator to create a beam of deuterons on a Carbon-13 target. The emitted neutrons will then be detected at various angles using an array of 8 liquid scintillator neutron detectors. Several deuteron energies around 300 keV will be used to measure how the cross section varies with energy.
The REU student involved in this project will take part in setting up the experiment, calibrating the detectors with radioactive sources, running the experiment and analyzing the data.
Background Studies for a Coherent Neutrino-Nucleus Scattering Experiment
Advisor: Dr. Kate Scholberg
Coherent neutral current neutrino-nucleus elastic scattering is a process in which a neutrino interacts with a nucleus, giving it recoil kick. Although the probability for such a process to occur is relatively high, the process has never before been detected because typical nuclear recoil energies are very small. However, detection of the process may be within the reach of the new generation of low-threshold detectors. Because the rate of the process can be quite precisely predicted, a deviation of measurement from prediction could indicate new physics beyond the Standard Model. A promising prospect for the first detection of this process is an experiment at a high flux stopped-pion neutrino source such as the Spallation Neutron Source at Oak Ridge National Laboratory in Tennessee.
This project will involve participation in ongoing neutron background measurements at the SNS using scintillation detectors. The work will include both data analysis and detector simulation studies, with the aim of evaluating the sensitivity of new technology to tiny neutrino-induced recoils. The student will gain experience with a variety of simulation and data analysis software tools. Programming experience will be useful but is not required.
Investigation of p-process Nuclei
Advisor: Dr. Hugon Karwowski
The p-process is the stellar mechanism for producing certain proton-rich nuclei that are not produced in other processes. The primary reaction mechanism of the p-process is the photodisintegration of heavy seed nuclei, moving nuclei to the proton rich side of the valley of beta-stability. However, it is still not well understood, with critical data needed, much of which depends on the availability of monoenergetic gamma rays. This summer we will perform nuclear resonance fluorescence mesurements on 142Nd and 150Nd. The primary motivation for these measurements is to better understand the effect of gamma-ray strength function, and particularly the pygmy resonance, on the p-process.