ISNAP Seminars - Abstracts 2016 Spring

Topic: 89Zr (n,γ) cross sections from a surrogate reaction approach

Dr. Shuya Ota ( Lawrence Livermore National Laboratory ) / January 18, 2016

The surrogate reaction approach is an indirect method for determining nuclear reaction cross sections which cannot be measured directly or predicted reliably. While recent studies demonstrated the validity of the approach for studying fission cross sections of short-lived actinides, its applicability for radiative neutron capture reactions ((n,γ)) is still under investigation. We studied the decay of excited 90Zr nuclei produced by 90Zr(p,p'), 91Zr(p,d), and 92Zr(p,t) surrogate reactions, respectively, in order to study the effect of the production mechanism on the decay of a compound nucleus and to infer the 89Zr(n,γ) cross sections. The experiments were carried out at the K150 Cyclotron facility at Texas A&M University with a 28.5-MeV proton beam. The reaction deuterons were measured at forward angles of 25-60 with the STARS array of three segmented Micron S2 silicon detectors. The compound nuclei with energies up to 8 MeV above the neutron separation thresholds were populated. The coincident γ-rays were measured with the Livermore Texas Richmond (LiTeR) array of five Compton-suppressed HPGe clover detectors. We will present results of emission probabilities from the different three reactions, which showed the 90Zr(p,p') reaction produces fewer γ rays associated with transitions involving high spin states (J = 6-8 hbar) than the other two reactions, suggesting that inelastic scattering preferentially populates states in 90Zr that have lower spins than those populated in the transfer reactions investigated. Theoretical approaches to obtain the 89Zr(n,γ) cross sections from these emission probabilities will be also discussed.

Topic: Special Nuclear Seminar:
Nucleosynthesis in Type I X-ray Bursts: Exploring the αρ-Process through High Precision (ρ,t) Measurements

Mr. Alexander Long ( University of Notre Dame ) / January 22, 2016

Shortly after their discovery in 1979, x-ray bursts were determined to be thermonuclear runaways occurring on the surface of neutron stars in binary systems with H/He rich companion stars. During these explosive events thermonuclear burning is driven by the αρ-process (a sequential series of (α,ρ) and (p,γ) reactions along the proton rich side within the sd-shell nuclei), and the rp-process (a series of (p,γ) and β+-decays riding along the proton drip line up the A = 100 mass region). Current x-ray burst sensitivity studies have revealed that certain (α,ρ) reactions along the αρ-process have a direct influence on the early rise-time structure of x-ray burst light curves.

Lacking experimental data, most of these (α,ρ) stellar rates have been calculated using statistical models, such as Hauser-Feshbach. Recently, it has been pointed out that the level density in many of the compound nuclei along the αρ-process may be too low to support this statistical approach, resulting in over predictions of stellar (α,ρ) rates used in x-ray burst models.

In this talk, I will discuss the recent efforts by our group at Notre Dame in trying to indirectly measure important (α,ρ) reaction rates through high precision (p,t) reaction measurements. More specifically, I will present results from our latest (p,t) experiment at iThemba LABS, where we indirectly measure the 26Si(α,ρ) and 34Ar(α,ρ) reaction rates by investigating α-unbound states in the compound nuclei 30S and 38Ca, respectively.

Topic: Direct Measurements of the 23Na(ρ,γ)24Mg Cross Section Underground at LUNA

Mr. Axel Boeltzig ( Gran Sasso Science Institute L'Aquila, Italy ) / January 25, 2016

Asymptotic Giant Branch (AGB) stars play a fundamental role in the determination of the observed abundances of light mass elements. One limiting factor for the uncertainty of AGB model predictions is the poor knowledge of the cross sections of the relevant proton capture reactions, in particular for the yield of light elements.

The reaction 23Na(ρ,γ)24Mg links the NeNa and the MgAl cycles of stellar burning. The reaction rate in AGB star scenarios is influenced significantly by the resonance of this reaction at 138 keV (assuming the most recent upper limit or value for the resonance strength from a possible observation of this resonance), the non-resonant cross section and the tails of resonances at higher energies. To constrain the reaction rate, cross section measurements over a larger energy window are required, leading to a global analysis of all available data.

Current measurements at LUNA, the Laboratory Underground for Nuclear Astrophysics, aim at a direct observation of the 138 keV resonance with a segmented bismuth germanium oxide detector, as well as the measurement of the non-resonant cross section and additional information with a High-Purity Germanium detector for low proton energies up to 400 keV.

High energy measurements up to above 1 MeV are planned at NSL to complement this data; first data taking will start soon. The current status of the experimental efforts will be presented, covering the setup and the preliminary results of measurements at LUNA as well as planned measurements at NSL.

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Topic: (α,ρ) reaction Measurements with ANASEN

Mr. Kevin Macon ( Louisiana State University ) / February 1, 2016

Type I X-ray bursts are the most common stellar explosions in the Galaxy, occurring on the surface of neutron stars in binary systems. Slow (α,ρ) reactions on certain Tz = (Z-N)/2 = +1 nuclei influence energy generation in the explosion and the time evolution of the system. We are developing and applying creative new techniques to better determine reactions that are important in stellar explosions like X-ray bursts. The principle challenge in directly measuring (α,ρ) reactions is to construct a thick helium gas target to achieve high reaction yields with maintaining good energy resolution. The Array for Nuclear Astrophysics and Structure with Exotic Nuclei (ANASEN) is an active-target/detector which is designed for direct measurements of (α,ρ) reactions with maximum sensitivity, allowing measurements with radioactive ion beam intensities of 105 ions/s. I will report on the development of ANASEN and the initial experimental campaigns in addition to highlighting other technical developments and experimental results.

Topic: Examining nuclear shell structure effects on isoscalar giant monopole resonance and nuclear
           incompressibility near A~90

Dr. Yogesh Gupta ( University of Notre Dame ) / February 8, 2016

Nuclear incompressibility is a fundamental quantity characterizing the equation of state (EOS) of nuclear matter. Experimentally, it may be determined from the compressional "breathing mode" of nuclear density oscillation, the isoscalar giant monopole resonance (ISGMR) in finite nuclei. Analyses of the ISGMR have implied that the determination of nuclear incompressibility is independent of the choice of the nucleus, provided that approximately 100% energy weighted sum rule (EWSR) fraction is exhausted in the ISGMR peak; this condition is satisfied for sufficiently heavy nuclei (A> 90).

In a recent work from Texas A & M group, it was claimed that the ISGMR strength distributions vary in a dramatic manner in nuclei near A~ 90 region. In particular, the A=92 nuclei, 92Zr and 92Mo, emerged quite disparate from the others. These results, if correct, imply significant nuclear structure contribution to the nuclear incompressibility in this mass region. To further examine these surprising and highly intriguing results, inelastic scattering of 385-MeV α particles has been measured on 90, 92Zr and 92Mo nuclei using the "Grand Raiden" spectrometer at RCNP, Japan. Results obtained from multipole decomposition analysis would be discussed.

Topic: Low-Energy 20Ne(p,γ)21Na Cross-section Study with the 5U-4 St. Ana Accelerator

Ms. Stephanie Lyons ( University of Notre Dame ) / February 15, 2016

In stars whose temperature is greater than 0.05 GK, hydrogen burning can proceed via the NeNa cycle, which in important for the nucleosynthesis of Ne, Na, and Mg isotopes. The first reaction in this cycle is 20Ne(p,γ)21Na, which also has the slowest proton capture reaction rate [1], thereby influencing the rest of the cycle and, potentially, abundances of the other nuclei that are synthesized in the cycle. The stellar reaction rate for 20Ne(p,γ)21Na is dominated by direct capture and the high energy tail of a subthreshold resonance. The aim of this work is to understand the direct-capture component of this reaction. Using Notre Dame’s recently commissioned 5U-4 accelerator, the 20Ne(p,γ)21Na cross-section has been measured relative to the 1169 keV resonance at low energies. The resonance strength of the 1169 keV resonance was also independently determined. Improvements to previous cross-section measurements will be discussed [2].

[1] Iliadis et. al. The Astrophysical Journal Supplement Series 134, 151 (2001)
[2] Rolfs et. al. Nuclear Physics A 241, 480 (1975)

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Topic: Experimental study of level density, gamma-strength and optical potentials for reaction cross section calculations

Dr. Alexander Voinov ( Ohio University ) / February 22, 2016

Accurate knowledge of nuclear structure quantities such as level densities, gamma-strength and optical potentialsis crucial for predicting reaction cross sections. Current knowledge of these quantities is based on limited experimental information. It results in large uncertainties in reaction cross section and reaction rate calculations needed in different areas of basic physics and applications including r- and s- processes in astrophysics.

Current activity and progress in experimental studies of these nuclear structure quantities will be discussed and experimental results will be presented.

Topic: Inferring neutron capture rates of short-lived isotopes

Dr. Sean Liddick ( NSCL, MSU ) / March 14, 2016

Nuclear reactions where an exotic nucleus captures a neutron are critical for a wide variety of applications, from energy production and national security, to astrophysical processes and nucleosynthesis. Neutron capture rates are well constrained near stable isotopes where experimental data are available; however, moving far from the valley of stability, uncertainties grow by orders of magnitude. This is due to the complete lack of experimental constraints, as the direct measurement of a neutron-capture reaction on a short-lived nucleus is extremely challenging. Indirect techniques have been developed to address this experimental shortcoming. The most recent indirect technique, the so called b-Oslo method, takes advantage of the beta decay of a short-lived radioactive nucleus to populate high energy states in the daughter. The subsequent gamma-ray cascade is detected using a total absorption spectrometer. Using an iterative technique to extract the first gamma-ray emitted in each cascade along with appropriate experimental normalizations, the nuclear level density and gamma-ray strength function are extracted. The two quantities are inserted in a Hauser-Feshbach model to constrain the neutron capture rate. An outline of the technique along with a recently obtained validation and extenstion to neutron-rich nuclei relating to the weak r-process will be presented.

Topic: GRETINA, the gamma-ray energy tracking array, and its recent science results

Dr. I-Yang Lee ( Lawrence Livermore National Laboratory ) / April 4, 2016

Gamma-ray detector with good energy resolution has been one of the essential instruments for the study of nuclear structure. To push these studies toward the exotic nuclei near the particle stability line, we need detectors with higher peak efficiency and good peak-to-total ratio. In addition, radioactive ion beams needed for such studies are often produced by the projectile fragmentation method. They have high velocities, and detectors must provide adequate position resolution for accurate Doppler correction. To meet these requirements, the new concept of gamma ray energy tracking array was developed. GRETINA, with 1p solid angle coverage, is the first implementation of this concept. GRETINA was completed at LBNL and started physics operation in 2012. It has been used at NSCL at MSU and ATLAS at ANL for a large number of experiments addressing diverse topics from nuclear structure to nuclear astrophysics.

In this talk I will describe the concept of gamma-ray energy tracking and the technology developed for GRETINA. A few representative experiments showing the breadth of the science and the power of the instrument will be discussed. Finally, the plan toward the full 4p array GRETA will be presented.

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Topic: Putting together the pieces of the puzzle for double beta decay

Prof. Dieter Frekers ( University of Münster, Germany ) / April 18, 2016

I will talk about efforts how to get information about the nuclear physics part involved in ββ decay, here for both decay types, the two-neutrino and the neutrinoless decay. The nuclear physics input is generally given in terms of the "nuclear matrix element", and the nuclear matrix elements are presently the most critical items to estimate the decay rate of ββ decay, in particular the one for the neutrinoless decay variant, which is the one proportional to the neutrino mass.

One piece of the puzzle can be obtained by charge-exchange reactions using hadronic probes. They give information about the Gamow-Teller (GT) strength distribution in the nucleus. Although these connect primarily to the two-neutrino type of ββ decay, there are nuclear properties being unveiled, which are equally important for the neutrinoless decay. My show-case examples will be 76Ge, 100Mo and 136Xe.

Another piece of the puzzle will be concerned with extending charge-exchange reactions to higher multipoles. I will show that hadronic processes may also be well suited for ex-tracting the weak interaction 2- spin-dipole (i.e., first forbidden) strength distribution. The spin-dipole transition connects to the operator, which appears in the description of the neutrinoless ββ decay.

Finally, I will describe a precision mass measurement performed with the JYFLTRAP system on the A=96 triplet 96Zr, 96Nb and 96Mo, where the single β decay Q-value of 96Zr has been at the center of attention. The 96Zr nucleus also undergoes ββ decay, and I will show that the four-fold forbidden single β decay can provide one of the most direct tests to models aiming at a description of the neutrinoless ββ decay.

Topic: TBA

Dr. Xilin Zhang ( University of Washington ) / April 25, 2016

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Topic: TBA

Dr. Dawn Shaughnessy ( Livermore National Laboratory ) / May 2, 2016

Topic: TBA

Prof. Nadia Fomin ( University of Tennessee ) / May 9, 2016

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