Audrey Wright TUNL REU 2013 University of Texas at Austin Adviser: Dr. Calvin Howell Other Group Members: Dr. Alexander Crowell, Dr. Mitzi Boswell Evaluating.

Evaluating the Cross Section of 9Be Near Threshold

Audrey WrightTUNL REU 2013University of Texas at AustinAdviser: Dr. Calvin Howell Other Group Members: Dr. Alexander Crowell, Dr. Mitzi BoswellEvaluating the Cross Section of 9Be Near ThresholdSupernovae: Elemental Crock PotsR-process: a rapid chain of nuclei capturing neutrons to produce a heavier nucleusNeeds region with high neutron flux and appropriately high temperature

Image: NASA, Hot Topics |International Year of AstronomyKnowing this cross section gives scientists insight into an important nucleosynthesis reaction that takes place in supernovae. Supernovae are elemental crock pots. Much of the scientific community agrees that core-collapsing supernovae are the best candidate for the site of the r-process: a rapid neutron capture chain of nuclear reactions that produces many of the heavy-neutron elements on the periodic table. What makes these bodies a prime suspect for housing the r-process is their high neutron flux and appropriately high temperature that fosters nucleosynthesis but still provides stability to the newly fused heavy nuclei. The r-process takes place in a matter of seconds during a critical point in the supernova lifetime before the supernova explodes and ejects these nuclei out into the interstellar medium.

2Scientific MotivationBlue elements come from r-process in supernovae

+ 8Be 8Be + n 9Be + 9Be + 12C + n

Where r-process begins!!Image: Nerlich, Steve. "Astronomy Without A Telescope Alchemy By Supernova."Universe Today Reverse Reaction: 9Be + 8Be + nThe reaction here, 8beng9be, is the reaction that bridges a stability gap in nucleosynthesis between A=5 and A=8. This reaction also kick-starts the r-process from the previous nucleosynthesis process. The rate of this reaction is believed to set the pace for the rest of the r-process. The r-process produces many elements that are critical to our lives (all the ones you see in blue!) but the process itself is very difficult to study. We on Earth do not live in an environment with high neutron flux or one that supports the unstable nuclei that are produced intermediately in the r-process so studies into the process are very difficult. Luckily, we have a way of circumventing that! By studying the reverse reaction 9begn8be we can learn about the forward reaction 8beng9be. Luckily, we also have a High Intensity Gamma Ray source to provide collimated, polarized gamma beams to drive this reaction.

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9Be Energy LevelsTransitions from

and their relative contributions to the reaction 9Be + 8Be + n

Also the contribution of 9Be + 5He+ +n

Image: TUNL Nuclear Data Evaluation , 2004In the experiment, were going to look at these spin states of the 9Be nucleus and the transition from each state to 8Be. The goal of this experiment is to determine the cross section for each of these reactions and also the relative contributions of each in the overall cross section of 9Be in the r-process. We will use gamma ray energies that correspond to each of these transitions to make our measurements. We use a quantity called Asymmetry to determine the cross section. We will also study the contribution of the reaction 9be(g,n)2alpha to the production of 9Be near threshold as well. 5The ExperimentSetup:

(gamma beam goes into page)Tested at many -energy levelsTwo possible reactions: 1: 9Be + 8Be+n2: 9Be + + + n

NeutronDetector || NeutronDetector9BeDetector 2: =90, =90Detector 1: =90, =0The experiment was conducted in the winter of 2003 by a then-grad student Mitzi Boswell. Here is the setup of the experiment: a 9be sample is placed along the axis of an incoming well-collimated gamma beam. In the plane of the beams polarization we have one liquid scintillator and there is another in a plane perpendicular to the beams polarization. The asymmetry between the neutrons detected in each plane is proportional to the cross section of the reaction. Many gamma energy levels were used in the experiment that probed each spin state mentioned previously. Also, we were able to collect data for both possible nuclear reactions of 9Be. The use of organic liquid scintillators gives us the ability to distinguish between different types of particles that interact with our detectors: gamma rays will undergo Compton scattering while neutrons will scatter off of nuclei in the scintillator. With these sensitive readings, we can separate neutrons coming from the decay of 9Be to 8Be and two alpha particles AND we can distinguish noisy gamma rays from our neutron samples easily.

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My purpose: Data AnalysisAsymmetry

N|| and N are the integrals beneath the main peaksContributions from each channelChannel 1: 9Be + 8Be+n Channel 2: 9Be + ++n

Energy increases With the experiment done, all that was left to do is data analysis and then to draft up the paper. This summer, I helped resurrect this project and take it steps closer to completion by analyzing the data left behind. The data came in the form of histograms compiled Paw++, the precursor to Root. Ultimately we want to compute asymmetry between the event counts in the perpendicular detector and the parallel detector. To get these counts, we must separate the counts from the two decay channels and then compare this extracted data. We will use time of flight histograms to analyze our data. Time of flight is proportional to the energy of the detected particles so we can separate parts of this histogram into different sets of events. On this diagram, energy increases from left to right. The main peak you see is the neutron peak from 9Be decay to 8Be because this reaction has a higher energy than the neutrons from the two-step transition via the 5He decay channel.

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Data Analysis (cont.)First, we make cuts on the data. N-peakAmBe calibration source for n detection (Pulse Height)Pulse Height vs Pulse Shape SpectrumPHPSPHBefore we make any of those calculations though, we must first make the proper cuts on our data to focus our analysis only on the decay events and block out noise from background or the gamma ray. We used a 59 keVee AmBe source to calibrate the neutron detectors because it emits a large amount of neutrons that are similar in energy to those we will encounter in our actual experiment. We used a pulse height spectrum from the AmBe source to determine the range of pulse height we would observe. Then, with this lower bound already in place, we shift the cut on our pulse shape distribution to take up as much of this neutron cloud as we can and eliminating much of the noise from this gamma detection.

8Data Analysis (cont.)

Energy increases CountsThen we move on to our time of flight spectrum. As you can see, the main peak has a Gaussian shape so I fit each histogram from each detector for each geometry and gamma energy levels to a normal distribution using a program in Fortran. Then I calculated the integral of each of these fits and compared that value with the area under the full foreground to obtain the number of events that occurred for the 5He decay channel. All of this information is compiled into a gigantic spreadsheet that represents basically all of the work I did this summer.

9Preliminary ResultsPrediction from EM Multipole Analysis of 9Be(g,n)Ex (MeV) JEM Channel A(90)4.703/2+E18Be + n-0.753.055/2+E18Be + n+0.502.801/2-M18Be + n 0.002.435/2-M1E28Be + n8Be + n -0.50+0.285/2+5/2-AsymmetryPreliminary results of the measured asymmetry at theta = 90 degrees as a function of gamma-ray energy is shown in this plot. The asymmetry was computed using the geometrical average of the counts collected in two detector geometries. The error bars represent the statistical uncertainty. The table insert is the predicted asymmetry from a multiple analysis of the photodisintegration process assuming the 8Be + n final channel. The measured asymmetry is consistent with the predictions of the multipole analysis. For the narrow 5/2- resonance at 2.43 MeV, the data suggest that the electromagnetic transition is E2.

that gigantic spreadsheet and weeks of fitting histograms in Paw++ is this set of data: the Asymmetry. We calculated several different Asymmetry values. What you see here is a value of asymmetry that minimizes discrepancy between the two neutron detectors. (Discuss that giant error bar?).

10SummaryNext stepsOne step closer to understanding the world and how we came to be here!Acknowledgements The next step is to convert this Asymmetry into a cross section and then to publish the work!With that knowledge, were one step closer to understanding the mysterious r-process and more about our universe and all its wonder.

11ReferencesImages:"Hot Topics | International Year of Astronomy 2009."NASA International Year of Astronomy 2009 - IYA - Events, Topics, and Discoveries. NASA, 1 May 2009. Web. 30 July 2013. http://astronomy2009.nasa.gov/topics_nov.htm,Nerlich, Steve. "Astronomy Without A Telescope Alchemy By Supernova."Universe Today Space and astronomy news. N.p., 14 Aug. 2010. Web. 30 July 2013. ."Energy Level Diagrams A=9."Triangle Universities Nuclear Laboratory : 2013. N.p., n.d. Web. 30 July 2013. < http://www.tunl.duke.edu/nucldata/figures/09figs/menu09.shtml#1959>. Cowan, John J., and Friedrich-Karl Thielemann. "R-Process Nucleosynthesis In Supernovae."Physics Today57.10 (2004): 47. Print.Cross-Section Measurement of 9Be( ,b) 8Be and implications for + + n 9Be in the r process C. W. Arnold, T. B. Clegg, C. Iliadis, H. J. Karwawski, G. C. Rich, J. R. Tompkins, C. R. Howell. Physical Review, c 85, 2012 Measurement of the 9Be(g,n) Cross Section Near Threshold;M. Boswell, A.E. Champagne, A.S. Crowell, C. R. Howell, H. J. Karwowski, A.P. Tonchev. TUNL Progress Report XLIII 2003-2004 p. 135, Section 9.4.1TUNL Nuclear Data Evaluation Project: http://www.tunl.duke.edu/nucldata/ (Jill!)

Thank you!Any Questions?