The 26g Al(p, ) 27 Si Reaction at DRAGON Heather Crawford Simon Fraser University TRIUMF Student Symposium July 27, 2005

Outline The astrophysical importance of the 26g Al(p, ) 27 Si reaction Overview of the DRAGON facility and its role in astrophysics at TRIUMF The principle of direct measurement of the resonance strength of this reaction using inverse kinematics at DRAGON Methods of beam normalization and its importance in calculating resonance strength

Why Study Astrophysics? All elements are produced through nuclear reactions in stars and novae or supernovae explosions; most are radiative capture reactions (i.e. (p, ), (n, ) or (, )) Astrophysics studies these reactions, to understand the origin of the chemical elements… to understand where we come from!

Astrophysical Importance of 26g Al(p, γ ) 27 Si Reaction 26g Al is directly observable in supernovae by orbiting gamma telescopes, due to the characteristic gamma emitted during its beta decay This allows comparison of observed nuclear abundances with values calculated using theoretical models Accurate models require good knowledge of resonance energies and strengths

Mg-Al System 26 Si 26 Mg 24 Mg 25 Mg 27 Al 26 Al 25 Al 28 Si 27 Si Characteristic MeV Gamma Ray from 26g Al Decay 4.16s Only direct method of destruction for 26g Al aside from its beta decay is radiative proton capture 7.18s0.717 Myr

Detector of Recoils And Gammas Of Nuclear Reactions (DRAGON) DRAGON is a recoil mass separator used in the study of (p, γ ) and ( α, γ ) reactions

DRAGON Gas Target and BGO Gamma Array Windowless gas target maintains H 2 or He gas at 4-8 Torr Surface barrier detectors within target detect elastically scattered protons 30 BGO detectors surround target and detect prompt gammas during reactions

DRAGON Separator DRAGON uses two stage separation; improves beam suppression and reduces background 26g Al(p, ) 27 Si: separate Si recoils from the beam and contaminants First Stage Second Stage

DRAGON End Detectors 26g Al(p, γ ) 27 Si: MCP (micro-channel plate) used in conjunction with a DSSSD (double-sided silicon strip detector) DSSSD gives number, energy, position and local timing information (with the MCP) MCP produces a signal as ions pass through (timing signal) Other experiments at DRAGON make use of an ion chamber, which gives particle ID information, timing and energy information

Direct Measurement of 26 Al(p, ) 27 Si Reaction Using Inverse Kinematics Intense radioactive 26 Al beam is incident on 6 Torr H 2 target with approximately 202 keV/u of energy Particles pass through target reaching resonance energy (188 keV in center of mass frame) near middle of target -- some react to produce 27 Si recoils, most pass straight through Recoils emerge with ~ same momentum as beam, with a small angular spread ( ~ 15 mrad) 26 Al Gas Target H2H2 27 Si γ

Reaction Rate and Resonance Strength Cosmic reaction rates are dominated by narrow resonances which occur within the Gamow window Narrow resonances are characterized by a resonance strength, ωγ Resonance strength is determined by measuring the thick target yield, given by the following equation: M = mass of target, m = mass of projectile, = de Broglie wavelength, = stopping power

Requirements for Thick Target Yield Determination Determination of the thick target yield requires accurate knowledge of: 1) Number of reactions that occur, determined through γ - heavy ion coincidence 11 recoils were observed over the entire 3 week run 2) Number of beam particles incident on the gas target, which requires… Beam Normalization

Beam Monitors within DRAGON Within DRAGON are a number of potential beam monitors: Rutherford scattered protons in gas target Current on the left mass slit Faraday cup readings Beta monitor Contamination NaI and HPGe monitors at the mass slit box Leaky beam on the DSSSD

Evaluating Possible Beam Monitors for Normalization Beta monitor, contamination detectors and DSSSD: of little use for beam normalization Current of Left Mass Slit: good beam variation profile, but prefer alternative Scattered Proton Monitor: excellent monitor when properly set Faraday cup upstream of target: best measurement of absolute beam intensity

Beam Normalization to Faraday Cup Reading Faraday cup readings are most reliable -- normalize other monitors to faraday cups Use values near beginning and end of each run to establish a normalization factor Integrate monitor responses over entire run, and use the normalization factor to determine the equivalent integrated response on the faraday cup

# scattered protons # of incident beam particles, gas pressure, 1/T 2 Rutherford Scattering into Surface Barrier Detectors in Gas Target Rutherfords Formula: Normalization factor can be determined that is independent of beam energy and gas pressure, defined as below:

Rutherford Scattering into Surface Barrier Detectors in Gas Target Take Δ t to be 300 seconds, and look at SB trigger rate; if ~ constant, calculate R with integral of proton peak for first 300 seconds, and FC reading from beginning of run. R = × Al·Torr/{proton·(keV/u) 2 }

Rutherford Scattering into Surface Barrier Detectors in Gas Target Use this value of R with integral over entire run to determine number of incident beam particles. Integral = # 26 Al particles over entire run = 1.42 × 10 12

Left Mass Slit Beam Monitor Left mass slit reads a current due to a portion of the beam being deposited there MIDAS records the current reading every 30 seconds Establish normalization factor using ratio of FC to left mass slit readings at beginning and end of each run Average Normalization Factor = 0.601

Left Mass Slit Beam Monitor Integrate left mass slit values over entire run … Then multiply by normalization factor to find integrated charge on faraday cup in coulombs, and convert to 26 Al particles… # 26 Al particles on target = 1.43 × 10 12

Comparing Normalization Methods… Comparing the two methods, we see a difference in this case of less than 1% For over 60 runs where both methods were used, the average difference was ~ 5% When one method cannot be applied, the other method can be trusted to yield an accurate normalized beam Rutherford Scattered Proton Monitor Left Mass Slit # 26 Al particles incident on target (15094): 1.43 × × 10 12

Whats Next? Calculation of the resonance strength, ωγ DRAGON has requested additional beam time for 26g Al(p, γ ) 27 Si to reduce the error on the experimental resonance strength

Summary A good knowledge of the 26g Al(p, γ ) 27 Si reaction is important in developing models for the production of 26g Al Cosmic reaction rates are determined by narrow resonance reactions; these are characterized by a resonance strength, ωγ Resonance strength can be determined directly by measuring thick target yields using DRAGON Beam normalization is critical to determining thick target yield There are a number of ways to normalize beam, which provide results in very good agreement with one another

Thanks to the DRAGON group