University of Colorado Nuclear reactions in stars, and how we measure their reaction rates in the lab. R. J. (Jerry) Peterson University of Colorado Jerry.Peterson@Colorado.edu

“Kitchens in the Cosmos” Nuclear fusion reactions generate the energy that drives stars. Nuclear fusion reactions in stars generate many heavy nuclei, up to A=56 (14 times 4). Stellar explosions, initiated by gravity, create heavier nuclei, with high fluxes of neutrons, protons, alpha particles (4He). All of these reactions involve ‘reaction rates’ which increase with temperature. Source of that fusion energy?

Curve of binding energy

A better curve of binding energy fusion fission

Fission Fission energy is from the electrostatic repulsion of two positive nuclei formed by nuclear fission.

The Coulomb barrier to fusion Like charges repel, so positive particles need to have kinetic energy to get near enough for nuclear processes to happen. This is a barrier penetration problem.

Maxwell Boltzmann thermal distribution T9=temperature in 109 K

keV

Coulomb barrier penetration probabilities=cross sections GK=109K =T9

S-factor These reactions are dominated by the Coulomb barrier, so remove it— s(E)= S(E) exp(-2ph) / E h=Sommerfeld parameter=Z1Z2e2 / (4pe0) hbar v v= relative velocity = SQRT(2E/A) E= center of mass energy A=reduced mass Numerically s(E)=S(E) exp(-0.9895 Z1Z2 SQRT(A/E)/E Units of S(E)—MeV barn

P-P chain 26.73 MeV

The CNO Cycle 26.73 MeV

Reaction Rates [AB] = R(T) [A] [B] [13N] = R(T) [proton] [12C] Example— 12C (p.g) 13N (g.s.) The start of the stellar CNO cycle to burn hydrogen to helium in stars. [13N] = R(T) [proton] [12C] In nuclear units—R(T) = NA <s(E) v>, in cm3 mole -1 sec -1 s(E) = radiative capture cross section v= relative velocity NA = 6.02 x 10 23—Avogadro’s number. Averaged over a thermal distribution of energies/velocities at temperature T. NA <s v> = NA (8 / p m (kT)3)1/2 ∫0∞ E exp(-E/kT) s(E) dE

Nucleosynthesis pathways Neutron slow process—capture a neutron on A, to A+1, which may decay. Repeat. Neutron fast process- capture a neutron on A, and before A+1 can decay, capture another. Repeat to the drip line. Proton process (there are lots of protons in stars, with mechanisms to accelerate them)-capture a proton on A, making A+1 Alpha process (there are also many alphas in stars)-capture an alpha on A to make A+4. (More Coulomb repulsion, but efficient)

The usual way Make a thin sample (thin is hard at low beam energies, since energy losses dE/dx are large) Put a single energy proton or alpha beam on the sample. Measure the cross sections ds/dW at some angles to integrate over all angles for s for that reaction. Repeat at more energies. Integrate <s v > for the reaction rate.

Thick TargetYields Beam into a thick sample, stopping the beam. Measure number of final radioactive nuclei produced, by off-line gamma counting. On a natural target, several reactions may be measured, with different gamma energies and halflives.. Yield= final nuclei per beam particle, at energies from the beam energy down to zero. Y(E) = NA/A ∫0 Ebeam [-dE/d(rx)] -1 s(E) dE

Thermonuclear Reaction Rates at any T NA <s v> = NA (8 / p m (kT)3)1/2 ∫0∞ E exp(-E/kT) s(E) dE = ∫0∞ W2(E) s(E) dE And yield: Y(E) = NA/A ∫0 Ebeam [-dE/d(rx)] -1 s(E) dE = ∫0Ebeam W1(E) s(E) dE Integrate by parts: NA <sv> = ∫0∞ Y(E) d[-W2(E)/W1(E)]/dE dE = ∫0∞ f(E) dE NA<sv> = ∫0Emin f(E) dE + ∫EminEmax f(E) dE + ∫Emax∞ f(E) dE. Few protons above Emax, small cross sections below Emin for a given T9. Thanks to Paul Ingalls

The experiment Measure thick target radioactive yields Y(E) from Emin to Emax, by counting protons into a thick (stopping) target and counting the radioactivity of the final nucleus in a well-calibrated gamma spectrometer. From tables of dE/dx in that material, and for some assumed Maxwell-Boltzman temperature, compute R(T). Vary that temperature until the uncertainties due to unmeasured energies gets too large.

Realities We used a variable energy (28 MeV protons) cyclotron on odd harmonics to reach proton energies down to a very few hundred keV. A cyclotron is a splendid momentum analyzer. Correct for radioactive half-lives during the bombardment and counting. Count gammas several times, to be sure of the right half-life.

12C(p,g)13N First ingredient of the CNO cycle 13N decay with t1/2=9.97 minutes by positron emission (no nuclear gamma rays) That positron annihilates on a regular electron to make two 511 keV photons Cross sections also well determined by thin target methods

First Result 12C(p,g)13N 10-9 Yield 13N per proton 1 MeV

What are the jumps? The yield must be a monotonically rising curve. At resonant energies exactly, cross sections rise. A test of the method, and a calibration.

Thin target (usual) data. 1 MeV Rolfs and Azuma :Nucl. Phys. A 227, 3 (1974).

Phys. Rev. C 64, 065804

Typical data

Danger! 56Fe(p,g)57Co (271.1 days) (91.7%) Or 57Fe(p,n)57Co (2.1%) Opens at lab. energy of 1.65 MeV. Nuclear Structure— Isobaric analog state at Coulomb energy difference from target ground state. At a resonant beam energy of 1.275 MeV for the p-wave ground state analog.

95Mo(p,g)96Tc vs 96Mo(p,n)96Tc Pure 95Mo

Thermonuclear Reaction Rates Recap NA <s v> = NA (8 / p m (kT)3)1/2 ∫0∞ E exp(-E/kT) s(E) dE = ∫0∞ W2(E,T) s(E) dE And yield: Y(E) = NA/A ∫0 Ebeam [-dE/d(rx)] -1 s(E) dE = ∫0Ebeam W1(E) s(E) dE Integrate by parts: NA <sv> = ∫0∞ Y(E) d[-W2(E,T)/W1(E)]/dE dE = ∫0∞ f(E) dE NA<sv> = ∫0Emin f(E) dE + ∫EminEmax f(E) dE + ∫Emax∞ f(E) dE.

C. Angulo et al., Nucl.Phys. A 656, 3 (1999) 12C(p,g)13N C. Angulo et al., Nucl.Phys. A 656, 3 (1999)

Test vs. newest thin target data N. Burtebaev et al. Phys. Rev Test vs. newest thin target data N. Burtebaev et al. Phys. Rev. C 78, 035802 (2008) 12C(p,g)13N

Ditto-- 4He-induced reactions 2H,3He-induced reactions for plasmas (UCB and CSM) (backwards, to determine temperatures) Proton yields, reaction rates, N A Roughton et al. Atomic Data and Nuclear Data Tables 23, 177 (1980) 4He yields and rates N A Roughton et al., At. Data Nucl. Data Tables 28,341 (1983)

Conclusions- The thick target method has been used to mass-produce nuclear reaction rates with protons and 4He, measuring the induced radioactivity, with half-lives as short as 0.6 sec (41Sc). Results agree with other methods.

acknowledgements Norbert Roughton, Clyde Zaidins, Carl Hansen, Martin Fritts Tom Intrator/plasma diagnostics. F. E. Cecil (CSMines)