Twenty micrograms of 26Al was used to produce two types of Al2O3 targets by evaporation of the oxide. 26Al, which requires the so-called p and/or rp processes for substantial 26Al production. Subsequent discoveries of the 1809-keV radiation in the galaxy imply an equilibrium 26Alj27Al ratio of about 10-5, which is about a factor of 50 higher than what can be explained by supemova production (CLA87).
At peak temperatures used in supernova calculations (Tg ~ 3), statistical models for the reaction rates are generally reliable due to the large number of states involved. However, in places with lower temperatures, experimental knowledge of the reaction rates becomes crucial. In general, experimentally determined reaction rates in the Mg-Al region can seriously conflict with model calculations below about Tg = 0.5. Four of these key nuclei are stable, and the availability of suitable targets has allowed significant progress to be made in determining their reaction rates.
If analogous states could be identified, knowledge of the spin in 27Al would constrain the lp in the 26Al(p, I')27Si reaction. Since the reduced proton widths of the low-lying resonances seen in Wang's work are unknown, such a comparison is impossible. Better limits for the lower states could be obtained with the 26AleHe, d?7Si reaction, which should proceed as a first-order direct proton transfer, giving an angular distribution typical of /-transfer and having a cross section proportional to the reduced proton width.
WANB9
No background was expected from the Pt supports due to the high Z . However, boron was found as a contaminant in Pt and proved troublesome. They were then brought to red heat from the opposite side with a Bunsen burner to convert the aluminum to its oxide. Disassembling the targets and heating them to red heat in the atmosphere removed the carbon as expected.
If the carbon were on the surface, the target Al20 3 profile should be preserved (ignoring the barrier on the surface carbon) although shifted to higher beam energies due to energy loss in the upper carbon. After a final wash in chemistry to remove the oxalate, the Ah03 was taken up in 10 pl of 3% HCl and transferred to a small well in the carbon boat. A target would occasionally be destroyed if the carbon separated from the copper when exposed to 2200 oc boat, but a 70%.
Turning the target shifts the peaks of Al and 0 by the difference in thickness of the two carbon films. Due to the previously described targeting problems, the € -dependent resonant efficiency of Y was suspect. Indeed, all 26Al(p, 1 )21 Si resonances observed with a germanium detector appear as a function of the 2164-keV line excitation.
High Voltage
Current Integrator
- target considerations
- experimental setup
- analysis
The angular distribution of the outgoing deuteron reflects the [-transfer from the proton, while its energy identifies the residue. Since the flux of each deuteron group is proportional to the proton width of the residual state, we have the basic ingredients to determine r P •, i.e. En l, and something proportional to ()2. CHASJB) argue that the uncertainty arising from the model dependence of the extracted C2S in special cases can be avoided by scaling up to the C2S of known states.
A plot of E versus b.E allowed the identification of the exiting particles (Figure 4.2), while the position of the focal plane determined their momenta. Because of the large background elsewhere, no new information could be obtained for the other states in 27Si. The initial shapes for these arrays were derived from the reference-target fit, which was constrained to give the known relative strengths of the 28Si arrays.
The fit center of the 7653-keV band at 5 degrees reflects the limitations of this analysis; however, the centroid of this peak obtained by direct subtraction is consistent with a target mass of 26.). The data would not allow definitive extraction of [-transmissions, although some pure [-transmissions could be excluded. This precluded the use of the scaling arguments given by Champagne et al. An upper limit was set for the 7589 keV group, which had never been observed, assuming l = 0.
The significance of a proton resonance at 196 keV prompted further experiments to determine the spin of the 7653-keV state in 27Si and thus determine the proton's !-transfer and consequent Coulomb barrier penetration factor.
This, and the realization of efficiencies less than 100%, means that at higher beam energies where many background resonances have significant yield (Figure 3.3), the possibility of extracting identifiable lines becomes questionable. Natural target activity (~ 90 differences/sec), background events from the chamber, 22Na beam contamination from a previous unrelated, cosmic experiment. To accurately predict the observed curves, it was necessary to include details of the geometry, including the detector's stainless steel jackets, the reflective material surrounding the crystals, and the details of the target chamber.
Requiring coincidence between two detectors removed much of the boron background from the region of interest and provided a statistically clearer resonance shape. The integrated area of the excitation function A could be determined for most resonances by simple background subtraction. However, due to the large background for the 196-keV resonance, it was necessary to create a template that could then be adapted to the excitation function by varying only the height and a linear background (Figure 5.5).
For the 196 V resonance one can see in Figure 5.9 that most of the spectrum can be accounted for by the triple cascade determined above; the line at 2.9 MeV requires a different deexcitation mode. 406-ke V resonant returns from the target to that of a solid 27Al blank, removes any dependence on the resonance strength, but requires knowledge of the dE/dx of protons in Al. To verify that it arose from 26Al(p, r?1Si), the total energy spectrum resulting from this coincidence requirement was examined at the peak of the resonance yield.
By removing the rear shield (not shown), the inner stand could be rolled out of the paraffin housing to facilitate target changes.
Paddles Nal Detectors
Based on the comments of Prantzos and Casse, we have determined the temperature below which, for a given density, the upper limit (crv) for the 26Al(p, 1)27Si reaction would allow beta decay of 26Al to dominate destruction. To determine this temperature, the effective beta-decay rate must be evaluated by including ground-state coupling to the isomeric state at 228 keV, which has a half-life of 6.4 seconds. Ward and Fowler (WARSO) showed that due to the short half-life of 26Alm and in the absence of other destruction reactions, 26Alo and 26Alm fall out of thermal equilibrium below T9 ~ 0.4.
Using their estimates of the branching ratios of the 417- and 1058-keV levels, we find that the initially formed 26Al 'in the ground state has an effective beta decay rate given by. This rate is equated to the proton decay rate for the opaque medium to provide the curves in Figure 6.2. In the region below these curves, the natural decay of 26Al is the main destruction pathway, making the details of the proton reaction rate irrelevant.
The significance of the measured (uv) also depends on the dynamics of the stellar environment: the reaction can be ignored if its rate is slow compared to other time scales. The lifetime of the nucleus versus reaction with a proton given by (pXHN Akv})-1 also creates a lower limit on the equilibrium time for flux through the reaction network. The lifetime of 26Al against protons and beta decay is given in Figure 6.3 for different environments.
Unfortunately, even when the time scales of nuclear reactions are short compared to thermodynamic changes, the effects of delayed j3 decay, details of freezing and depletion of seed material in the Mg-Al region require dynamic calculations to obtain realistic calculations. Results.
In two separate balloon flights, the field of view determined by the antiincidence detectors was rv 15° in one case and rv 87° in the other. With the detector pointed at the galactic center, the difference in the net 1809-keV 1-beam count rate between these flights was 120%—not consistent with a point source. A reanalysis of the Mahoney, Share, and MacCallum data assuming a point source gave an equivalent current flow about five times smaller than that reported by Ballmoos et al.
For reference, they also present the high-energy 1-ray distribution obtained from COS B which was used in the initial analysis of the 1809-keV 1-rays by Mahoney et al. If that source were novae, it would require a rate of 3,000 per year in the central region of the galaxy—75 times more than expected for the entire galaxy. Whether such an efficient conversion of 25Mg to 26Al is possible depends mainly on the reaction rates of 25Mg(p,1)26AI and 26Al(p,1)27Si, while the feasibility of using
Current limits for these rates, normalized to the analytical expression given for the 26Al(p, 1 )27Si rate in section 6.1, are presented in Figure 6.6. The importance of the 26Al(p, 1)27Si reaction rate in determining the origin of the 26Al observed in our galaxy led us to re-examine the 26Al(p, 1 )27Si generation function below 1 MeV, which resulted in a significantly better determination. of (O'v) at the lower temperatures. The limits from A) are normalized to our statistical model calculations for the reaction rate.
The dotted line is given by the analytical expression found in Section 6.1 and represents our best "guess" for the actual rate.
Average lifetime of 26Al0 • This figure illustrates the time scales involved for the destruction of 26Al either by protons or through (3+ decay. Resonance energies for the current work were scaled from energies determined for 27Al(p, 1)28Si resonances of (MAA7s ), as reported in (END78) It then asks for cascade from a resonant level and tracks each possible cascade sequence along with its probability.
These differences account for the different branching ratios we obtained compared to Buchmann et al. For the 793 keV resonance, the 6065 keV line corresponding to the transition to the 2164 keV level does not appear. For the 859 keV resonance, the 5382 keV line corresponding to the transition to the 2910 keV level does not appear.
Furthermore, the lines at 1383 and 1538 ke V had excessive returns (about double) for the presented decay scheme. The slope voltage window used to define the 929 keV resonance spectrum was then indicated in this fit and the fraction of the 2910 keV yield due to the lower resonance was determined. This quantity was compared to the 2910 keV yield in the 928 keV resonance spectrum, and their ratio was used to correct all other lines.
So, for the weaker branches we assign a 50% uncertainty, while for the stronger ones we assign a typical "unknown" strength of 20% to each.
JOH 72
KAY 60 KOU 74
LEI 85 MAA 78
MAH 84
MAH 85
Mayer-Hasselwander et al., “Large-scale distribution of galactic gamma-rays observed by COS-B,” Astr. Casse, “On the Production of 26Al by Wolf-Rayet Stars: Galactic Yield and Gamma-ray Line Emissivity,” Ap. Schmalbrock et al., “Proton threshold states in 27Si and their implications for the hydrogen combustion of 26Al,” Nucl.
