The {12 C(alpha,gamma) 16 O} cross section at astrophysical energies is an incoherent sum of the electric dipole (E1) and electric quadrupole (E2) components. The E1 component is dominated by a broad level at 9.6 MeV and a subthreshold level at 7.12 MeV (alpha particle threshold 7.16 MeV) and the interference between them. The E2 component is dominated by a subthreshold level at 6.92 MeV, and direct capture, and the interference between these two terms. The beta-delayed alpha spectrum of 16 N provides a unique way to determine the alpha-particle width of the 7.12 MeV level, which is crucial to our understanding of the E1 cross section at low energies.
We have measured the alpha-particle spectrum using the rotating arm apparatus built for the mass-8 beta-alpha correlation measurements. The deuteron beam (30 microA at 3.5 MeV) from the FN tandem was incident on a rotating target which consisted of {Ti 15 N} on a Ni backing. When the beam was on, the recoiling 16 N nuclei were collected by a carbon catcher foil of 10 or 20 micrograms/sq.cm. When the beam was off, the catcher was transferred to the counting area. There are two silicon surface barrier detectors in the counting area. The energies of the alpha-particles and the carbon nuclei were detected simultaneously. The slow timing between the two particles was recorded at the same time to determine the random coincidences between two counters. The lifetime of the decay, measured by clocking each event relative to the arm rotation, is found consistent with the known lifetime of the 16 N nuclei. In addition, the absolute efficiency to measure carbon ions and detector response to low energy carbon ions were also studied using the 12 C(p,p) 12 C reaction. Our results are consistent with previous measurements of TRIUMF(1) and Yale(2).
The data were analyzed in an R-matrix formalism using a 3-level approximation. In Fig. 1.1-1, we show an R-matrix fit to our data. Possible l = 3 contribution is included in the fit.
The main uncertainties in the E1 component of the {12 C(alpha,gamma) 16 O} cross section were the uncertainty in the alpha-particle width of the subthreshold 7.12 MeV level and also the uncertainty in the sign of the interference between 7.12 MeV level and 9.6 MeV level. The beta-delayed alpha spectrum of 16 N provides a unique way to determine the alpha-particle width of the 7.12 MeV level, as discussed in the previous report,1,2,3 but provides no new information on the sign of the interference in the {12 C(alpha,gamma) 16 O} reaction. The E2 component is obtained by measuring the angular distribution of the gamma rays, and the extrapolation to astrophysical energies is also largely uncertain.
Although the combined four sets of previous cross section measurements4,5,6,7 favors constructive interference in the E1 {12 C(alpha,gamma) 16 O} reaction, examination of individual data sets reveals inconsistency among them especially regarding the sign of the interference in the E1 channel. Since the alpha-particle width of the subthreshold level is determined by the 16 N data, the constructive and destructive interference in the E1 channel of the {12 C(alpha,gamma) 16 O} reaction should differ by a factor of two at energy E c.m. = 1.2 MeV. A 10% measurement at this energy should allow us to distinguish between two interference schemes. We plan to do measurements in two steps; step one is a two-detector assembly at 90 degrees to determine the E1 component (mainly the sign); step two is a measurement of the angular distribution of the gamma rays to determine the E2 component.
Recently, technological advancements have made available a high intensity pulsed ion beam and large volume germanium counters. We plan to surround the Ge counter with a NaI(Tl) annulus and plastic scintillator. The Ge counter will be running in escape mode (requiring coincidence with 511 keV gamma rays in the NaI(Tl) counter). Therefore the Ge counters provide good energy resolution, and escape gamma rays in the NaI(Tl) provide excellent timing relative to the pulsed beam. We expect a significant background reduction relative to previous measurements. Using two Ge counters (8 cm long and 8 cm in diameter), the total efficiency of this detector system is 0.001. For a 120 microgram/sq.cm. target, the expected count rate is 4.5/day at E c.m. = 1.2 MeV. The E2 contamination in the 90 degree detector arrangement is less than 10% for the entire accessible energy range. We expect the beam independent background to be negligible.
We are currently testing such a concept using a smaller Ge and BGO counters. We also studied the beam independent background in such a system. We plan to study the beam related background in the near future.
J.G. Cramer and P.B. Cramer*
Gamma ray bursts (GRB) constitute an outstanding puzzle of contemporary astrophysics. In recent satellite experiments GRB have been observed to occur in a time scale of milliseconds to seconds at the rate of a few per day from directions uncorrelated with the galactic plane and to deliver the remarkably large integrated energy of about 6 MeV per sq.cm of detector area. No radio, optical, or X-ray counterparts of GRB have been observed, lending ambiguity to their origin and distance scale. The leading scenarios for GRB source locations are: (1) GRB are at galactic halo distances, perhaps produced by unusual neutron stars or quiet supernovas, or (2) GRB are at cosmological distances, perhaps produced by merging neutron stars. If GRB are galactic and are emitted in an angular cone with a half-angle Theta (expressed as a fraction of Pi), the energy release of each GRB is 10**51 Theta**2 ergs (or, with Theta =1, roughly the mass of Mars' largest moon Phobos converted entirely into gamma ray energy). On the other hand, if GRB are cosmological, each GRB has an energy release of 10**51 Theta**2 ergs (with Theta =1, this is roughly a Jupiter-mass converted completely into gamma rays in a few seconds). In either case, both the generation and the transport of this quantity of energy constitute very formidable and unsolved theoretical problems.
We here consider the possibility of using the motion-induced dipole moment of the GRB distribution as an indication of the GRB source. COBE data shows that the Earth is moving through the microwave background radiation with a velocity of Beta (sub CB) = 1.23 x 10**-3 in the direction
II II l = 264.4º +/- 0.3º and b = 48.4º +/- 0.5º in galactic coordinates.On the other hand, the Sun moves with respect to the galactic center with a velocity of Beta (sub gal) = 7.92 x 10**4 in the direction
II II l = 91.1º +/- 0.4º and b = 0º in galactic coordinates.These vectors thus point in nearly opposite directions, making an angle of about 130º with each other.
The solid angle of a moving source is modified by its proper motion, an effect well known in nuclear and high energy physics. This solid angle modification will induce a dipole component in the GRB angular distribution of magnitude 2 Beta with its peak pointing in the direction of Beta, i.e. the GRB events will be distributed on the sky with a probability given by P(Theta,phi) = 1 + 2 Beta cos Theta. Consider an observed sample of N observations of GRB, with the ith event detected at a polar angle of Theta (sub i) with respect to the source velocity direction of interest and with an observational efficiency epsilon (sub i). The estimated boost is <Beta(sub abs)> = SUM(from i=1 to N) Omega (sub i) cos Theta (sub i) / SUM(from i=1 to N) Omega (sub i), where Omega (sub i) = 1/epsilon (sub i). Assuming Omega (sub i) =1 and treating the dipole component as small, the values of (Omega (sub i) cos Theta (sub i)) will fall on a uniform flat distribution of values between -1 and +1. The mean of such a distribution is zero and its standard deviation is {EMBED EQUATION |} = 1/{EMBED EQUATION |}. Therefore, for N observations, the observed standard deviation of {EMBED EQUATION |} will be {EMBED EQUATION |} = 1/{EMBED EQUATION |}.
We conclude that a determination of the dipole distribution at the 1{EMBED EQUATION |} level for the COBE velocity would require 220,000 GRB observations and for the galactic velocity vector would require 532,000 GRB observations. Unfortunately, the most recent catalog of GRB events contains only 1121 events. Thus, because the proper motion of the solar system is relatively small and the number of detected GRB is still quite small, it is not feasible to use the expected dipole distribution as an indication of GRB origin.
J.G. Cramer, R.W. Forward,(a) M.S. Morris,(b) M. Visser,(c) G. Benford(£) and G.A. Landis(§)
Visser has suggested a wormhole configuration, a flat-space wormhole that is framed by a variant of the cosmic string solutions of Einstein's equations with a negative string tension of -1/4G and therefore a negative mass density. The inflationary phase of the early universe might produce closed negative-mass string loops framing stable Visser wormholes or expand microscopic wormholes in the Planck-scale spacetime foam to macroscopic dimensions, thereby creating stable natural wormholes.
When a massive object passes through a wormhole, from back-reaction the entrance mouth should gain mass and the exit mouth lose mass. If the mass flow occurs in the early universe from a high to a low density region, the exit wormhole mouth could acquire a stellar-scale negative mass. We have christened such objects "gravitationally negative anomalous compact halo objects'' (GNACHOs).
We have considered the gravitational lensing of such objects as a way of detecting them. We find that the lensing of a negative mass is not analogous to a diverging lens. In certain circumstances, it can produce more light enhancement than does the lensing of an equivalent positive mass.
The intensity modulation of a background star that occurs when the GNACHO lensing mass crosses near the source-detector line of sight shows light enhancement profiles that are characteristically bounded by two caustics, each of which provides a very sharp increase in light intensity. Between these caustics is an umbra region with no transmitted light at all. This light enhancement profile is qualitatively different from that of a positive lensing mass of the same magnitude and geometry. In particular, the negative mass light curves is much sharper, showing stronger but briefer light enhancements, and a precipitous drop to zero intensity in the central region.
Our calculations show that objects of negative gravitational mass, if they exist, can provide a very distinctive light enhancement profile. Since three groups are presently conducting searches for the gravitational lensing of more normal positive mass objects, we have suggested that these searches be broadened so that the signatures of the objects discussed above are not overlooked by over-specific data selection criteria and software cuts. While the analysis of this brief report is phrased in terms of wormholes, the observational test proposed is more generally a search for compact negative mass objects of any origin. The question of whether quantum field theory is consistent with negative mass in a spacetime that is asymptotically flat and semiclassical near infinity is as yet undecided. Observation of GNACHOs would give an experimental and definitive answer to this question. We recommend that MACHO search data be analyzed for evidence of GNACHOs.
This work has been accepted for publication in Physical Review D, and is available as electronic preprint astro-ph/9409051 on World Wide Web at http://babbage.sissa.it.
E.G. Adelberger and A. Garcia*
This work was a byproduct of our study of {EMBED EQUATION |} decay at ISOLDE, which is presented elsewhere in this report. The {EMBED EQUATION |} activity was produced in a fluorinated Ti target, and data were taken with both A = 36 (for {EMBED EQUATION |}) and A = 55 (for {EMBED EQUATION |}) beams. The A = 36 beam was dominated by {EMBED EQUATION |}; the A = 55 beam was extremely pure but had such a low intensity (because the fluorine leak in the target had expired) that we detected only the intense proton group corresponding to the superallowed decay of {EMBED EQUATION |}.
The delayed proton energy scale was calibrated using well-known proton groups from {EMBED EQUATION |} and {EMBED EQUATION |} decays. We found that the superallowed {EMBED EQUATION |} group had a lab energy of {EMBED EQUATION |}keV, which corresponds to a mass excess of {EMBED EQUATION |}keV for the lowest T = 2 level in {EMBED EQUATION |}. When combined with the well-known masses of the {EMBED EQUATION |}, {EMBED EQUATION |}, and {EMBED EQUATION |} members of the isospin quintet and the less well-known mass of {EMBED EQUATION |}, our result provides one of the most precise tools of the isobaric multiplet mass equation (IMME) {EMBED EQUATION |}. The results of fitting these data to the IMME (plus extensions containing {EMBED EQUATION |} and {EMBED EQUATION |} terms) are shown in Table 1.5-1. Table 1.5-1. Coefficients of multiplet mass equation for the lowest quintet in A = 36. ______________________________________________________________________________ a (keV) b (keV) c (keV) d (keV) e (keV) {EMBED EQUATION |} {EMBED EQUATION |} ______________________________________________________________________________ -19378.4{SYMBOL 177 \f "MT Symbol"}1.0 -6043.8{SYMBOL 177 \f "MT Symbol"}1.3 200.5{SYMBOL 177 \f "MT Symbol"}0.7 1.5 0.22 -19377.6{SYMBOL 177 \f "MT Symbol"}1.5 -6044.5{SYMBOL 177 \f "MT Symbol"}1.6 199.1{SYMBOL 177 \f "MT Symbol"}1.9 0.8{SYMBOL 177 \f "MT Symbol"}1.0 2.4 0.12 -19.3771{SYMBOL 177 \f "MT Symbol"}1.5 -6043.6{SYMBOL 177 \f "MT Symbol"}1.3 197.6{SYMBOL 177 \f "MT Symbol"}2.5 0.6{SYMBOL 177 \f "MT Symbol"}0.5 1.6 0.21 -19377.1{SYMBOL 177 \f "MT Symbol"}1.5 -6039.2{SYMBOL 177 \f "MT Symbol"}3.8 195.4{SYMBOL 177 \f "MT Symbol"}3.1 -4.2{SYMBOL 177 \f "MT Symbol"}3.4 2.7{SYMBOL 177 \f "MT Symbol"}1.8
E.G. Adelberger, B.A. Brown,(a) Z. Janas,(b) H. Keller,(b) K. Krumbholz,(c) V. Kunze,(c) P. Magnus F. Meissner,(c) A. Piechaczek,(b) M. Pfützner,(£) E. Roeckl,(b) K. Rykaczewski,(c) W.-D. Schmidt-Ott,(c) W. Trinder(b) and M. Weber(b)
We have used the FRS projectile fragment separator at GSI to study the decay of {EMBED EQUATION |}. Beta delayed {EMBED EQUATION |}'s and protons were observed using the same detector setup we used for the {EMBED EQUATION |} decay study discussed in the following report.. A secondary {EMBED EQUATION |} beam with an intensity of 0.25 atoms/s was produced by a primary beam of 300 MeV/u {EMBED EQUATION |} impinging on a 1g/cm2 {EMBED EQUATION |}target. A total of 2.8 x 104 {EMBED EQUATION |} atoms were implanted during the experiment. The {EMBED EQUATION |}lifetime, {EMBED EQUATION |} ms, was extracted from the time distribution of proton events with energies above 2.5 MeV, mainly originating from the superallowed transition. We observed strong transitions to {EMBED EQUATION |} emitting states in {EMBED EQUATION |} at 1112.8(4) and 1619.0(2) keV, and to proton decaying {EMBED EQUATION |} states at 3370(41), 4287(39), 4451(33), 4687(37), 5947(47) and 6798(71) keV.
Fig. 1.6-1 compares our measured B(GT) values to shell model predictions using the USD interaction. We find a situation strikingly similar to that observed in {EMBED EQUATION |} decay(1) where the theory with "quenched" GT operators gives a good account of the strengths to transition at low energy ({EMBED EQUATION |} 3.5 MeV), but predicts much too little strength at higher energies. In fact the B(GT) integrated strength up to our experimental cutoff (see Fig. 1.6-1) agrees much better with that predicted by the unquenched theory. There are now three examples of high energy-release {EMBED EQUATION |} decays ({EMBED EQUATION |}, {EMBED EQUATION |}, and {EMBED EQUATION |}) where the shell model fails, in similar ways, to predict the distribution of GT strength. This systematic shortcoming poses an important problem for nuclear structure calculations.
This work has been published in Physics Letters B.
The observation of significant discrepancies between mirror B(GT) values extracted from {EMBED EQUATION |}{EMBED EQUATION |} decay and {EMBED EQUATION |} studies (mentioned elsewhere in this report) motivated this measurement that provided absolute intensities of the previously observed(1) {EMBED EQUATION |}-delayed proton groups and, for the first time, detected the beta-delayed gamma rays following {EMBED EQUATION |}decay. In addition we obtained a precise measurement of the {EMBED EQUATION |} halflife.
The FRS projectile fragment separator at GSI produced a secondary {EMBED EQUATION |} beam of about 30 atoms/s, from the bombardment of a {EMBED EQUATION |} Be target with a 300 MeV/u beam of {EMBED EQUATION |}. A total of 2.6 million {EMBED EQUATION |} ions were stopped in the central element of a 3 counter Si telescope that was surrounded by 2 large-volume Ge detectors. Beta delayed protons were detected in the central Si counter, and delayed gammas were detected by the Ge detectors in coincidence with a {EMBED EQUATION |} pulse in the outer two Si counters.
The {EMBED EQUATION |} halflife was obtained from the time distribution of the intense proton group corresponding to the superallowed decay. Our value of 181(1) ms is two standard deviations larger than the previously accepted result. A high-quality spectrum of {EMBED EQUATION |}-rays following {EMBED EQUATION |} decay (see Fig. 1.7-1) showed peaks at 1370.9(2), 2750.4(2) and 3239.3(2) keV with branching ratios of 2.1(1)%, 2.8(1)%, and 4.8(2)% respectively. Combining these results with the delayed proton intensities from ref. 1 we find that the 3239 keV state of {EMBED EQUATION |} has {EMBED EQUATION |}. This is a surprisingly large value for a level that lies nearly 1.4 MeV above proton threshold, and explains most of the discrepancy noted earlier regarding the mirror B(GT)'s for this level.
We have used our results to recompute the cross section for {EMBED EQUATION |} neutrinos on a {EMBED EQUATION |} detector. Our result {EMBED EQUATION |} is consistent with Bahcall's standard value of {EMBED EQUATION |}.
Comparison of the isospin analog B(GT) values from our {EMBED EQUATION |}-delayed proton work on {EMBED EQUATION |}(1) and a new, high-resolution study(2) of {EMBED EQUATION |} showed some large discrepancies. The discrepancies at low {EMBED EQUATION |} could arise if {EMBED EQUATION |}-decay competed successfully with proton decay for a few of the low-lying final states in {EMBED EQUATION |}.(3) We studied {EMBED EQUATION |}-delayed {EMBED EQUATION |} emission in {EMBED EQUATION |} at ISOLDE and found unexpectedly significant {EMBED EQUATION |}-ray branches from 2 of the unbound daughter states in {EMBED EQUATION |}.
Our {EMBED EQUATION |} source was produced using a {EMBED EQUATION |} beam from the ISOLDE general-purpose on-line isotope separator at the CERN PS/booster. The fluorinated A = 56 beam had very little radioactive contamination. In particular, it had virtually no {EMBED EQUATION |} activity that in an A = 37 beam is so intense that it would have been impossible to do our experiment. The fluorination process was very efficient; the A = 56 beam had about 30 {EMBED EQUATION |} ions per second, 50% of the {EMBED EQUATION |} intensity observed in the A = 37 beam. The {EMBED EQUATION |} beam was focused onto a three-element particle telescope that was surrounded by an annular eight-segment NaI detector that covered {EMBED EQUATION |}. We observed {EMBED EQUATION |}-delayed {EMBED EQUATION |} branches of (1.5{SYMBOL 177 \f "MT Symbol"}0.4)%, (3.6{SYMBOL 177 \f "MT Symbol"}0.8)%, and (4.4{SYMBOL 177 \f "MT Symbol"}0.6)% to {EMBED EQUATION |} levels at 1.37, 2.75, and 3.24 MeV. These results are consistent with Ge detector data taken at GSI (see Section 1.7 of this report). Combining this {EMBED EQUATION |}-delayed {EMBED EQUATION |} work with our earlier {EMBED EQUATION |}-delayed proton results1 we find the B(GT) values listed in Table 1.8-1. It is apparent that although the larger anomalies have essentially been eliminated, problems remain at the factor of 2 level.
This work was recently published in Physical Review C.(4)