Search Results (63 total)

We have set limits on contributions of scalar interactions to nuclear β decay. A magneto-optical trap provides a localized source of atoms suspended in space, so the low-energy recoiling nuclei can freely escape and be detected in coincidence with the β. This allows reconstruction of the neutrino momentum, and the measurement of the β-ν correlation, in a more direct fashion than previously possible. The β-ν correlation parameter of the 0⁺→0⁺ pure Fermi decay of ³⁸K^m is a˜=0.9981±0.0030+0.0032 / -0.0037, consistent with the standard model prediction a˜=1.

A new technique, full neutrino momentum reconstruction, is used to set limits on the admixture of heavy neutrinos into the electron neutrino. We measure coincidences between nuclear recoils and positrons from the beta decay of trapped radioactive atoms and deduce the neutrino momentum. A search for peaks in the reconstructed recoil time-of-flight spectrum as a function of positron energy is performed. The admixture upper limits range from 4×10^-3 to 2×10^-2 and are the best direct limits for neutrinos (as opposed to antineutrinos) for the mass region of 0.7 to 3.5 MeV.

Proton spectra from the ²⁰Ne(n,p)²⁰F reaction induced by 298 MeV neutrons have been measured for angles between 14° and 32°, angles at which the excitation of the stretched 6^- states is expected to be observed. The results are presented and are compared with those from the ²⁰Ne(p,n) reaction study of Tamimi et al. [Phys. Rev. C 45, 1005 (1990)] and the calculations of Carr et al. [Phys. Rev. C 45, 1145 (1990)].

Using the charge exchange facility and second arm spectrometer at TRIUMF, we have measured the ³¹P(n,p) double differential cross section for 198 MeV incident neutrons, at scattering angles of 0–30°, and excitation energies up to 30 MeV. Via a multipole decomposition analysis we have extracted both the Gamow-Teller strength distribution and the GT transition probabilities to low-lying ³¹Si bound states. Comparison to a shell model calculation, using the full 1s-0d space and universal SD interaction, shows reasonable agreement in the strength distribution, but reduced summed strength in the experiment (∑B_GT=2.32±0.20 for E_x<~10 MeV) relative to the theory (∑B_GT=3.33 for E_x<~10 MeV). We discuss the role of configuration mixing and Pauli blocking on the ³¹P³¹Si GT strength distribution.

Double differential cross sections from the ¹²⁰Sn(n,p)¹²⁰In and ¹⁸¹Ta(n,p)¹⁸¹Hf reactions at 298 MeV and the ²³⁸U(n,p)²³⁸Pa reaction at 318 MeV have been measured for excitation energies up to 50 MeV in the residual nucleus. These data, together with the previously published data from the ⁹⁰Zr(n,p)⁹⁰Y and ²⁰⁸Pb(n,p)²⁰⁸Tl reactions at 198 MeV, have been analyzed for spin-isovector resonances of multipolarities less than 7, using the multipole decomposition method. The strengths due to spin-isovector excitations of multipolarity less than 4 have been extracted. The anomalous behavior of the extracted spin-isovector quadrupole strength with target mass number is discussed with reference to the calculations of Leonardi et al. The cross section due to quasifree processes was calculated and subtracted from the data. The data after this subtraction were reanalyzed for spin-isovector resonances and the strengths due to multipolarities up to 3 were extracted. The strengths due to spin-isovector dipole and octupole excitations were compared to values calculated, for 1ħω transitions only, using the sum rules of Macfarlane. The behavior with target mass number is well represented by these sum rules.

Cross sections have been measured up to 35 MeV excitation for the ⁷⁶Se(n,p)⁷⁶As reaction at five laboratory angles between 0° and 15°. The incident neutron energy was 198 MeV. The distribution of Gamow-Teller (GT) strength at excitation energies less than 10 MeV was deduced by carrying out a multipole decomposition of the data. The GT strength below 6 MeV excitation was found to be less than 0.9 units, in qualitative agreement with a quasiparticle random-phase approximation calculation. The present results have been compared with data from the ⁷⁶Ge(p,n)⁷⁶As reaction to obtain an estimate of a lower limit for the lifetime of ⁷⁶Ge for ββ decay. The estimate is in qualitative agreement with the measured lifetime.

We report measurements of the ²³Na(n,p) differential cross section at an incident energy of 198 MeV and angles from 0° to 24° using the TRIUMF Charge Exchange Facility. From these data we determine Gamow-Teller (GT) transition probabilities to low lying 1/2⁺, 3/2⁺, and 5/2^{+ 23}Ne states and the GT⁺ strength distribution up to 25 MeV excitation energy. The values of B_GT⁺ to discrete states, and the GT⁺ strength below 10 MeV, are found to be in reasonable agreement with a full 1s-0d shell model calculation with a normalization factor of about 0.74. The GT⁺ strength above 10 MeV suggests the removal of strength from lower to higher excitation energies. We also compare the ²³Na(n,p) data with ²³Na(μ^-,ν) data and find agreement between the values of B_GT⁺ to discrete levels extracted from the (n,p) and (μ^-,ν) reactions. The general consistency of the (n,p) and (μ^-,ν) data, and the full 1s-0d shell model calculation, give confidence in a recent extraction of the weak pseudoscalar coupling from μ^- capture on ²³Na. Finally, using both β^--decay and μ^- capture data, we obtain unit cross sections from the ²³Na(n,p) measurement.

Double differential cross sections and analyzing powers over the quasielastic region have been measured using the (p→,n) reaction on ¹²C and ^natPb. The data were obtained at proton energies of 495 and 795 MeV and cover momentum transfers from 1.0–2.1 fm^-1 at 495 MeV data and from 0.0–3.5 fm^-1 at 795 MeV. The cross section data are compared to results from a nonrelativistic reaction model that includes contributions from two-step scattering. The analyzing power measurements are compared to a recent relativistic plane-waves model.

Measurements of the (n,p) reaction cross section on ^60,62,64Ni have been made at 198 MeV for θ_lab=0–20° in 4° steps. Multipole analyses have been performed on the resulting spectra and Gamow-Teller (GT) strength distributions have been obtained. These GT distributions have been found to occur as strong peaks at low excitation energy in the final Co nuclei and extend weakly upwards to 30 MeV of excitation. Comparisons of the data with renormalized shell-model calculations show reasonable agreement at low excitation, though the calculations predict very little strength at excitation energies above about 6 MeV.

Cross sections for the reactions ⁵⁵Mn(n,p), ⁵⁶Fe(n,p), and ⁵⁸Ni(n,p) have been measured at an incident energy of 198 MeV, with protons observed over a range of energies corresponding to excitations of up to about 35 meV in the residual nuclei ⁵⁵Cr, ⁵⁶Mn, and ⁵⁸Co, respectively. Measurements were carried out at center-of-mass angles between 1.6° and 19.9°. A multipole analysis of the results yielded the distribution of Gamow-Teller (GT) and spin-dipole strength for each target. The total GT strength below an excitation energy of 8.5 MeV was 1.7 units for ⁵⁵Mn, 2.8 units for ⁵⁶Fe, and 3.8 units for ⁵⁸Ni. Shell model calculations of the GT strength distribution, carried out in a restricted vector space, show fair to good agreement with the data up to an excitation energy of 8.5 MeV, but overestimate the total strength by a factor of between 3 and 4.

The (n,p) reaction has been studied on the nuclei ⁵¹V and ⁵⁹Co at an energy of 198 MeV. Spectra were measured at laboratory angles of 0°, 4°, 8°, 12°, 16°, and 20° up to an excitation energy of 35 MeV in the final nuclei ⁵¹Ti and ⁵⁹Fe. A multipole analysis of the data up to 30 MeV was carried out to identify Gamow-Teller (ΔL=0,ΔJ^π=1⁺) and spin dipole (ΔL=1,ΔJ^π=0^-,1^-,2^-) strengths. GT strength is concentrated in a resonance with centroid energy of 5.2 MeV in ⁵¹Ti and 4.1 MeV in ⁵⁹Fe. The spin-dipole strength appears as a broad resonance with centroid energy about 16 MeV in both nuclei. Shell model calculations of the GT strength reproduce the energy distribution reasonably well, but the calculated strength exceeds the measurement by a factor of about four.

Differential cross sections were measured and compared to distorted-wave impulse-approximation calculations for the J^π=4^-(νd_5/2,πp_3/2^-1) state excited in the ¹²C(n,p)¹²B (4.52 MeV) reaction, the J^π=6^-(ν,f_7/2πd_5/2^-1) state excited in the ²⁸Si(n,p)²⁸Al (5.175 MeV) reaction, and the J^π=10^-(νh_11/2,πg_9/2^-1) state excited in the ¹²⁰Sn(n,p)¹²⁰In (≃0.0 MeV) reaction at 300 MeV. We obtained a normalization factor of 0.68 for the J^π=10^-(νh_11/2,πg_9/2^-1) state excited in ¹²⁰Sn(n,p)¹²⁰In (≃0.0 MeV) reaction; this is the first ‘‘1ħω’’ stretched state known in this mass region.

A complete set of polarization-transfer observables has been measured for quasifree (p→,n→) reactions on ²H, ¹²C, and ⁴⁰Ca at a bombarding energy of 495 MeV and a laboratory scattering angle of 18°. The data span an energy-loss range from 0 to 160 MeV, with a corresponding momentum transfer range of q_c.m.=1.7–1.9 fm^-1. The laboratory observables are used to construct partial cross sections proportional to the nonspin response and three orthogonal spin responses. These results are compared to the transverse spin response measured in deep inelastic electron scattering and to nuclear responses based on the random phase approximation. The polarization observables for all three targets are remarkably similar and reveal no evidence for an enhancement of the spin-longitudinal nuclear response relative to the spin-transverse response. These results suggest the need for substantial modifications to the standard form assumed for the residual particle-hole interaction.

Cross sections for the ³He(n,p) reaction were measured at angles of 2.7°, 7.5°, 14.4°, 28.2°, and 34.9° (c.m.) at an incident beam energy of 288 MeV. Outgoing protons were observed to energies corresponding to a Q value of -40 MeV. The reduced cross section for the Gamow-Teller component of the ground-state isobaric-analog transition was determined to be σ^=σ(q=0)/B_GT=6.9±0.2 mb/sr. This value is significantly smaller than that observed for nuclei in the mass range 6≤A≤13. A multipole analysis of the data indicates that the cross section of the continuum up to a Q value of -20 MeV involves mainly ΔL=1 spin dipole transitions. The analysis determines an upper limit of 0.06 units of Gamow-Teller strength in transitions to excited states between 7 and 16 MeV, but is consistent with no Gamow-Teller strength in this region. Distorted wave impulse approximation calculations using transition amplitudes from a large-scale shell model calculation provide a good fit to the measured ground-state cross section, but fail to account for the magnitude of the measured cross sections to the continuum below 30 MeV excitation.

The (p,n) reaction has been measured for ¹²C and ⁵⁴Fe targets at 290 MeV for a laboratory angle of 20.4° and at 420 MeV for a laboratory angle of 24.0°. This paper presents new cross section data and analyzing powers at excitation energies near the quasifree peak. The data are compared with recently developed nonrelativistic calculations and with extended relativistic calculations that include random phase approximation correlations.

Results from a complete set of polarization-transfer observables for quasifree (p→,n→) scattering at 495 MeV are reported. Measurements were carried out on CD₂, carbon, and calcium targets at a laboratory scattering angle of 18° using the new neutron time-of-flight facility at LAMPF. The ratio of spin-longitudinal to spin-transverse responses at a momentum transfer of approximately 1.72 fm^-1 is extracted from the data and compared to distorted-wave calculations incorporating a random-phase approximation description of the nuclear response. No enhancement is observed in the experimental ratio.

We studied the energy dependence of the excitation of the ‘‘1ħω,’’ J^π=6^-, state in the ²⁸Si(p,n)²⁸P(4.95 MeV) reaction and the ‘‘0ħω,’’ J^π=9⁺, state in the ⁸⁸Sr(p,n)⁸⁸Y(1.48 MeV) reaction at E_p=100, 200, 300, and 400 MeV. Differential cross sections for these reactions were measured and compared to distorted-wave impulse-approximation (DWIA) calculations. The ratio of the ⁸⁸Sr to the ²⁸Si DWIA normalizations is independent of beam energy, indicating that the systematic difference in DWIA normalizations reported earlier between 0ħω and 1ħω stretched states is a nuclear-structure effect. Additionally the DWIA normalization for each reaction is independent of the beam energy indicating that the necessary ingredients for DWIA calculations to describe these transitions consistently are well understood.

Measurements of vector analyzing power have been carried out for the (d→,t) reaction on targets of ⁵⁴Cr, ⁵⁶Fe, and ⁶⁴Ni at deuteron energies of 15, 16, and 18 MeV. Data were taken over the angular range 10°–55° for transitions to low-lying states of J^π=(1/2,3/2, 5) / 2 ,7/2)^- using a position-sensitive counter telescope in the focal plane of an Enge spectrograph. Transitions to states of j^π=3/2^- or 7/2^- show only slight dependence on incident energy, while those to states of j^π=1/2^- or 5/2^- show a greater dependence, and greater variations between different states. Distorted-wave Born approximation calculations show at best qualitative agreement with the data. The results indicate that measurements of VAP for the (d→,t) reaction at low energies will not provide accurate estimates of j mixing for transitions on odd-A targets.

The zero-degree cross section for the ¹³C(p,n)¹³N reaction has been measured at 200 MeV incident energy, with an energy resolution of 800 keV. Of particular interest is the transition to the ¹³N(15.06 MeV, 3/2^- T=3/2) level, since a previous measurement of this cross section is inconsistent with that for the isobaric analog level excited in the ¹³C(n,p)¹³B_g.s. reaction. The present measurement yields a cross section significantly less than that previously reported, and in agreement with that for the (n,p) reaction.

The reaction ¹⁵N(n,p)¹⁵C was studied at a neutron energy of 288 MeV using the TRIUMF (n,p) charge-exchange facility and a high-pressure gas target. The angular distributions for spin dipole (ΔL=1) transitions to the states in ¹⁵C at energies 0 and 0.740 MeV, as well as for higher excitation energies, were measured and the results were compared with distorted-wave impulse approximation calculations. The measured distribution of the spin dipole strength agrees well with shell-model predictions, indicating that a rather simple model provides a satisfactory description of the ¹⁵N ground state, and of positive-parity states in ¹⁵C up to about 18 MeV excitation. The magnitude of the peak cross sections (at ≃7°) is described well by the calculations when the theoretical cross section is renormalized by a factor of 0.7. The calculated cross sections near 0° are generally smaller than experimental data. If this is a general feature of ΔL=1 transitions, it suggests that estimates of Gamow-Teller strength based on a multipole decomposition of measured cross sections may be too high.

Tests of isospin symmetry in (n,p), (p,p’), and (p,n) reactions at 280 MeV populating the T=1 isospin triads in A=6 and A=12 nuclei have been performed. Distorted-wave impulse approximation (DWIA) calculations for the A=12 triad where the known ft asymmetry is included in the analysis show good agreement with experimental (n,p) and (p,p’) data. Angular distributions for ⁶Li(n,p) and ⁶Li(p,p’) cross sections differ significantly at finite values of momentum transfer (q) and do not agree with DWIA calculations. However, this discrepancy may become negligible upon extrapolation to q=0.

Cross sections at 0° for the pure Fermi (0⁺, 2.31, MeV) and Gamow-Teller (1⁺, 3.95 MeV) states in the ¹⁴C(p,n)¹⁴N reaction have been measured at E_p=494, 644, and 795 MeV using the LAMPF neutron time-of-flight facility with a new 617-m neutron flight path. The measured cross sections per unit transition strength, σ^, provide a measure of the isovector spin-flip and non-spin-flip central components of the effective nucleon-nucleon interaction. Cross sections and the ratio R²=σ^_GT/σ^_F are compared to lower-energy measurements and to calculations using a free NN t matrix.

Doubly differential cross sections of the reaction ⁹⁰Zr(n,p)⁹⁰Y have been measured at 198 MeV for excitations up to 38 MeV in the residual nucleus. An overall resolution of 1.3 MeV was achieved. The spectra show qualitative agreement in shape and magnitude with recent random phase approximation calculations; however, all of the calculations underestimate the high excitation region of the spectra. A multipole decomposition of the data has been performed using differential cross sections calculated in the distorted-wave impulse approximation. An estimate of the Gamow-Teller strength in the reaction is given. The isovector spin-flip dipole giant resonance has been identified and there is also an indication of isovector monopole strength.

Differential-cross-section distributions for the ⁷Li(p,n)⁷Be(g.s.+0.43-MeV) reaction have been measured for E_p=80, 120, 160, 200, 494, 644, and 795 MeV. These distributions have been integrated and normalized to independent values for the total cross section obtained from parametrizations of ⁷Li(p,n) activation measurements. Zero-degree cross sections obtained from this normalization are reported.

Angular distributions of the ⁵⁴Fe(p,n)⁵⁴Co and ⁵⁴Fe(n,p)⁵⁴Mn cross sections have been measured to test the Gamow-Teller sum rule [S_--S₊=3(N-Z)] in a case where the Gamow-Teller strength is large for both channels. The results for S_- and S₊ are compared to several models which have moderate success in describing the data. Large scale shell-model and quasiparticle random-phase-approximation calculations correctly predict the distribution of Gamow-Teller strength but overestimate the total strength. A model that approximates the nuclear surface to be a semi-infinite slab describes the cross sections well in the quasielastic scattering region if 2p-2h correlations are included.