I.V. Mitchell

Stopping power measurement for 19F, 24Mg, 27Al, 32S and 35Cl at energies ∼0.2 to ∼3.5 MeV/nucleon in Ti, Fe, Ni, Cu, Ag and Au

Abstract Stopping powers have been measured for beams of 19 F, 24 Mg, 27 Al, 32 S and 35 Cl for energies ∼0.2 to ∼3.5 MeV/nucleon in Ti, Fe, Ni, Cu, Ag and Au to an accuracy of ±4%. The results are compared with previous experimental values and with the semiempirical tabulation of Northcliffe and Schilling. A stopping power table for these six materials containing all projectiles in the range 8 ≤ Z ≤ 20 is presented.

Angular momentum dependence of 200Pb fission studied by comparison of 19F on 181Ta with 16O on 184W

Abstract The time development of fission in highly excited Pb nuclei has been studied by the crystal blocking technique. Thin Ta crystals were bombarded with 19 F ions in the energy range 90–120 MeV and the yield of fission fragments was measured for emission directions close to a strong axis. The experimental blocking dips are compared with calculated dips containing a superposition of two components, corresponding to short- and long-lived compound nuclei. The information extracted is the energy dependence of the relative amount of fission which comes from compound nuclei with lifetimes of τ≳3 × 10 −17 s. The total fission cross section and angular distribution of fission fragments were also measured for 19 F bombardment of 181 Ta in the energy range 84.3–114.7 MeV and for 16 O bombardment of 184 W in the energy range 83.4–107.9 MeV. The results of the three types of measurements have been interpreted through comparison with statistical model calculations that follow the spin and energy distribution of compound nuclei through the neutron evaporation cascade. The 19 F + 181 Ta measurements, when compared with the present 16 O + 184 W cross section and angular distribution measurements and earlier lifetime measurements for 16 O + W, yield information on the spin distribution for the compound nucleus and its influence on the fission process.

Lifetime measurements in 22Ne, 28Si and 31P relevant to the interpretation of the Z2 variation in low velocity DSAM results

Abstract Accurate lifetimes have been measured for low-lying levels in 22 Ne, 28 Si and 31 P by bombarding 4 He implanted targets with beams of 19 F and 28 Si ions. Mean lifetimes determined by fitting Doppler-broadened γ-ray lineshapes were (E x in MeV , τ in ps ): 22 Ne (1.275, 5.15 ± 0.31; 3.357, 0.324 ± 0.009), 28 Si (1.779, 0.667 ± 0.035), 31 P (1.266, 0.70 ± 0.07; 2.234, 0.363 ± 0.024 ). The lifetime values for the 3.357 MeV level in 22 Ne and the 2.234 MeV level in 31 P are used to calibrate low velocity DSAM lifetime data for these two levels and to obtain scaling factors to theoretical electronic stopping powers for Ne and P ions.

The use of 4He ion-implanted targets to obtain accurate nuclear lifetimes from analysis of doppler broadened lineshapes

Abstract The inverse reactions 4 He( 24 Mg, 4 He) 24 Mg and 4 He( 35 Cl, p) 38 Ar have been used to measure the mean lifetimes of the first excited states of 24 Mg and 38 Ar. The Doppler broadened lineshapes were analyzed using scaled experimental stopping powers and yielded results of 1.82 ±0.14 ps for the 1.37 MeV level in 24 Mg and 0.72 ± 0.05 ps for the 2.17 MeV level in 38 Ar.

Electric quadrupole moments of the first excited states of 28Si, 32S and 34S

Abstract The reorientation effect in Coulomb excitation has been used to measure the following static electric quadrupolemoments: Q 2 + ( 28 Si ) = +17.5 ± 2.9 e · fm 2 , Q 2 ( 32 S) = −11.5 ± 4.8e · fm 2 and Q 2 + +( 34 S) = +6.3 ± 4.3 e · fm 2 . Energy spectra from the 208 Pb+ 28 Si, 32, 34 S reactions at E = 105–122 MeV were obtained with the Chalk River Q3D spectrometer. These data were fitted with accurate B ( E 2; 0 + → 2 + ) values determined from a weighted average of all previous measurements combined with those measured by the Doppler shift attenuation method by using the inverse reactions 4 He( 32, 34 S, α) where the mean 2 + 1 state lifetimes were found to be 0.236 ± 0.016 ps ( 32 S) and 0.442 ± 0.026 ps( 34 S).

A DBLA measurement of the lifetime of the first excited level in 18O and a critical comparison of lifetime results by different methods

Abstract An accurate measurement of the lifetime of the 1.982 MeV level in 18 O has been obtained by bombarding 4 He and 1 H implanted targets with beams of 34 and 47 MeV 18 O ions. The mean lifetime determined by fitting the Doppler-broadened γ-ray lineshapes with experimentally determined stopping powers was 2.80 ± 0.07 ps. A detailed comparison of the mean lifetime values obtained by different techniques for this level and several other sd shell transitions suggest that recent Coulomb excitation B ( E 2; 0 + → 2 + ) values for 18 O are ≈ 10% too small.

A measurement of the local density of 3He-implanted metal foils by the doppler shift attenuation method

Abstract A new experimental method has been devised to measure density changes in the helium implanted region of metal foils. The principle is demonstrated for the case of 35 keV 3He ions implanted into Zr, Nb and Au foils, and it is shown that large swelling occurs for fluences from 2 to 7 × 10 21 3 He/m 2 .

Lifetime measurements of levels in 26Si and the ratio of neutron to proton transition amplitudes for mirror nuclei

Abstract The mean lifetimes of the 1796, 2784, 3333 and 4138 keV levels of 26 Si have been measured using the Doppler shift attenuation method. The ratios of neutron to proton transition matrix elements determined from these data and additional data from mirror transitions in 26 Mg are compared with recent data from inelastic pion scattering and with shell model calculations. The isoscalar strenghts are compared with α-scattering data.

Lifetime measurements of levels in 34Ar: Isoscalar and isovector matrix elements for E2 analogue transitions

Abstract Mean lifetimes have been measured for the low-lying levels in 34 Ar( T z = − 1) excited by the bombardment of 3 He-implanted Au targets with a beam of 80 MeV 32 S. The lifetimes determined by fitting Doppler-broadened γ-ray lineshapes are as follows ( E x in keV, τ in fs): 2091, 460 ± 60; 3288, 280 ± 50; 3871, > 270 and 4128, 1301 +170 −130 . The resulting transition matrix elements for the 2 + 1 → 0 + 1 and 2 + 2 → 0 + 1 transitions are compared with those for the analogue transitions in 34 S ( T z = + 1) and 34 Cl( T z = 0) to determine the isoscalar and isovector components. The component values are compared with theoretical calculations and with values of the ratio of neutron to proton matrix elements determined by inelastic scattering with strongly interacting probes.

Lifetimes of low-lying levels in 18F

Abstract Mean lifetimes of levels in 18 F have been measured using the Doppler-shift attenuation method and the inverse reaction 3 He( 16 O, p) 18 F. Targets of 3 He implanted into Al, Nb, and Au foils were employed in the measurements. The Doppler-broadened lineshapes observed at 0° to the beam were analyzed to obtain the following lifetime values: 0.971 ± 0.030, 0.605 ± 0.029 and 0.435 ± 0.041 ps for the 1.70(1 + ), 2.52(2 + ) and 3.36(3 + )MeV members of the K π = 1 + rotational band, 5.12 ± 0.56, 0.403 ± 0.018 and 1.91 ± 0.17 ps for the 2.10(2 − ), 3.13(1 − ) and 3.79(3 − ) MeV members of the K π = 0 − bands, and 〈1.2, 2.7 +4.1 −2.7 and 20 ± 2 fs for the 3.06(2 + , T = 1), 3.72(1 + ) and 3.84(2 + ) MeV states, respectively.