D. H. Wilkinson

Isotopic spin selection rules IX: The 9.58 MeV state of 16O

Abstract The reaction 12C(α, γ)16O through the 1–state of 16O at 9.58 MeV is detected and the radiative width measured as 6 MeV. This corresponds to ∥M∥2 = 1 × 10×5 in Weisskopf units or to a contamination of the predominantly T = 0 state with T = 1 or the ground state with T = 1 of α0 2(1)×3∼10×4. Alternatively if the transition is ascribed to a mixing-in by the Coulomb perturbation of the T = 1 state at 13.09 MeV we find HC∼0.035 MeV.

Parity conservation in strong interactions: The7Be(n, α)4he reaction

The abundance of evidence for lack of conservation of parity in weak interactions ralses the question as to whether parity is conserved in strong interactions. The spin of the Be/sup 7/ nucleus is 3/2 and the parity odd. Since the centrifugal barrier inhibits thermal neutron interactions with orbital angular momentum greater than zero, the parity of the initial system formed by the absorption of a thermal neutron must be odd. The symmetry and parity operators are identical for identical, spinless partioles and so a Be/sup 8/ state of odd parity cannot break up into two alpha particles if purity is conserved. The Q value of the Be/sup 7/(n, gamma )He/sup 4/ reaction (from the known masses) is 18.98 Mev. The disintegration of Be/sup 7/ by slow neutrons into two 9.5 Mev alpha particles was therefore sought as an indication of violation of Parity conservation. An upper limit of 25 mb was set for the (n, alpha ) cross section. An upper limit of 1 x 10/sup -7/ is also set for the square of the amplitude of the parity non-conserving component of the wave function. The results are not sensitive to whether the parity nonconserving forces conserve isotopic spin. (A.C.)

Beta-decay of16N: Conservation of spin and parity in16o

The degree to which parity is conserved in strong inter actions was studied and F/sup 2/, the intensity of the component determined. A thick target of TiN/sup 15/ was irradiated with a beam of deuterons at 1.1 Mev. The consequent N/sup 16/ decay was examined. It was found that alpha particles are emitted following the beta decay of N/sup 16/. These alpha particles came from the decay of the broad 1- state in O/sup 16/. Results suggested a value of 7 x 10/sup -12/ for F/sup 2/. (M.C.G.)

Isotopic spin selection rules XI: The 8.06 and 6.23 MeV states of 14N

Abstract The decay scheme of the 8.06 MeV 1−T =1 level of 14N is reinvestigated using the reaction 13C(p, γ)14N at the 550 KeV resonance. The relative strength of the isotopic spin forbidden transition to the first excited state is 0.022 corresponding to α1 2(0)8.06∼0.061. Owing to the great strength to it of the transition from the 8.62 MeV O+ state it seems that the 6.23 MeV T = O state must be 1−. The decay of this state has been determined; it has been reached by this El transition from the 1.16 MeV resonance. The strength of the isotopic spin forbidden transition to the ground relative to the allowed transition to the first excited state is 0.31 corresponding to α0 2(1)6.23∼0.078.The closeness of these two isotopic spin impurities suggests that these two states may be each others chief contaminators and in this Sense the forbidden transitions from the one is the same radiation as the allowed transition from the other and vice versa. If this suggestion is correct H C∼0.45 Mev which large value sugges...

Parity conservation in strong interactions: The 7Be(n, α)4he reaction

The abundance of evidence for lack of conservation of parity in weak interactions ralses the question as to whether parity is conserved in strong interactions. The spin of the Be/sup 7/ nucleus is 3/2 and the parity odd. Since the centrifugal barrier inhibits thermal neutron interactions with orbital angular momentum greater than zero, the parity of the initial system formed by the absorption of a thermal neutron must be odd. The symmetry and parity operators are identical for identical, spinless partioles and so a Be/sup 8/ state of odd parity cannot break up into two alpha particles if purity is conserved. The Q value of the Be/sup 7/(n, gamma )He/sup 4/ reaction (from the known masses) is 18.98 Mev. The disintegration of Be/sup 7/ by slow neutrons into two 9.5 Mev alpha particles was therefore sought as an indication of violation of Parity conservation. An upper limit of 25 mb was set for the (n, alpha ) cross section. An upper limit of 1 x 10/sup -7/ is also set for the square of the amplitude of the parity non-conserving component of the wave function. The results are not sensitive to whether the parity nonconserving forces conserve isotopic spin. (A.C.)

Isotopic spin selection rules XII: The mirror reactions 6Li(d, n)7Be and 6Li(d, p)7Li

Abstract It is remarked that the mirror reactions 6Li(d, n)7Be and 6Li(d, p)7Li leading to the fist excited states of the mirror nuclei form an excellent testing place for the prediction following the assumption of charge-independence that such mirror reactions have equal intrinsic probability. This is because the reactions are almost non-resonant which makes for the good operation of the isotopic spin rules and because the protons are emitted well above the Coulomb barrier which makes the interpretation of the experimental neutron to proton branching ratio rather simple. It is also remarked that the experiment may be performed without observing the neutrons and protons by using the gamma rays from the excited states to label the two transitions; all higher excited states are unstable to heavy particle emission and so do not yield gamma rays, The branching ratio is determined at 31 deuteron energies between 200 Kev and 1.8 Mev. Over the whole range it agrees to within 15% with that expected from charge in...

Isotopic spin selection rules XIII: The 6.92 MeV state of 16O revisited

Abstract The status of the possible E1-E3 cascade between the 2 + state at 6.92 mev, the 3- state at 6.14 mev and the 0+ ground state of 16O is reviewed in the light of recent improvements in our knowledge of radiative transitions and isotopic spin impurities. It is concluded that the present limit of 5 × 10−3 on the relative intensity, although clearly demonstrating the efficacy of the isotopic spin selection rule, is not sharp enough to be a significant test for detailed models of the nuclear structure, for which purpose an improvement by an order of magnitude would be required. It is shown by a coincidence method, wing the reaction 19F(p, α)16O, that the relative intensity is less than 4 × 10−4. This corresponds to α0 2(1) < 2 × 10−3, a low figure.

Isotopic spin selection rules XIV: Transitions between mirror states in 13C and 13N

Abstract The 3.68 mev 3/2 - state of 13C is shown, by a coincidence technique to cascade via the 3.09 mev 1/2+ state with a probability of (6−5±1.0) × 10−3. The rule that Corresponding E1 transitions in mirror nuclei have the same reduced speed, i.e. are mirror transitions, is used to deduce the speed of this weak branch from data available in 13N and hence to determine the radiative width of the M1 transition to ground from the 3.68 mev state. This is (0.40–0.75) ev. The speed of this transition, which is not a mirror transition to the corresponding one in 13N, is compared with the prediction of the independent particle model. Agreement is obtained within the range of the intermediate coupling parameter a/K that satisfies other data in the A =13 system.

The properties of beryllium-11

Ajzenburg and Lauritsen (1955) suggested that the energy difference between the ground states of Be/sup 11/ and B/sup 11/ should be about 11.5 Mev and that Be/sup 11/ should therefore be stable by about 0.5 Mev against neutron emission. In this case the Q-value for the reaction Fink (1958) discovered a beta activity of half life 14.1 plus or minus 0.3 seconds induced in boron by the 14-Mev neutrons and ascribed it to Be/sup 11/. The reaction has been studied by the authors in the same manner. The assignment to Be/sup 11/ has been confirmed, and certain properties of decay have been determined. (A.C.)