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Search of parity violation effects in neutron reactions on natural Lead. A. I. Oprea, C. Oprea, P.V. Sedyshev, Yu. M. Gledenov Frank Laboratory of Neutron Physics (FLNP) Joint Institute for Nuclear Research (JINR) 141980 Dubna, RF. Bucharest – EuNPC 17-21 September 2012.

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slide1

Search of parity violation effects in neutron reactions on natural Lead

A. I. Oprea, C. Oprea, P.V. Sedyshev, Yu. M. Gledenov

Frank Laboratory of Neutron Physics (FLNP)

Joint Institute for Nuclear Research (JINR)

141980 Dubna, RF

Bucharest – EuNPC 17-21 September 2012

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PV effects. Abstract

Parity violation effects (PV) in nuclear reaction were discovered in the ’60 years of the last century in the capture of thermal transversal polarized neutrons by 113Cd nucleus. In this reaction experimentally was measured a non zero asymmetry of emitted gamma quanta and the results was interpreted by the existence of weak non leptonic interaction between nucleons in the compound nucleus. This first experimental result gave a serious impulse of theoretical and experimental developments of parity violation question in nuclear reactions.

The weak interaction acts in the background of strong interaction (with order of magnitude higher) and therefore it is very difficult to observe and evidence it. One possibility is the evaluation of asymmetry effects induced by PV phenomena.

For neutrons scattering there are a few asymmetry effects (like polarization of incident neutron beam, spin rotation and emitted neutrons asymmetry of incident transversal polarized neutrons) explained by the presence of weak interaction.

In natural Lead were observed an unexpected high value of neutron spin rotation due to the PV phenomena. The natural Lead contains four isotopes and the main contribution to the PV effects is given by 204Pb. Further to explain the high value of neutron spin rotation it was supposed the existence of a new negative P resonance with energy EP = - 16 eV.

In this work were estimated the PV effects in scattering in order to extract the weak matrix element and to verify the existence of the new negative resonance of 204Pb nucleus.

Bucharest – EuNPC 17-21 September 2012

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PV effects. Introduction

Weak interaction

- discovery 1934 – neutron and proton decay

- muon decay

Tanner – first theoretical results on parity violation (PV) between nucleons

N. Tanner, Phys. Rev., Vol. 107, p. 1203, 1957

Feynmann & Gell – Mann – Hypothesis of weak interaction universality

R.P. Feynmann, M Gell-Mann, Phys. Rev., Vol. 109, p.193, 1957

Experiments which have demonstrated the existence of weak interaction between nucleons

Yu. Abov and collaborators (LNF-JINR Dubna) 1964

Yu. G. Abov, P. A. Krupchitsky, Yu. A. Oratovsky, Phys. Lett., Vol. 12, № 1, p. 25, 1964

They have measured an asymmetry of emitted gamma quanta in the capture of thermal transversal

polarized neutrons in the reaction

- measured non zero asymmetry explained by existence of weak interactions between nucleons in CN

Bucharest – EuNPC 17-21 September 2012

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PV effects. Theory

Flambaum & Sushkov

Formalism of the compound nucleus (CN) mixing states with the same spin and opposite parities

To explain asymmetry effects they introduced PV reaction amplitude of type:

= weak matrix element between S and P states of СN

The main idea of the formalism

-the nuclear reaction is going by formation of a CN

-CN is described by resonance states (the most simple S and P states with energy ES and EP)

- If exist resonance states with the same spin and opposite parities then it is possible to observe PV effects in nuclear reaction due to the weak interaction between nucleons

- This weak interaction acts in the background of strong nuclear interaction and therefore is very difficult to measure (Hweak << Hstrong, Hweak / Hstrong 10-210-4)

One possibility is to evaluate asymmetry effects because they are proportional with the weak matrix element WSP

- S,P = resonant states of CN formed by interaction of neutron with target nucleus with orbital momentum l=0 respectively l=1

Bucharest – EuNPC 17-21 September 2012

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PV effects. Theory (2)

This formalism has explained the PV effects of Abov experiment and also allowed to extract in the 35Cl(n,p)35S reaction the weak matrix element

Abov experimental PV effect

Obtained by authors in 35Cl(n,p)35S reaction with thermal and resonant neutrons

Low energy neutron scattering (reaction) – thermal and resonant neutrons with energy up to keVs or tens of keVs depending on nuclei

Thermal neutrons

Cold neutrons

Parity conserving amplitude (S and P)

Bucharest – EuNPC 17-21 September 2012

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PV effects. Definitions

For neutron scattering on nuclei can be observed the following PV effects

Definitions

- Asymmetry of emitted neutrons initially transversally polarized

= differential cross section of scattered neutrons with spin up (↑) and down (↓) related to the initial neutrons spin direction.

- f+, f- = scattering amplitude on zero degree direction with positive (+) and negative (-) neutron helicity

- s-, s+ = total cross section with negative (-) and positive (+) helicity

- N = number of target nuclei on volume unit

- l = neutron wave length

Spin rotation

Spin rotation on the unit length

Longitudinal polarization

Bucharest – EuNPC 17-21 September 2012

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PV effects. Natural Lead & Isotopes

Natural Lead contains four isotopes: 204Pb (1.43%), 206Pb (24.15%), 207Pb (22.4%), 208Pb (52.4%)

PV effects, as was demonstrated, are given mainly by 204Pb isotope

G. A. Lobov – Yadernaya Fizika, v. 63, № 8, p. 1465-1468, 2000

PV effects evaluation

Two levels approximation – in the case of thermal and resonance neutrons –

204Pb we use 2 resonance (S, P) with determined energy and parity

S, P resonance – CN formed by interaction of neutrons with orbital momentum l=0,1 with target nucleus

Mughabghab S.F., Divadeenam M., Holden N.E., Neutron Cross Sections, NY Academic Press, 1981, v.1,2

Evaluation procedure of a, F, and P

– Born approximation (Hweak << Hstrong, Hweak / Hstrong 10-210-4)

- optical theorem

- stot = total cross section, f(0) = reaction amplitude in the forward direction, k=1/l = neutron wave number, l = neutron wave length

V.E. Bunakov, V. P. Gudkov, Materials of XVIth Winter School of Leningrad Institute for Nuclear Physics, p. 32-52, UDK 539.172, Leningrad, USSR, 1981 (in Russian)

Bucharest – EuNPC 17-21 September 2012

slide8

PV effects. Theoretical evaluations of the effects

Two levels approximation

= due to the potential scattering

Bucharest – EuNPC 17-21 September 2012

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PV effects. Theoretical and experimental values

Theoretical evaluations in the two levels approximation using the neutron resonance parameters from Atlas (Mughabghab)

Experimental values after correction for 204Pb

Very difficult to measure

Very big discrepancy between theory and experiment

Lobov explanation

- PV effects are given by 204Pb isotope

and most important

- High experimental values of PV effects are given by the presence of a new “negative” P resonance, EP = -16 eV

Experimental values on natural Lead

Bucharest – EuNPC 17-21 September 2012

slide10

PV effects. Theoretical values with new “negative” resonance

Lobov

Ep = -16 eV – New values (but the same widths)

Weak matrix element

WSP = 2 ∙10-4 eV (Bunakov)

Resonance parameters

ES=-2980 eV, EP=-16 eV

The difference remains very high

Bucharest – EuNPC 17-21 September 2012

slide11

PV effects. Analysis of theoretical and experimental data

Values of EP and WSP (and widths) are important for the evaluation of PV effects

To obtain the position of P resonance EP and the value of the weak matrix element WSP the authors propose a method to extract them

- We use the expressions of longitudinal polarization and spin rotation on length unit

- we form a system of two equation and using the existing experimental values numerically it is possible to obtain the position of P resonance and weak matrix element

Second set

First set

With results

With results

If the PV effects have the same sign with maybe is not necessary a new P resonance, its enough to increase the weak matrix element

If the PV effects have different sign with an matrix element of order of 10-4 eV a negative P resonance can exist

Bucharest – EuNPC 17-21 September 2012

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PV effects. Conclusions

- New experimental data for neutron scattering on natural Lead and their isotopes are needed

- Polarized neutrons with energies of hundreds of eV are very difficult to obtain but comparison of longitudinal polarization and spin rotation with different parameters can give an answer on the existence of a new P resonance (from figure we have different shapes of energetic dependences)

- To improve our method on extraction of weak matrix element it is necessary new data on the neutron capture

- an analogue coefficient to longitudinal polarization of emitted neutrons P is the asymmetry of emitted gamma quanta Ag

- Agit will be easier to measure experimentally as is not affected by potential scattering

- it will be useful to evaluate the contribution of other resonant state of compound nucleus

- also it will be of interest to interpret experimental data in the frame of others approach explaining PV effects

- Our experimental group from FLNP – JINR Dubna has the methodic and experience to measure very low effects (of order of 10-8)

Bucharest – EuNPC 17-21 September 2012

slide13

PV effects. THANKS

THANK YOU VERY MUCH

THANKS AND RESPECTS TO THE ORGANIZERS

Bucharest – EuNPC 17-21 September 2012