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Light Nuclei at Neutron Stability Border

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Structure of the Lightest Nuclei on the Border of Neutron Stability

Tech Area / Field

  • PHY-ANU/Atomic and Nuclear Physics/Physics

Status
3 Approved without Funding

Registration date
18.07.2000

Leading Institute
Joint Institute of Nuclear Research, Russia, Moscow reg., Dubna

Supporting institutes

  • VNIIEF, Russia, N. Novgorod reg., Sarov

Collaborators

  • Oak Ridge National Laboratory / Physics Division, USA, TN, Oak Ridge\nUniversita Degli Studi di Messina, Italy, Messina\nCEA, France, Ganil\nInstitut de Recherches Subatomiques, France, Strasbourg\nInstitute of Physical and Chemical Research (RIKEN) / RIKEN Accelerator Research Facility, Japan, Saitama, Wako\nCEA / DSM / DAPNIA/CEN Saclay, France, Saclay\nGSI, Germany, Darmstadt\nVanderbilt University, USA, TN, Nashville

Project summary

Deep learning of physics regularities governing the structure of the lightest neutron-drip nuclei is one of the nowadays-nuclear physics hot problems. Many research groups concentrate their efforts in this field, especially because the highest excess of neutrons in respect of the proton number has been achieved for the lightest nuclei. An entirely new phenomenon – neutron halo being a rarefied neutron matter surrounding a dense nuclear core – was discovered in some of the most neutron-rich, light nuclei. Observation of new precise information about the properties of these halo nuclei and the synthesis and study of even more neutron-excess nuclear systems beyond the neutron-drip line promise to broaden considerably our knowledge about the nuclear matter and such its exotic form, which is the neutron halo. The beams of radioactive neutron-excess nuclei (6Не, 8Не, 9Li, 11Li etc.), that recently became accessible in a number of research centres in Europe, USA, Japan and Russia, increased considerably the potential of nuclear physicists in solving these problems.
A conclusive step in this direction will be the use of a tritium target on the beams of exotic neutron excess nuclei including also the beam of accelerated tritium nuclei – tritons. The tritium nucleus is the lightest and, at the same time, the most neutron-excess one, which one can use as a target, bombarded with nuclear beams. Combinations of beams of exotic nuclei, notable for their neutron excess, with a tritium target would be the interacting nuclear systems outstanding for their enormous neutron excess. In this respect, the unique 8Не+3H system keeps an absolute record. Extremely favourable Q values characteristic for these reactions result in their high cross section values and, as a result, in high count rates for reaction products. Reaction products complementary to the neutron drip-line nuclei of interest are light particles (protons, deuterons, tritons and particles) much more favourable from the point of view of detection conditions as compared to the reaction products that one obtains with heavier targets. High count rates and the best detection conditions allow one to obtain new data of utmost precision about the structure of neutron halo in nuclei and about ground state energy and excited levels of such halo nuclei as 5Н, 8Не, 10Не, 11Li. The tritium target used at the beams of exotic nuclei ensures a break-through in a region of new, quasi-stable nuclei and nuclear systems lying beyond the neutron drip line. Isotopes of hydrogen 5H and helium 10He belong to this region. High precision data about the life-times of these nuclei would be of a key meaning for ones understanding of stability limits of even more heavy isotopes of hydrogen and helium.
The project goal is to carry out experiments dedicated to the observation of new high-precision data on neutron-excess nuclei with Z=1, 2 and 3 by means of a specially created set-up involving a liquid tritium target installed on a radioactive nuclear beam line. In particular, energy spectra of protons emitted as a result of the transfer of neutron pairs from the target tritium nuclei to the projectile nuclei of 3Н and 8Не will be measured in order to deduce information about the life times of neutron-excess nuclei 5H and 10He. New excited states and data on manifestations of neutron pair correlation will be obtained for these nuclei and halo nuclei 8Не, and 11Li. The probability to find in neutron-excess nuclei 5H, 8He, 10He and 11Li, respectively, subsystems identical with free 3Н, 6Не, 8Не and 9Li nuclei will be deduced from these data. The high data precision will be obtained by reducing uncertainties in the energy and trajectory angles of inpidual bombarding ions to the values better than 0,3 % и 0,2, respectively. Energy spectra and angular distributions of protons emitted in these reactions will be measured with a precision, which at least will be adequate to the measurement accuracy of these beam parameters.
A tritium target intended for radioactive nuclear beams will be created for the first time in the framework of this Project. This target will be a unique set-up from the point of view of the noted above outstanding prospects in the study of drip-line neutron excess nuclei. It will be unique also due to the fact that this will be the only set-up of this kind possessing distinctive features that are necessary for such a target to be used at radioactive beams.
Conflicting requirements of high count rates and high precision in the energy and angles of reaction products inevitably lead to a liquid-tritium target having a material thickness of 0.3 – 0.4 mm homogenous in its whole area within 2 – 3 %. A target area of about 0.5 cm2 must fit typical diameter values of radioactive nuclear beams. Detection conditions of reaction products leaving the target dictate the need of thin target windows made of well-stretched metallic foils welded to the body of the target cell (5 – 10 of stainless steel). The strict limits on the thickness homogeneity result in the target working at low pressure (50 – 400 Torr). One should cool the target to about 19 K constraining temperature stability within 0,2 К in order to keep stable liquid phase (to avoid vaporisation or solid phase formation).
Important are environmental safety conditions imposed on the device of the liquid-tritium target. One should keep the tritium volume in additional vessels providing at least three protection barriers avoiding inadmissible tritium leaks. Firm absorption of tritium leaks in the volumes of second and third protection barriers should be guaranteed. The working conditions of particle detectors installed within the volume of the third barrier result in restrictions that are by one order of magnitude more hard in respect of tritium contamination as compared to the limitations imposed by the environmental safety conditions.
The tritium target set-up satisfying all these conditions will be an entirely new device unexampled in the world practice.

This project suggests developments along three principal directions:

development and manufacture of a liquid tritium target and prerequisite set-ups and units that are necessary to provide the installation and exploitation of the whole set of devices on beams of the ACCULINNA separator;

development and manufacture of devices providing, for inpidual nuclei in radioactive nuclear beams of the ACCULINNA separator, the measurement and control of the energy and trajectory angles with a precision of better than 0,3 % and 0,1, respectively;

getting new, high precision data on the two-neutron transfer reactions between projectile nuclei of tritium and target nuclei of 3H, 6He, 8He, and 9Li and the observation of new data about the properties of neutron-excess nuclei 5H, 8He, 10He, and 11Li.



The following scientific and engineering problems will be solved in the framework of the proposed Project :
The design, manufacture and installation of a liquid tritium target on the radioactive beam line of separator ACCULINNA.
Beams of radioactive nuclei 3Н, 6Не, 8Не, 9Li will be produced at the ACCULINNA separator and systems intended to measure and control the energy and trajectory angles of inpidual beam nuclei with a precision of better than 0,3 % and 0,1, respectively.
Development and manufacture of a gas handling system for the liquid tritium target providing the environmental safety for the work on the radioactive beams with tritium.
Energy spectra and angular distributions will be measured for protons and complementary products formed as a result of nuclear reactions occurring in bombardments of the tritium target with the beams of 3Н, 6Не, 8Не, 9Li nuclei.


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