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Two proton Correlations in the decay of 45Fe

Krzysztof Miernik1, Wojciech Dominik1, Zenon Janas1, Marek Pfützner1, Leonid Grigorenko2, Carrol Bingham3, Henryk Czyrkowski1, Mikolaj Cwiok1, Iain Darby3, Ryszard Dabrowski1, Thomas Ginter4, Robert Grzywacz3,5, Marek Karny1, Agnieszka Korgul1, Waldemar Kusmierz1, Sean Liddick3, Mustafa Rajabali3, Krzysztof Rykaczewski5 Andreas Stolz4.

Physical Review Letters 99 192501 (2007)

Ever since the discovery that certain species of nuclei spontaneously disintegrate, radioactive decay has been an important probe into the properties of atomic nuclei. Ground-state proton radioactivity, first predicted by V.I. Goldansky in 1960 (and unobserved until the discovery of 151Lu in 1982), plays a significant role in the studies of very neutron-deficient nuclei. This phenomenon occurs when the nucleus proton separation energy becomes negative, defined as the "proton drip-line", allowing for the emission of a constituent proton. In addition to predicting single proton emission, Goldansky further predicted that ground-state 2-proton radioactivity should also exist for nuclei near the proton drip-line for which single proton emission was energetically forbidden. The initial predictions and further refinements thereafter, suggested the mass (A) equals 50 region to be the most promising with 45Fe being the optimum candidate.

After discovery of 151Lu it took a further 14 years for experimental techniques to become sufficiently advanced that three ions of 45Fe could be identified among the fragmentation products resulting from a 58Ni beam bombarding a Be target. The first spectroscopic information on 45Fe was obtained separately at the GSI and GANIL laboratories observing 5 and 20 events respectively. While these experiments were able to measure the sum decay energy of this nucleus, they were unable to identify the individual protons and test the relationship between them. Recently a further experiment at GANIL was able to record a few events observing directly the individual protons but was still unable to determine the decay mechanism.

Predicted 2-proton opening angles for 45Fe.The figure shows simulations by L. Grigorenko for 200 events.

The challenge for these experiments is to detect and identify true 2-proton ground-state radioactivity rather than the sequential emission of two protons. In addition to a direct observation of 2-proton decay an important question is what information may reveal structural details of the nuclear wavefunction and of the interaction between nucleons within the atomic nucleus. Several models exist to describe the character of 2-proton radioactivity. These can be discriminated by measuring the angle between the two protons (see figure above).  In order to do this the individual protons must be separately observed and their energies and their angular distribution measured.

To meet this challenge the Warsaw University Institute of Experimental Physics working with their colleagues from the field of High Energy Particle Physics used the modern technology of digital imaging to develop a new type of gaseous detector -  the Optical Time Projection Chamber (OTPC) in which images of ionising particle trajectories are recorded optically. The OTPC (illustrated right) has an ionisation chamber (top), where ions and their decay products are stopped, which is filled with a counting gas. Primary ionisation electrons drift in a uniform electric field toward a double-stage amplification structure formed by parallel mesh flat electrodes where charge multiplication and emission of UV photons occurs. The photons are wavelength shifted using a thin luminous foil to the optical range where they are captured by a Charge Coupled Device (CCD) camera and a photomultiplier tube (PMT). The digital photograph taken by the CCD camera (top right) represents the projection of the particles' tracks on the luminescent foil. The signals from the PMT (bottom right) are digitised providing information on the drift time of the electrons, which is related to the position along the axis normal to the image plane. The pioneering research on gaseous detectors incorporating a position sensitive optical readout was performed by Georges Charpak et al in the late 1980's and this is the first application of such a device to nuclear physics studies.

The OTPC has an implantation volume of 20x20x42cm3 filled with a counting gas mixture of 66% He, 32% Ar, 1% N2 & 1% CH4. The wavelength shifting foil is covered with a thin layer of sodium salicylate (NaC7H5O3). The digital camera has a 2/3" 1 MegaPixel matrix and an integrated image intensifier with a gain of up to 2000.

The search for conclusive proof of 2-proton radioactivity was undertaken in an experiment at Michigan State Universities' National Superconducting Cyclotron Laboratory (NSCL). Ions of 45Fe were produced by using a fragmentation reaction in which an enriched beam of 58Ni ions were accelerated to approximately 50% of the velocity of light and impinged on a  natural Ni target. The Iron ions were separated from other unwanted reaction products and unreacted beam by the A1900 spectrometer and were individually identified in-flight using their characteristic Time-of-Flight (ToF) and energy-loss (ΔE). A diagram of the layout at NSCL is shown beneath.

 

In the S1 vault the OTPC assembly, nicknamed the "cannon" was positioned to accept the incoming ions. The cannon was set at angle to the incoming ions in order to maximise the active gas volume through which the decays could be observed. The device was designed such that the high voltage charge amplification and imaging systems were activated only when a 45Fe ion entered the chamber. In this way the corresponding CCD photo and the light collection profile from the PMT could be definitively assigned to each ion.

Images and time profiles showing the decay of 45Fe via 2-proton radioactivity are shown beneath.

Shown CCD photos (top) and light collection profiles from the PMT (bottom) for 2-proton decays from 45Fe. Left to right are images from 2-proton decay, 2-proton followed by β-delayed proton emission and 2-proton decay followed by β-delayed 2-proton emission.

Analysis of the results from this experiment has led to the conclusion that 2-proton decay of 45Fe has a 3-body nature ruling out the possibilty of a diproton decay and indicates that the ground state of 45Fe is characterised by a significant mixture of p2 and f2 configurations.

For more details, particularly regarding interpretation of the result please see our letter in Physical Review Letters. If you have any questions about this work please feel free to contact one of the authors.

ALL IMAGES ON THIS PAGE ARE COURTESY OF MAREK PFUTZNER

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1 Institute of Experimental Physics, Warsaw University, Warsaw, Poland,

2 Joint Institute for Nuclear Research, Dubna, Moscow Region, Russia,

3 Department of Physics and Astronomy, University of Tennessee, Knoxville, TN,  USA,

4 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, TN, USA,

5 Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA.


This work was supported by a grant from the Polish Ministry of Science and Higher Education number 1 P03B 138 30, the U.S. National Science Foundation under grant number PHY-06-06007, and the U.S. Department of Energy under contracts DE-FG02-96ER40983, DEFC03-03NA00143, and DOEAC05-00OR22725. A.K. acknowledges the support from the Foundation for Polish Science.