ISOLDE Proposal # IS538 Precision measurement of the half-life of In large and small lattice environments A. Ray Variable Energy Cyclotron Center 1/AF, Bidhan Nagar, Kolkata – India

Collaborators P. Das, S. Bhattacharyya VECC, Kolkata, India S. Lahiri, A. Goswami SINP, Kolkata, India A. De Raniganj Girls’ College, Raniganj, West Bengal, India Y. Blumenfeld, K. Johnston, M. Kowalska CERN, Geneva, Switzerland P. Reiter and MINIBALL Collaboration University of Cologne, Cologne, Germany

Nuclear decay rate independent of external environment Electron capture nuclear decay rate in principle susceptible to external environment Decay rate (half-life  52 days) of found to be susceptible to external environment. About 0.2% decay rate change observed among different 7 Be compounds. Decay rate of 7 Be implanted in different media found to change by ≥1% in some cases. Explanation: 2s electron density at 7 Be nucleus  3% of total electron density at the nucleus. 7 Be loses more 2s valence electrons in a medium of high electron affinity  lower 2s electron density at nucleus  Lower 7 Be electron capture decay rate. DFT calculations agree.

Recent Measurements of 7 Be half-life Implanted in different materials. A.Ray et al., Phys. Lett B455, 69 (1999). Ray et al. found Half-life of 7 Be in Au longer than that in Al 2 O 3 by  0.7% E. B. Norman et al., Phys. Lett. B 519, 15 (2001). Z. Liu et al., Chinese Phys. Lett. 20, 829 (2003). Norman et al., found Half-life of 7 Be in Au longer than that in Ta by (0.22  0.13)% Half-life of 7 Be in Au longer than that in graphite by (0.38  0.09)% Liu et al. (Chin. Phys. Lett. 22, 565 (2005)) found 7 Be decay rate in Pd faster by  0.8% compared to that in Au. Y. Nir-El et al., (Phys. Rev C75, (R), (2007)) found 7 Be decay rate in Cu faster than that in Al 2 O 3 by  0.32%.

Change of electron capture nuclear decay rate under compression General Interest Compression of atom Valence electrons affected Effect on core electrons: DFT calculations predict increase of energies of core electrons because of crystal potential, but use free atom boundary condition. Boundary conditions on core electrons Wave function  at ∞ Compressed 7 Be atom; WIEN2K (DFT calculation) [Lee and Steinle-Neumann, EPSL 267, 628 (2008)] Valence 2s electron density at nucleus increases due to compression. 1s electron density at nucleus decreases due to screening by 2s electrons. Net Result Increase of electron density  0.1% at 270 kbar. Experimental Observation (0.6 ±0.2)% at 270 kbar for 7 Be O crystal.  0.8% for 7 Be(OH) 2 gel at 270 kbar.

Change of electron capture nuclear decay rate of many electron atomic systems under compression Implications in different areas such as geophysics, astrophysics etc.  8 TW power produced by 40 K electron capture nuclear decay in the Earth’s core. Important in determining the thermal and tectonic evolution of the earth. Important for earth and planetary science. Core collapse of massive stars Important to understand electron capture nuclear decay rate under compression

Compression of larger many electron atomic system ( 40 K, 109 In, 110 Sn etc.) Valence electron density at the nucleus is extremely small. 5s electron density at the nucleus is about % of the total electron density at the nucleus. DFT codes (TB-LMTO, WIEN2K) predict extremely small (non-observable) change of electron density at the nucleus due to the implantation of such ions in a small lattice. Recent Experimental observation: [A. Ray et al., Phys. Lett B679, 106 (2009)] Decay rate of 109 In in smaller Au lattice increases by  1% compared to that in the larger Pb lattice. Energy of 109 In in Au much larger (  keV) than that in Pb  significant relative compression of 109 In in Au.

Density functional codes used TB-LMTO code assumes a frozen core for the inner orbital electrons. Does not really calculate the electron density at the nucleus WIEN2K code (Vienna) is supposed to be a fuller calculation. Gives electron density at the nucleus WIEN2K and TB-LMTO do similar calculations in principle. Basis states different. TB-LMTO and WIEN2K codes

WIEN2K basis states (for valence orbitals) Atomic wave function inside muffin-tin sphere Plane wave outside the muffin-tin sphere. Smooth matching at the boundary surface of Muffin-tin sphere. Inner core electrons Inner electrons see spherical part of crystal potential. Otherwise free atom solution. Wave function vanishes at infinity. WIEN2K code Neither TB-LMTO nor WIEN2K predicts the observed increase of electron density at the nucleus due to the compression of the atom.

Very little data available Two sets of data on electron capture rate of 7 Be (under compression). One set of data on electron capture rate of 109 In, 110 Sn (under compression). Observed increases of electron capture rate under compression >> predictions of DFT code. Such large increases possible only if the core electron density at the nucleus increases. DFT codes considers core electrons in nuclear and crystal potential, but assumes free atom boundary condition. Recent calculation by V. Flambaun (Relativistic Hatree-Fock +correlation ) also shows very small increase of electron density at the nucleus similar to DFT codes. Important to clarify experimental situation for many electron atomic systems, because of its potential applications in many areas and for improvement of theoretical calculations. Increase of electron capture rate under compression

Motivation for measuring the electron capture rate of 109 In Under compression Important to clarify the experimental situation regarding the compression of many-electron atomic systems because of their many potential applications. Improvement of DFT codes, HF codes? Propose to measure the increase of electron capture rate of 109 In In Pt and Ta compared to that in Pb. Expect to see bigger effect, because the available interstitial spaces in Pt and Ta smaller than those in Au. Expected compression should be more than in Au. Availability of a pure 109 In beam from ISOLDE, CERN should enable us to perform a cleaner experiment. Earlier experiment performed by implanting 109 In ions along with other radioactive ions produced In 20 Ne + 93 Nb reaction.

Proposed experiment 109 In is one of the most intense beam at ISOLDE, CERN. Intensity of 60 keV 109 In beam  10 7 particles/sec. GLM beamline to be used. Chamber used by IS500 to be used. Implantation target to be kept at -20 kV. 109 In ions would be incident with about 80 keV energy. Implantation depth > 100 Angstrom. Expect to be in the bulk region. No radioactive contaminant beam. Problem of stable impurity ions in 109 In beam. (Like to keep heavy impurity Ions < 10 4 pps to reduce damage of lattice structure of catcher foils. ) Implantation targets: Pb, Pt, Ta

Three catcher foils to be used 1)Pb (lattice parameter  5 Angstrom) 2) Pt (lattice parameter  3.9 Angstrom) 3)Ta (lattice parameter)  3.3 Angstrom 2 hour implantation run with each catcher foil Minimum intensity of 109 In  5  10 6 ions/sec (energy  80 keV ) Measurement with each catcher foil to be repeated. Expect to bring down statistical error  (0.15% -0.2 %) level. Beam time requirement: 3 number of 8 hour shifts.

109 In implanted foil counted along with a 60 Co source using MINIBALL detector system. Count rate in each detector of MINIBALL array to be kept  cps. Time keeping to be done using a precision pulser. Dedicated electronics and DAQ of MINIBALL array to be used. Singles spectra collected for 15 minutes. Stored and run restarted for next 15 minutes. Data collection will continue for hours. Ratio of peak areas of 203 keV  -line to the sum of 60 Co  -lines keV and keV to be monitored with time. Background expected to be free of any contaminant  -line.

Count rate estimate Considering ISOLDE SC yield of 109 In = 5  10 6 ions/sec. The number of 109 In implanted in 2 hours  3  ions. The total number of counts of 203 keV  -ray photons in the photo-peaks of MINIBALL array In 15 minutes is = 5  10 7 counts. Total  - ray count rate from the source 5  10 5 counts per sec. Singles count rate per detector cps; (higher initially). Measurements to be done for Pt, Ta and Pb targets one by one and then repeated. Attempt to determine lifetime of 109 In in different catcher foils within (0.1%-0.15%) accuracy.

Summary Study of increase of electron capture nuclear decay rate under compression is interesting. Has implications in many areas. Very little data exists. Observed change >> DFT or HF calculations Experiment should be done with pure radioactive beam to avoid background problem. Intense 109 In beam available at ISOLDE, CERN. 3 shifts of beam time required. Minimum intensity of 109 In beam  5  10 6 ions/sec.