ISOLDE Proposal # IS538

Precision measurement of the half-life of In large and small lattice environments

A. Ray

Variable Energy Cyclotron Center1/AF, Bidhan Nagar, Kolkata – 700064 India

In109

Collaborators

P. Das, S. Bhattacharyya VECC, Kolkata, India

S. Lahiri, A. GoswamiSINP, Kolkata, India

A. DeRaniganj Girls’ College, Raniganj, West Bengal, India

Y. Blumenfeld, K. Johnston, M. KowalskaCERN, Geneva, Switzerland

P. Reiter and MINIBALL CollaborationUniversity 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 7Be compounds.

Decay rate of 7Be implanted in different media found to change by ≥1% in some cases.

Explanation: 2s electron density at 7Be nucleus 3% of total electron density at the nucleus. 7Be loses more 2s valence electrons in amedium of high electron affinity lower 2s electron density at nucleusLower 7Be electron capture decay rate. DFT calculations agree.

)21( 227 ssBe

2)0(e

Recent Measurements of 7Be half-life Implanted in different materials.

A. Ray et al., Phys. Lett B455, 69 (1999). Ray et al. found Half-life of 7Be in Au longer than that in Al2O3 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 7Be in Au longer than that in Ta by (0.22 0.13)%Half-life of 7Be in Au longer than that in graphite by (0.38 0.09)%

Liu et al. (Chin. Phys. Lett. 22, 565 (2005)) found 7Be decay rate in Pd faster by 0.8% compared to that in Au.

Y. Nir-El et al., (Phys. Rev C75, 012801(R), (2007)) found 7Be decayrate in Cu faster than that in Al2O3 by 0.32%.

Change of electron capture nuclear decay rate under compression

General InterestCompression of atom Valence electrons affectedEffect 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 electronsWave function 0 at ∞

Compressed 7Be 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 7BeO crystal. 0.8% for 7Be(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 40K 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 (40K, 109In, 110Sn etc.)

Valence electron density at the nucleus is extremely small.5s electron density at the nucleus is about 0.00001% of the totalelectron density at the nucleus.

DFT codes (TB-LMTO, WIEN2K) predict extremely small (non-observable) change of electron densityat 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 109In in smaller Au lattice increases by 1% comparedto that in the larger Pb lattice. Energy of 109In in Au much larger ( keV) than that in Pb significant relative compression of 109In 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 spherePlane 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 7Be (under compression).

One set of data on electron capture rate of 109In, 110Sn (under compression).

Observed increases of electron capture rate under compression >> predictionsof 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 ) alsoshows 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 109In Under compression

Important to clarify the experimental situation regarding thecompression of many-electron atomic systems because of theirmany potential applications. Improvement of DFT codes, HF codes?

Propose to measure the increase of electron capture rate of 109InIn Pt and Ta compared to that in Pb. Expect to see bigger effect, because the available interstitial spaces in Pt and Ta smallerthan those in Au. Expected compression should be more than in Au.

Availability of a pure 109In beam from ISOLDE, CERN should enableus to perform a cleaner experiment. Earlier experiment performedby implanting 109In ions along with other radioactive ions produced In 20Ne + 93Nb reaction.

Proposed experiment

109In is one of the most intense beam at ISOLDE, CERN.

Intensity of 60 keV 109In beam 107 particles/sec.

GLM beamline to be used. Chamber used by IS500 to be used.

Implantation target to be kept at -20 kV. 109In 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 109In beam. (Like to keep heavy impurityIons < 104 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 foilMinimum intensity of 109In 5 106 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.

109In implanted foil counted along with a 60Co sourceusing MINIBALL detector system.

Count rate in each detector of MINIBALL array to be kept 15000 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 fornext 15 minutes.

Data collection will continue for 20- 30 hours.

Ratio of peak areas of 203 keV g-line to the sum of 60Co g-lines - 1173 keV and 1332.5 keV to be monitored with time.

Background expected to be free of any contaminant g-line.

Count rate estimate

Considering ISOLDE SC yield of 109In = 5 106 ions/sec. The number of 109In implanted in 2 hours 3 1010 ions.

The total number of counts of 203 keV g-ray photons in the photo-peaks of MINIBALL arrayIn 15 minutes is = 5 107 counts.

Total g- ray count rate from the source 5 105 counts per sec.

Singles count rate per detector 10000 cps; (higher initially).

Measurements to be done for Pt, Ta and Pb targets one by one and then repeated.Attempt to determine lifetime of 109In 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 109In beam available at ISOLDE, CERN.

3 shifts of beam time required. Minimum intensity of 109In beam 5 106 ions/sec.