( 1) Institute for Nuclear Research of the RAS, Russia

1. TAU-4 installation intended for long-term monitoring of a half-life value of the 212Ро E.N. Alexeev1, Yu.M. Gavrilyuk1, A.M. Gangapshev1, A.M. Gezhaev1, V.V. Kazalov1, V.V. Kuzminov1, S.I. Panasenko2, O.D. Petrenko2, S.S. Ratkevich2. (1)Institute for Nuclear Research of the RAS, Russia (2)V.N.Karazin Kharkiv National University, Ukraine The work supported by the Presidium of the RAS under the “Physics of Basic Interactions and Atomic Technologies ” Basic Research Program ICPPA-2018 (Moscow, MEPHI, 22-26 October 2018)

2. Tasks of the experiment 1. Search for decay constant time variations. No theoretical predictions about decay constant time variations exist nowadays. We have only experimental results. 2. Test of an exponetiality of the fundamental decay law. Deviation from an exponentiality at a region of small values of a decay curve argument has been predicted by some theoretical models. In particular, it could appear due to the theoretically proved Zeno quantum effect. Estimations of rare processes (proton decay, double beta-decay, cosmic chronology and others )

3. Experiments Investigated isotopes:3H – 239Pu Observed periods of variation: 1 day – 13.5 years Amplitudes:n∙(10-2 – 10-4) (n- a few units)

4. 108Ag, 133Ba, 154Eu, 85Kr, 226Ra, 90Sr: 1 year variations, A = (0.068 – 0.088)% Ā = (0.081±0.007)% (11σ) P. A. Sturrock, E. Fischbach and I. Jenkins arXiv:1408.3090 Limitations 198Au:1 year period - A < 0.02% (99% C.L.) (J.C.Hardy, J.R. Goodwin and V.E. Jacob arXiv:1108.5326) 137Cs: (a few hours – 1 year) period - A< 0.0096% (95% C.L.) (E. Bellotti et al, arXiv:1108.5326) 40K: ~ 1 year period - A< 0.0061% (95% C.L.) (E. Bellotti et al, arXiv: 1311.7043)

5. 1. Count rate instability (background; electric and Decay rate variations = F{ magnetic fields; temperature; pressure; humidity; } aging; source-detector characteristics …) 2. Half-life variations Decay rate measurements → Life time measurements 226Ra (T1/2=1600 y) as a source of 214Po →…. →214Bi→(β, Т1/2= 19.9 min)→214Po*→γ→ 214Po (α,Т1/2=164.3±2.0 μs)→ 214Po (Т1/2=163.46±0.04μs) E.N.Alexeev, Yu.M. Gavrilyuk, A.M. Gangapshev et al. “Results of a search for daily and annual variations of the 214Po half-life at the two year observation period”. Physics of Particles and Nuclei, 47(6), (2016), 986- 994 214Po (Т1/2= 163.58±0.29(stat.)±0.10(syst.)) µs G. Bellini, J.Benziger, D.Bick et al. “Lifetime measurements of 214Po and 212Po with the CTF liquid scintillator detector at LNGS”. Eur. Phys. J. A (2013) 49:92

6. 229Th as a source of 213Po 229Th (α, T1/2= 7340 years) → 225Ra (β, T1/2= 14.5 days) → 225Ac (α, T1/2= 10.0 d) → 221Fr (α, T1/2= 4.8 min) → 217 At (α, T1/2=3.23∙10-2 s) →213Bi (β, T1/2=46 min) → 213Po (α, T1/2= 4.2∙10-6 s) →209Pb (β, T1/2= 3.3 h) → 209Bi β α γ Fig.1. Decay schemes 213Bi and 213Po γ(440 keV, 26.1%/dec.) + (β + α) 213Po (Т1/2=3.705±0.001μs) E.N.Alexeev, Yu.M. Gavrilyuk, A.M. Gangapshev et al. “Search for Variations of 213Ро half-life” Physics of Particles and Nuclei, 49(4), (2018), 557- 562 213Po (Т1/2= 3.65±0.4) µs J. Wawryszczuk, K. Ya. Gromov, V. B. Zlokazov et al. “New measurement of the half-life of 213Po” Physics of Atomic Nuclei 61(8), (1998), 1322-1324

7. d=14 mm Plastic PETP 2.5 μm film “Goodfellow” Glue 0.05 mm d=3 mm PS-disc 229Th Fig.2.(226Ra ) 229Th-source + PS-detector

8. γ α β Amplitude, a.u. Time (channels), 1 ch=10 ns Fig.3. Schematic view of TAU-2 installation Fig.4. Sample of the 213Bi -213Po decay event TAU-2  4900 m w.e. NaI(Tl)×2 - 150×150 mm 25 cm PE+1mm Cd+(15 cm+15 cm Pb) A ≈ 50 Bk

9. B – Gallium Germanium SN-Telescope D D KAPRIZ • TAU-1, (L=620 m, T=(20±2)oC) DULB-4900 TAU-2, (L=3670 m, T=(26.5±0.2)oC) Fig.5. Schematic view of BNO underground laboratories

10. 1. Solar-daily variation (24 hours) of the 214Po half-life 214Po half-life [a.u.] Interval step, [hours] Fig.6.(▼) Solar-daily variation of half-life of 214Po obtained by means of Inner Moving-Average method (IMA method). (1) Approximation by τ(t)/τ0 =[1+A∙sin(ω∙(t-φ))], ω = 2π/24[h-1]; A = 3.4∙10-4 ; φ =19h. (2) τ(t)=τ0 ∙[1+5.3∙10-4∙sin((2π/24) ∙(t-1))]{1=19+6-24} Amplitude of the solar-daily variation is A = (5.3±0.3)∙10-4

11. Experimental results

12. 212Po(T1/2= 2.9∙10-7 с, α) 232Th (Т1/2=1.4∙1010 лет, α)→ 228Ra (Т1/2=6.7 лет, β)→228Ac (Т1/2=6.1 час., β)→ 228Th (Т1/2=1.9 лет, α)→ 224Ra (Т1/2=3.6 дней, α)→ 220Rn (Т1/2=55.3 с, α)→216Ро (Т1/2=0.158 с, α)→212Pb (Т1/2=10.6 час., β)→212Bi (Т1/2=60.5 мин, β(64%) + α(36%))→212Ро (Т1/2=2.99∙10-7 с, α) + 208Tl (Т1/2=3.05 мин.,β)→208Pb (стаб.) Th(NO3)4 β α (β + α) - ~59.8% / decay 212Bi 1 delay coincidence/13.7 single decay Fig.7. Decay schemes 212Bi and 212Po

13. DETECTOR Plastic Scint. +Th(NO3)4 PMT Cylinder with mirror walls D=90 mm 250 mm Fig.8. Schematic view of TAU-4 detector Low background shield – 15 cm Pb

14. Experimental results Fig. 9. Block diagram of a DO starting pulse former intend for a double delayed pulses sampling. (S) - splitter, (D) – discriminator, (TIS) - time interval shaper (СС) - coincidence circuit, (SPS) - starting pulse shaper

15. Experimental results Fig.10. Samples of selected useful events with the two coincident pulses. Count rate – 18 s-1

16. Experimental results Fig.11. Spectra of α-particles (left) and β-particles (right).

17. Experimental results Fig.12. Distribution of delay time between β-pulse (start) and α-pulse (stop) collected at 680 hours. 1/²{Y-f(t)}² →min; f(t) =A∙exp(-t/)+ B τ = 294.09±0.07 ns

18. Experimental results Fig.13. Dependence of a half-life value on a delay’s low threshold for a fixed upper threshold (2600 ns). Fig.14. Dependences of a half-life values on a delay’s upper threshold for the two fixed low thresholds (100 ns and 300 ns).

19. Experimental results Fig. . Dependences on time relative values of the212Ро half-life measured at a ground (30 May- 02 July 2018) and an underground (04 July – 10 August 2018 ) locations obtained for the decay curves collected at 12 hours.

20. 1. Solar-daily variation (24 hours) of the 212Po half-life Fig. .Solar-daily variation of half-life of 212Po obtained by means of Inner Moving-Average method. (■) Ground measurements (680 hours); (•) Underground measurements (564 hours). A=(7.5±1.7stat±3.3sist)∙10-4 = (7.5±4.1)∙10-4

21. Experimental results

22. Conclusions 1. TAU-4 installation intended forlong-termobservationofthe212Ро half-lifeis created. Natural thorium used as a source of the mother’s chain. 2. Averaged value of the 212Po half-life measured by TAU-4 at 680 hours is equal to 294.09±0.07 ns. The value is in a good agreement with others experiments. 3. Solar-daily variations of the 213Po half-life searched out in the TAU-4 data. Amplitude is equal to (7.5±4.1)∙10-4. Plans: Continue a search for the origin of the half-life variations. 2. Accumulation and handling of the data from the TAU-4 setup with the 212Po source.