TAU-4 installation intended for long-term monitoring of a half-life value of the $^{212}$Po

Abstract

Description of the TAU-4 installation intended for long-term monitoring of the halflife value T 1/2 of the 212 Po is presented. Natural thorium used as a source of the mothers chain. The methods of measurement and data processing are described. The comparative results of short test measurements carried out in the ground (680 h) and underground (564 h) laboratories are given. Averaged value T 1/2 =294.09 ± 0.07 ns of the 212 Po half-life has been found for the ground level data set similar one for the underground data set. The solar-daily variations with amplitudes ASo = (11.7 ± 5.2) × 10 −4 for the ground data and ASo = (7.5 ± 4.1) × 10 −4 for the underground one were found in a series of τ values.

The half-life of $^{212}$Po

“…The data production was performed by using a constant-fraction discrimination method with the delay 1.8 ns and the multiplication factor 0.4 for the undelayed pulse (without integration). A fit of the obtained time distribution returns the half-life value T 1/2 = 295.09 (26) ns in agreement with the value T 1/2 = 295.10(26) ns obtained with the method described above. 6 Rise time of scintillation detector depends on several factors: photodetector sensitivity and time properties, the readout electronics bandwidth, scintillation material, size and geometry of the scintillator and reflector, energy and ionization density of particle (see, e.g., [37,38,39]).…”

“…Moreover, while 228 Ra has too small energy of β decay (Q β = 45.5 (6) keV [28]) well below the hardware threshold (see Sec. 3.1), presence of some amount of 228 Ac (β active daughter of 228 Ra with the decay energy Q β = 2123.8 (26) keV [28]) leads to a mild contribution to the random-pairs background. Thus presence of some amount of 228 Ra in the scintillator almost does not affect the half-life of 212 Po determination.…”

“…Typically the results are affected by large statistical uncertainties up to 17% 2 , and the systematic effects were not evaluated. Much larger statistics was gathered in recent measurement [26] where an external Th source was placed between plastic scintillators viewed by a photomultiplier (PMT) obtaining T 1/2 = 294.09 (7) ns. Unfortunately, systematic effects in the experiment were not estimated.…”

Annual variations of the $^{214}$Po, $^{213}$Po and $^{212}$Po half-life values

“…Though, such probability could not be excluded totally because of existence of a large quantity of active environmental factors (pressure, air humidity and temperature, layer of air ionization, earth magnetic field and others) and difficulties of its detection with the required level of sensitivity at a presence level of useful event count rates. It was found with a start of the 212 Po half-life measurement at the TAU-4 set-up and using of the Hamming filtering for a decay curve smoothing [6] that deviation of τ -values averaged at 2-4 hours from the one averaged at 1 week essentially exceeds the statistical errors. Such behavior of the investigated value at the 1 day scale showed a possibility to test a belonging of different installation components to an appearing of the variations.…”

“…A description of the set-up and first result are presented in the Ref. [6]. Isotope 212 Po is born in a 232 Th decay chain after the 212 Bi beta-decay: ... ⇒ 212 Bi[T 1/2 =60.5 min; β(64%) + α(36%)] ⇒ 212 Po(T 1/2 = 2.9 • 10 −7 s) + 208 Tl(T 1/2 = 3.05 min) ⇒ 208 Pb(stab.)…”