Long-term study of low energy counting rate with the Large Volume - PowerPoint PPT Presentation

Long-term study of low energy counting rate with the Large Volume Detector Gianmarco Bruno – LVD collaboration Gran Sasso National Laboratory & L’Aquila University July 3, 2009 Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 1 / 16

Large Volume Detector Main Features: Active Mass: M = 1kton 840 tanks: (1.0 × 1.0 × 1.5 m 3 ) 2520 pmts: 15 cm diameter Liquid Scintillator: C n H 2 n +2 n = 9 . 6, + 1g/l PPO + 0.03 g/l POPOP, ρ = 0.8 g/cm 3 Thresholds: E H ≃ 4MeV & E L ≃ 1MeV Goal: The detector is mainly designed to measure low energy ¯ ν e from stellar core collapse. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 2 / 16

Trigger modes LVD can operate at 2 different thresholds: The signals of Each PMT are discriminated at two thresholds resulting in 1 two possible levels of coincidence between a counter PMTs: H and L, corresponding to E H ≃ 4MeV and E L ≃ 1MeV. The H coincidence, in any counter, represents the scintillator trigger condition . The single tank low threshold rate is monitored by a system of 840 scalers. 2 The counting rate of each tank is measured during a time window of 10 s. The read out of this low priority data channel is enabled every 10 minutes by the: asyncronous monitoring trigger . At energies near E L the single tank counting rate is mainly due to: Rock radioactivity Building materials radioactivity Secondary particles generated by muons 222 Rn ( α, n ) − reactions Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 3 / 16

Correlation LTCR – Radonmeter The Low Threshold Counting Rate ( LTCR ) and the radonmeter data are clearly correlated. The last 500 days of data are shown in the picture. 100 400 counts per counter (Hz) ) 3 LTCR Rn concentration (Bq/m 90 350 Rn-meter 80 300 70 250 60 50 200 40 150 30 222 100 20 50 10 0 0 01/01/08 01/04/08 01/07/08 01/10/08 31/12/08 01/04/09 2 questions: LVD sensitivity to Rn contamination. What are we counting? Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 4 / 16

Calibration of a single counter Correlation coefficient 100 400 0.9 LTCR (Hz) ) 3 Rn concentration (Bq/m 90 350 80 0.8 300 70 250 0.7 60 50 200 0.6 40 150 30 222 100 0.5 20 50 10 0.4 -4 -3 -2 -1 0 1 2 3 4 0 0 13/07/08 14/07/08 15/07/08 16/07/08 17/07/08 18/07/08 Lag (hours) For each counter we determine the time lag at which the maximum of the LTCR (Hz) 80 cross-correlation function occurs, and 70 the sensitivity in terms of radon 60 activity. Sensitivities have been 50 evaluated by fitting the bivariate 40 distribution at the lag corresponding 30 0 50 100 150 200 250 300 350 400 Rn-meter (Bq/m 3 ) to the maximum correlation. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 5 / 16

Distribution of the calibration parameters The distribution of the calibration parameters (angular coefficient of the straight line) and the distribution of the delays obtained for all the counters are shown. Entries Entries 791 791 Entries Entries 791 791 counters counters Mean 0.264 Mean 0.264 200 Mean 1.046 Mean 1.046 80 RMS 0.1415 RMS 0.1415 RMS 0.3246 RMS 0.3246 180 70 160 60 140 50 120 100 40 80 30 60 20 40 10 20 0 0 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -1 -0.5 0 0.5 1 1.5 2 2.5 3 Radon sensitivity (Hz Bq -1 m 3 ) delay (hours) On average, a variation in Rn activity of 1 Bq/m 3 , corresponds to a variation in the single counters low threshold counting rate, of 0.3 ± 0.1 Hz. The average delay between a Rn-meter peak and the corresponding peak in the counter rate is: 1 ± 0.3 hours. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 6 / 16

222 Rn and its decay products Radium series: β β α α α 222 Rn 218 Po 214 Pb 214 Bi 214 Po 210 Pb − − − → − − − − → − − − − → − − − − → − − − → 164 µ s 3 . 82 d 3 . 10 min 26 . 8 min 19 . 9 min The parent radionuclide A decay according to A = A 0 e − λ A t exponential law producing atoms of type B: B change with a rate depending on: dB the parent decay 1 dt = A λ A − B λ B the decay of B itself 2 Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 7 / 16

Decay chain The more generale case of a n members dA 8 dt = − A λ A > > chain is described by a system of differential > > > > dB equation: > dt = A λ A − B λ B > < dC > > dt = B λ B − C λ C > > > > > > : . . . 222 Rn 1.0 218 Po The time evolution of the 0.8 signal has been calculated 214 Pb Activity H % L assuming that a certain 0.6 214 Bi quantity of radon (as 0.4 measured by the 0.2 Rn-meter) persists in the environment during 10 0.0 Time H min L 0 50 100 150 200 minutes. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 8 / 16

Decay chain The previously obtained curves can be regarded as pulse response functions of our system to a brief Rn injection. Discrete time convolution can be used to determine the output of a sampled data system from its input and pulse response. ∞ � y ( t ) = x ( t − n ) h ( n ) n =0 Thus, appliyng a convolution between each one of that curves and the radonmeter data series we can convert the activity of the radon in activity of the corresponding product. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 9 / 16

Shape 170 counts (arbitrary units) 160 Comparison between radonmeter data ( red 150 line ) and LTCR data ( black line ), during a time 140 130 period of intense variation in radon 120 110 concentration due to a scheduled switch off of 100 the ventilation system in the experimental hall. 90 80 196 196.5 197 197.5 198 time (days) Now the red line represent the activity of 214 Bi 170 counts (arbitrary units) calculated from the radon-meter data series. A 160 significantly better agreement is achieved, 150 140 explaining the delay measured, so we can state 130 that: 120 110 our counters can measure the Rn concentration 100 because gammas from 214 Bi ( E γ = 609 KeV 90 I γ = 46 . 1%) are detected. 80 196 196.5 197 197.5 198 time (days) Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 10 / 16

( α, n )-reactions Reference tank Reference tank LTCR (Hz) Events (Hz) 90 900 Gd loaded tank Gd loaded tank 80 800 70 700 60 600 50 500 40 400 30 300 20 200 Radon signal background 10 100 0 4 6 8 10 12 14 16 18 20 22 24 26 -1 0 1 2 3 4 5 6 7 8 9 Time (days) Energy (MeV) Counting rates versus time of a standard counter ( red line ) and a counter filled with Gd loaded scintillator ( blue line ) collected during a period of transition between high and low Rn contamination. Since we are studying the Rn contribution the spectra are background subtracted. From the two spectra, after Background subtraction, we can argue that the contribution of n from ( α, n )-reactions is negligeable. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 11 / 16

12 years of data series LTCR counts per counter H Hz L 120 100 80 Out[26]= 60 40 20 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 years 2000 Power Spectral Density 1500 1000 Out[26]= 500 0 0.0 0.5 1.0 1.5 2.0 Frequency H days - 1 L We report the counting rate collected since 1997 up to 2009 and the power spectral density obtained applying DFT algorithm. The discontinuity in the middle of 2003 is related to upgrade in the ventilation system. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 12 / 16

Weekly modulation MON TUE WED THU FRI SAT SUN 49 counts per counter (Hz) 48 47 46 45 44 1 2 3 4 5 6 7 8 days counts per counter (Hz) 72 70 68 66 64 62 1 2 3 4 5 6 7 8 days The counting rate behaviour of the average week is inverted before and after the discontinuity of 2003. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 13 / 16

Power Spectral Density (data since 1997 to 2003) The 4 peaks in simmetrical position 1 800 with respect to the 1 day − 1 Power Spectral Density frequency exceeding the 7 σ c.l. are 600 related to the daily signal structure. 400 2 The remaining two peaks represents Out[28]= the weekly modulation (0,14 day − 1 ) 200 and its higher harmonic. 0 Frequency H days - 1 L 0.5 1.0 1.5 2.0 3 In the low frequency spectrum the higher peak corresponds to a 1400 frequency compatible with an annual Power Spectral Density 1200 modulation: 1000 800 frequency: 365 ± 32 d 600 Out[37]= amplitude: 1.5 – 2.5 Hz 400 The peaks exceeding 3 σ c.l. in the 200 4 frequency region between 0.03 and 0 Frequency H days - 1 L 0.00 0.02 0.04 0.06 0.08 0.10 0.04 day − 1 correspond to a monthly periodicity and are under study (they could be related to a tidal effect). Notations: background level, 3 σ c.l., 7 σ c.l. Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 14 / 16

Annual Modulation h365 h365 52 offset offset 46.66 46.66 counts per counter (Hz) amplitude amplitude 0.7347 0.7347 51 frequency frequency 332.7 332.7 phase phase -0.4744 -0.4744 50 49 48 47 46 45 44 50 100 150 200 250 300 350 days Counting rate of 6 years averaged and fitted by sinusoidal function: k + A · sin (2 π ( t T + φ )) obtaining: T = 333 ± 32 d, maximum = 240 ± 32 d Gianmarco Bruno (INFN–LNGS) TAUP 2009, Rome July 3, 2009 15 / 16