Leaching Studies for the SNO+ Experiment

Transcription

1 Leaching Studies for the SNO+ Experiment Pouya Khaghani Laurentian University SNOLAB Users Meeting Symposium September 2 nd

2 SNO+ Physics SNOLAB, Creighton Mine (2070m 6000 m. w. e) Linear Alkyl Benzene (LAB) Physics Goals: Neutrino-less double beta decay ( 130 Te-loaded scintillator) Low energy solar neutrinos Geo and reactor anti neutrinos Supernova neutrinos Nucleon decay (water phase) 2

3 Backgrounds for the experiment Internal Backgrounds: Radioisotopes: U/Th chain, 40 K, etc Te cosmogenics ( 40 Co, 88 Y, ) Leaching of 222 Rn daughters into the detector μ External Backgrounds: Gammas from PMTs and AV ( 208 Tl, 214 Bi, ) Cosmic muons and fast neutrons from the cosmic muons 8 B solar neutrinos 2νββ decay of Tellurium n α, β γ n, p, 11 C

4 Backgrounds from the implanted radon daughters Scintillator target goal: 238 U ~ g/g [1] Level of 222 Rn in the lab air ~ 131 ± 6.7 Bq/m 3 [2] Implanted radon daughters in the acrylic are a source of background for SNO+ Backgrounds: 210 Bi: Solar Phase (Q β = 1.16 MeV) 210 Po: 0νββ search (α,n) interaction 2.2 MeV γ How quickly will the surface contaminants leach into the liquid? [1] M. Pallavicini, Nucl. Phys. B (Proc. Suppl.) 217, (2011) [2] Ian. T. Lawson, Radon Levels in the SNO+ Radon Reduction Room and Acrylic Vessel, SNO+ internal report,

5 Leaching of 222 Rn daughters as a first order process Molecular leaching as a first order process. dn(t) dt = k T N(t) Temperature dependency: k T = A e E a R T 5

6 Leaching rate measurements for SNO+ Bench-top measurements ( spiked acrylic samples): Activity of 210 Pb: high efficiency gamma counter (E = 46.5 KeV) Activity of 210 Po: Silicon alpha counter (E α = 5.3 MeV) In-situ measurements of the activity. Measurements were performed for different temperatures and into different media (UPW, LAB, Te+LAB, EDTA+UPW, etc.). The table is taken from [1] [1] SNO+ DocDB-1811-v1, Oleg Chkvoret, Results on leaching Pb-210 and daughters from acrylic vessel and spiked acrylic samples 6

7 The leaching model for SNO+ Fitting the model to the data points. Interpolate the leaching rate according to the temperature and the nature of contaminant. Determine the amount of desorbed contaminant through the equations. Output: discrete data set of activity + generate a plot 7

8 Estimating the activity of the desorbed 222 Rn daughters for a possible timeline Ethylenediaminetetraacetic acid (EDTA) suggested to accelerate the leaching process. Bismuth and polonium are in equilibrium with lead. Initial Activity ~ 2.3 ± 0.8 Bq/m 2 8

9 4 months of UPW at 12 C Ethylenediaminetetraacetic acid (EDTA) suggested to accelerate the leaching process. Bismuth and polonium are in equilibrium with lead. Initial Activity ~ 2.3 ± 0.8 Bq/m 2 i. 4 months UPW (12 C) 7

10 5 months of UPW + (0.027M) EDT at 12 C Ethylenediaminetetraacetic acid (EDTA) suggested to accelerate the leaching process. Bismuth and polonium are in equilibrium with lead. Initial Activity ~ 2.3 ± 0.8 Bq/m 2 i. 4 months UPW (12 C) ii. 5 months of UPW + (0.027M) EDTA (12 C) 7

11 6 months of liquid scintillator at 12 C Ethylenediaminetetraacetic acid (EDTA) suggested to accelerate the leaching process. Bismuth and polonium are in equilibrium with lead. Initial Activity ~ 2.3 ± 0.8 Bq/m 2 i. 4 months UPW (12 C) ii. 5 months of UPW + (0.027M) EDTA iii. iv. Water will be removed 6 months of liquid scintillator 7

12 5 years of Te-loaded scintillator Ethylenediaminetetraacetic acid (EDTA) suggested to accelerate the leaching process. Bismuth and polonium are in equilibrium with lead. Initial Activity ~ 2.3 ± 0.8 Bq/m 2 i. 4 months UPW (12 C) ii. 5 months of UPW + (0.027M) EDTA iii. iv. Water will be removed 6 months of liquid scintillator v. Liquid scintillator will be removed vi. 5 years of Te-loaded scintillator The activity of the radon daughters reduced by 90% in less than 3 years with the suggested timeline. 7

13 The lead assay on January 27 th, 2015 Jan 27 th -28 th : water assay performed (water-fill since Oct 11 th ) Water passed HTiO column and then counted Using high efficiency gamma counter, the specific activity of Pb-210 = Bq/m 3 Temperature: T avg = 18.4 C The leaching constant for 18.4 C was interpolated through the model k 18.4 C = [day 1 ] 9

14 Estimating the desorbed lead through the model Initial activity of acrylic ~ 2.3 ± 0.8 Bq/m 2 Leaching per unit area: dn a (t) dt dn w (t) dt = k T N a t λn a (t) = k T N w t λn w (t) Total desorbed lead = Integral of N w t over the surface A(t) = λ t/ τ V(t) 1 N w t iδτ h i+1 h i. 2πR Calculated desorbed lead over 120 days: A = Bq/m 3 10

15 Other possible sources of 210 Pb in water Lead concentration in a blank sample of UPW Water plant assay (August 2014) A = ± mbq/m 3 (negligible) Diffusion of the radon in the lab air into the water Radon level inside the acrylic vessel ~ ( ± ) (131 ± 6.7 )Bq/m 3 Partition coefficient of 222 Rn between UPW and air at 18.4 C κ = Fick s Law: C(z, t) t = D ΔC(z, t) C w z, t = κ C air (1 4 π n=0 1 2n + 1 e 2 n+1 2 π2 Dt 4h 2 sin 2n + 1 π z 2h ) A z, t = C w z, t dv 11

16 Diffusion of Radon into water as a source of lead Total activity of diffused radon into water: A w t = i=t/δτ i=0 0 h i C w z, t S z dz Generated a discrete data set to determine the amount of produced lead. 12

17 Results for the January lead assay Specific Activity of produced lead from the diffused radon in UPW: A = Bq/m 3 Desorbed lead + lead from air: A sum = Bq/m 3 Measured lead from the water assay: A = Bq/m 3 13

18 The lead assay May 20 th, m 3 of water passed through the filter (HTiO column) Eluted with HCl Sample was counted for 7.9 days ( 210 Pb peak= 46 KeV) Total Activity = ± Bq 18

19 Desorbed 210 Pb through the leaching model Assumptions: A int = 2.3 ± 0.8 Bq/m 2 T = 18 ± 1 ` A measured = 63 ± 17 mbq/m 3 A model = mbq/m 3 19

20 Conclusion: The leaching model works! The measured activities from lead assays were compatible with the predicted values by the model. Other sources such as the diffusion of 222 Rn from air into water is complicated due to the temperature gradient from the bottom of AV, but relatively low compared to leaching. Question? 14

21 Backup EDTA Ethylenediaminetetraacetic acid (EDTA) was suggested to accelerate the leaching rate. Has no effect on the leaching rate of 210 Bi and 210 Po UPW+ EDTA (0.25M) accelerates the leaching process of 210 Pb by factor of 30 15

22 Backup (Radon diffusion) Radon diffusion as a function of depth 16