Thermal neutron–induced γ-ray background in 124Sn

Journal Pre-proof Thermal neutron–induced γ-ray background in

124

Sn

G. Gupta, H. Krishnamoorthy, A. Garai, A. Mazumdar, V. Nanal, A. Shrivastava, R.G. Pillay PII:

S0969-8043(19)30363-X

DOI:

https://doi.org/10.1016/j.apradiso.2019.108923

Reference:

ARI 108923

To appear in:

Applied Radiation and Isotopes

Received Date: 28 March 2019 Revised Date:

23 September 2019

Accepted Date: 2 October 2019

Please cite this article as: Gupta, G., Krishnamoorthy, H., Garai, A., Mazumdar, A., Nanal, V., 124 Shrivastava, A., Pillay, R.G., Thermal neutron–induced γ-ray background in Sn, Applied Radiation and Isotopes (2019), doi: https://doi.org/10.1016/j.apradiso.2019.108923. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Thermal neutron–induced γ-ray background in

124

Sn

G. Gupta1 , H. Krishnamoorthy2,3 , A. Garai2,3 , A. Mazumdar2,3 , V. Nanal1,† , A. Shrivastava3,4 , R. G. Pillay1,∗ 1 Department

of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Mumbai 400005, India

2 India-based

3 Homi 4 Nuclear

Neutrino Observatory, Tata Institute of Fundamental Research, Mumbai 400005, India

Bhabha National Institute, Anushaktinagar, Mumbai 400094, India

Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

Abstract The thermal neutron-induced gamma-ray background in

124

Sn is investigated

in connection with neutrinoless double beta decay (0νββ) studies in this purpose, a 99.26% enriched 15

124

124

Sn. For

Sn sample was irradiated with a thermal

2

neutron fluence of 3×10 /cm in the Dhruva reactor at the Bhabha Atomic Research Centre, Mumbai. The gamma ray spectra of the irradiated sample were measured in a low background counting setup to study both long-lived and short-lived activities. The present data give an independent measurement of the half-life of

125

+

Sn∗ ( 32 ) and

125

−

Sn( 11 2 ) as 10.01(8) min and 9.63(2) d,

respectively. The impact of the observed high-energy gamma rays and the residual activity due to 125 Sb, on the background in the region of interest around the Qββ value of

124

Sn (∼2.291 MeV) is discussed.

Keywords: Thermal neutron capture, γ-ray background

I† [email protected] ∗ Present address: Department of Physics, Indian Institute of Technology Ropar, Rupnagar, India

Preprint submitted to Applied Radiation and Isotopes

October 5, 2019

1. Introduction It is important to understand and minimize the neutron-induced background arising from material in and around the detector in studies of rare decays such as neutrinoless double beta decay (0νββ) or dark matter searches [1, 2]. High 5

energy neutrons are produced in the fission and (α,n) reactions of trace elements nat U and nat Th in rocks and surrounding materials, and in cosmic muoninduced reactions. These neutrons can undergo various reactions in surrounding materials and can be thermalized. Thermal neutrons are also a potential background hazard because of (n,γ) reactions. In 0νββ, which is a second

10

order weak interaction process, event rates are very low since T1/2 > 1026 y and the neutron-induced background is a limiting factor for the sensitivity [3]. Feasibility study of TIN.TIN (The INdia-based TIN Detector), a Sn cryogenic superconducting bolometer, for the study of 0νββ in

124

Sn is in progress at

Tata Institute of Fundamental Research (TIFR) [4]. The fast neutron-induced 15

gamma-ray background in Sn and some other detector materials was investigated with a neutron flux of ≈ 105 n/cm2 /s and was reported in [5]. Thermal neutron capture by decays to

125

124

Sn leads to

125

Sn that decays to

125

Sb, which further

Te. A schematic of the reaction and decay processes is shown in

Fig. 1. It may be noted that neutron capture can lead to the population of 20

excited states of

125

Sn (up to 5.7335 MeV [6]). However, all these states decay

predominantly by gamma-ray emission either to the

3+ 2

state. It can be seen that the Q value of beta decay of Qββ (

124

state or to the 125

Sn is larger than

Sn) = 2.2911 ± 0.0015 MeV [7] and hence is a potential source of back-

ground. In addition, several gamma rays are generated in the decay of 25

and

125

in the

11 − 2

125

Sn

Sb. Verma et al. [8] have reported detailed gamma-ray measurements

124

Sn(n,γ) reaction, including those with very weak branching ratios. As

per the available spectroscopy data [9], gamma rays in the energy range from 1.5 to 2.275 MeV can be emitted in this decay chain with a branching ratio of up to 1.9%. These high-energy gamma rays are of concern, as they can adversely 30

affect the sensitivity of the 0νββ measurement in 124 Sn. With the motivation to

2

understand this neutron-induced background in

124

Sn, particularly at energies

above 2 MeV (i.e., close to the region of interest [ROI] for 0νββ),

124

Sn was

irradiated with thermal neutrons in the Dhruva reactor at the Bhabha Atomic Research Centre (BARC), Mumbai. Both short-lived and long-lived activities 35

were studied (∼10 min to 550 d) in a low background counting setup. Identification of gamma rays was confirmed by measurement of the half-life. This article presents details of the n-capture induced gamma-ray background in

3

124

Sn.

Figure 1: Schematic presentation of neutron capture of

124 Sn

and subsequent β − decay.

2. Experimental details and data analysis A 40

124

Sn sample (4 mm × 4 mm × 0.2 mm, 23.3 mg, 99.26% enrichment,

procured from ISOTOPE JSC, Russia) was irradiated in the Dhruva reactor (BARC, Mumbai) with a thermal neutron flux of 5× 1013 n/cm2 /s for 1 min. 4

From secondary ion mass spectrometry (SIMS) measurements, traces of other Sn isotopes were measured to be less than 20 ppm and metallic impurities were estimated to be less than 5 ppm. It may be mentioned that the natural isotopic 45

abundance of

124

Sn is 5.79%.

The gamma-ray measurements were done mainly in the TiLES (TIFR Low background Experimental Setup) [10]. This consists of a high efficiency highpurity Ge (HPGe) detector (relative efficiency 70%1 ) provided with an active shield (plastic scintillator for cosmic muon veto) and a passive shield (5 cm 50

oxygen-free high thermal conductivity Cu and 10 cm low activity Pb). Data were acquired with an N6724 CAEN digitizer with a sampling rate of 100 MS/s and 14-bit resolution and analysis was done with LAMPS (Linux Advanced MultiParameter System) [11]. Energy calibration of the TiLES was done with a standard

55

152

Eu source and was monitored with known background lines over

an energy range of 120–2615 keV. The energy resolution of the detector is ≈2.6 keV at 1408 keV. The dead time was measured with a 10 Hz pulser and was estimated to be ≈0.1%. The setup also has a provision for measurements in a close counting geometry with higher efficiency. The detector efficiency in the close counting geometry was estimated by Monte Carlo simulations with

60

GEANT4 [12] for the optimized model of the detector. Dokania et al. [10] have shown that the photopeak efficiency and the overall line shape of gamma rays are well reproduced with the optimized TiLES HPGe detector model used in the present simulations. Prior to irradiation, the radiopurity of the pristine 124 Sn sample was studied

65

in the TiLES for ≈ 14 d (see Fig. 2), and no additional gamma rays were observed above the background level within the measurement sensitivity of the TiLES. In particular, the background in the ROI close to Qββ (124 Sn) was found to be similar to the ambient background. 1 Efficiency

for 1332 keV at 25 cm relative to that of a 3 in. × 3 in. NaI detector.

5

Figure 2: The gamma ray spectrum of

As shown in Fig. 1, 70

125

+

124 Sn

before irradiation.

Sn*( 23 ) (T1/2 = 9.52 min) and

125

−

Sn( 11 2 ) (T1/2 =

9.64 d) are populated in thermal neutron capture reactions with cross sections of 130 mb and 4 mb, respectively [9]. The gamma ray spectrum of the irradiated sample was measured immediately after the irradiation (cool-down time, tc , ∼10 min) with a standard HPGe detector (∼30% relative efficiency) at 10 cm +

and is shown in Fig. 3. The intense peak arising from the short-lived 125 Sn(* 32 ) 75

is clearly visible at 331.9 keV. The half-life of 331.9 keV gamma ray was measured to be 10.01(8) min (see Fig. 3b), which is consistent with the literature 6

value [9]. No additional short-lived impurities were observed in the spectrum.

106 105

ln(counts / min)

Counts/0.7 keV

331.9 keV

104 103 102 (a)

15

T1/2 = 10.01(8) min

14

Tref 1/2 = 9.52(5) min

exp

13 12 11

(b) 500 1000 Energy (keV)

0 10 20 30 40 50 60 Time (min)

Figure 3: (a) Gamma ray spectrum of irradiated and (b) decay curve of Eγ = 331.9 keV

124 Sn

after tc = 10 min (Tcounting = 1 min)

+ (125 Sn*( 32 )).

The irradiated sample was then transported to TIFR and measurements were done in the TiLES. Spectra were recorded at a distance (l) of 12 cm 80

from the detector face to minimize the coincident summing effect and in close counting geometry (l ∼ 1–5 cm) for measurement of the background in the ROI due to summing. A typical spectrum is shown in Fig. 4, where gamma rays from the decay of

125

Sn and

125

Sb are indicated. The inset in the bottom

panel shows an expanded view of the 2250–2300 keV region, where the peak 85

at 2288.2 keV due to summing is clearly visible. It should be mentioned that the close counting geometry data were used only for identification of gamma rays and the intensities of gamma rays were extracted from spectra obtained at l = 12 cm. The identification of various gamma rays was further confirmed by tracking the half-life of individual gamma rays.

7

Figure 4: Gamma ray spectrum of the irradiated 124 Sn after tc ∼1 d in the TiLES (l = 9 mm, Tcounting = 16.3 h). The inset shows an expanded view of the region of interest and sum peak (#) at 2288.2 keV (DE: double escape, SE: single escape).

90

3. Results and discussion Tables 1 and 2 list the observed gamma rays from the decay of and

125

Sb, respectively. For

125

125

−

Sn( 11 2 )

+

Sn*( 32 ), all gamma rays other than 331.9 keV

have much lower intensities (see Fig. 3) [9]. The measured intensity relative to the most dominant gamma rays (i.e., 1067.1 keV for 95

125

125

Sn and 427.9 keV for

Sb) extracted from the l = 12 cm data are also listed in Tables 1 and 2, respec-

tively. Errors were computed by fitting the Gaussian peak and the background. Typical error in the peak centroid, inclusive of the fitting and the calibration error, is less than 0.5 keV. Fitted peak centroids are within ±0.5 keV of the corresponding values in [9]. Additionally, gamma rays corresponding to 100

which arise from the

27

24

Na,

Al(n,α) reaction, are also observed. However, the origin

of the aluminum could not be ascertained. The Compton background arising from

24

Na and other high-energy gamma rays was the main limiting factor for

measurement of weaker decay branches, and gamma rays with intensities less than ≈ 0.04% could not be observed at l = 12 cm. The present measurements 8

105

resulted in a more accurate value of T1/2 of 9.63(2) d for

125

−

Sn( 11 2 ), as shown

in Fig. 5. Table 1: Observed gamma rays from the decay of

125 Sn( 11 − ) 2

ref = 9.64(3) d) together (T1/2

with the measured intensity (Ir ) relative to the intensity of 1067.1 keV gamma ray.

Energy

Ir (%)

Ir (%)

Energy

Ir (%)

Ir (%)

(keV)

(measured)

(reference)

(keV)

(measured)

(reference)

270.6

1.8(3)

1.10(2)

1163.8

–

0.32(2)

332.1

14.1(4)

14.5(3)

1173.3

2.5(6)

1.87(4)

351.0a

9.7(6)

2.72(5)

1186.2

–

0.09(1)

469.9

15.9(2)

15.3(3)

1220.9

2.4(5)

2.76(6)

563.0

–

0.16(2)

1349.4

0.7(4)

0.61(2)

652.6

–

0.42(1)

1419.7

4.6(5)

5.02(10)

800.3

10.0(9)

11.00(22)

1557.3

–

0.042(10)

822.5

43(1)

44.1(9)

1591.4

2.3(9)

0.26(2)

893.4

3.8(7)

3.0(1)

1806.7

1.8(5)

1.53(3)

c

1.8(7)

0.76(4)

915.6

40(1)

42.6(9)

1889.9

934.6

2.8(4)

2.15(4)

1982.5

–

0.033(10)

1017.4

2.8(6)

3.30(7)

2002.1

20(1)

19.8(4)

100.0(2)

100

2201.0

--

0.40(2)

1089.2

56(2)

12.3(2)+47.3(9)

2275.7

2.2(6)

1.88(4)

1151.2

1.2(4)

1.18(2)

1067.1 b

a

Mixed with 351.9 keV gamma rays from the ambient background (214 Pb decay).

b c

Composite of 1087.7 keV and 1089.2 keV, which could not be resolved.

Intensity affected by coincident summing; see the main text for details.

9

Table 2: Observed gamma rays from the decay of

125 Sb

together with the measured intensity

(Ir ) relative to the intensity of 427.9 keV gamma ray.

Energy

Ir (%)

Ir (%)

(keV)

(measured)

(reference)

117.0

1.0(4)

0.887(9)

176.3

21.1(7)

23.11(5)

204.1

1.3(4)

1.070(21)

a

208.1

1.2(4)

0.837(14)

227.9

–

0.443(6)

321.0

1.3(2)

1.404(9)

380.5

5.6(4)

5.124(19)

402.0

–

0.021(2)

408.1b

1.1(1)

0.623(6)

427.9

100(1)

100

443.6

0.8(3)

1.035(6)

463.4

36.1(6)

35.45(10)

600.6

58(1)

59.62(16)

606.7

16.2(8)

16.83(6)

636.0

38.3(9)

37.9(3)

671.4

6.7(4)

6.049(19)

a

Mixed with 209.3 keV gamma rays from the ambient background (228 Ac decay).

b

Mixed with 409.5 keV gamma rays from the ambient background (228 Ac decay).

10

exp

ln(counts / hr)

ln(counts / hr)

9 8 7 6 5

exp

6

T1/2 = 9.63(2) d

T1/2 = 9.6(1) d

5 4 3 2

(a) 1067.1 keV

0

20 40 Time (d)

(b) 1889.9 keV

1 0

60

20 40 Time (d)

Figure 5: Decay curves for different gamma rays produced in the decay of keV, (b) 1889.9 keV

ref (T1/2

125 Sn:

60 (a) 1067.1

= 9.64(3) d).

The observed intensities of most of the gamma rays from the decay of and

125

125

Sn

Sb are consistent with the reference values, except for very weak (< 0.1%)

high-energy gamma rays such as those of 1889.9 keV. From the measured T1/2 110

of 1889.9 keV, which is in very good agreement with the reference value [9], it is evident that this peak arises from the

125

Sn decay and does not have

an extraneous contribution. As mentioned earlier, depending on the detector geometry, the measured intensity of high-energy gamma rays can be affected by coincident summing of multiple gamma rays in a cascade. A partial decay 115

scheme of

125

Sb, populated in the decay of

125

Sn, is shown schematically in

Fig. 6. A list of possible pathways of coincident summing of two gamma rays in a cascade resulting in a high-energy photopeak is given in Table 3 for a few energies of interest. The probability P0 for the photopeak detection of a direct γ0 transition with branching ratio Iγ0 and detector efficiency 0 can be given 120

as Iγ0 0 . A simple estimation of the coincident summing probability P12 of two gamma rays (γ1 , γ2 ) in a given cascade to the photopeak of Eγ0 (= Eγ1 + Eγ2 ) was carried out using GEANT4 simulations [12]. The simulations employed the

11

optimized TiLES detector [10] geometry with a distributed source of sample size of 4 mm × 4 mm, at a distance l from the detector face. An event consists 125

of two components: (i) generation of the first gamma ray in the cascade of energy Eγ1 , (ii) generation of the second gamma ray of energy Eγ2 with a probability fγ2 , which is the partial branching ratio of the γ2 emission from the corresponding level. Both these gamma rays are tracked in the detector and the energy deposited is recorded for Ns0 = 107 events. Both gamma rays are

130

emitted isotropically, ignoring angular correlations. The summing fraction is obtained from the intensity of the peak corresponding to Eγ0 in the simulated spectrum (Nsum ) as Psum = Nsum /Ns0 .

(1)

The probability of summing contribution to the photopeak of Eγ0 is then calculated as P12 = Iγ1 Psum , 135

(2)

where Iγ1 is the branching ratio of γ1 . Table 3 lists the simulated summing probability P12 at l = 12 cm and 9 mm together with the corresponding P0 . It is evident that for 1889.9 keV the coincident summing contribution is significant (∼50%) even at a distance of 12 cm and results in the higher observed intensity. The simulations further illustrate that the intensities of the

140

weak decay branches are affected by the coincident summing, for example for 1982.5 keV (Iγ = 0.0032%, P12 ∼ 10P0 ). Hence, the intensities of such weak transitions cannot be reliably extracted in the present measurements even at l=12 cm. More importantly, coincident summing contributes significantly at energies higher than 2 MeV, and its impact is discussed in the next section.

145

The contribution from summing of three gamma rays will be further reduced by the corresponding γ3 fγ3 and hence is neglected here. The absolute yield of 125

Sn was measured to be 1.36(±0.2) × 109 , which is found to be consistent with

the calculated value using the listed cross section and the incident neutron flux.

12

11/2+

2002.1 keV

9/2,11/2+ 9/2-,11/2-

822.5

800.3

893.4

9/2+

10 9.2

2275.8 keV

684.0 1186.2

1591.6 keV

7/2+,9/2+

1208.4

915.6

1089.5 keV 1067.3 keV

11/2+

9/2,11/2+

1982.9 keV

563.0

1419.9 keV

9/2+

1198.7

11/2-

1889.9 keV

469.9

2288.2 keV

1220.9 286.2

1419.7

15 7. 5/2+

1591.4

332.2 keV 1889.9 332.1

106 .1 7/2+

1982.5

2002.1

2275.7

0 keV

Figure 6: A partial level scheme of

125 Sb,

populated in the beta decay of

125 Sn.

Table 3: A list of possible pathways for coincident summing of two gamma rays in the decay of

125 Sn

for a few energies of interest. The corresponding branching ratio/partial branching

ratio in percent is indicated in parentheses for each gamma ray.

Elevel (Iγ0 )

l

P0

keV

mm

(%)

1889.9

120

1.7×10

−4

(0.074)

1982.9

120

7.1×10−6

(0.0032) 2240.7

2275.8

9

9

–

4.2×10−3

(0.18)

2288.2

9

–

Eγ1 (Iγ1 )

Eγ2 (fγ2 )

Psum

P12

keV

keV

(%)

(%) −3

1.0×10−7

1557.3 (0.0041)

332.1 (100)

2.5×10

822.5 (4.3)

1067.1 (100)

1.4×10−3

6.1×10−5

800.3 (1.1)

1089.2 (100)

1.3×10−3

1.4×10−5

469.9 (1.5)

1419.7 (29)

4.7×10−4

7.1×10−6

915.6 (4.1)

1067.1 (100)

1.4×10−3

5.9×10−5

893.4 (0.29)

1089.2 (100)

1.3×10−3

3.8×10−6

1173.3 (0.18)

1067.1 (100)

0.14

2.5×10−4

1151.2 (0.11)

1089.2 (100)

0.14

1.5×10−4

890.5 (0.009)

1349.4 (14.5)

<1×10−5

434.1 (0.024)

1806.7 (96.8)

<1×10−5

351 (0.26)

1889.9 (1.06)

<1×10−5

258.3 (0.01)

1982.5 (0.07)

<1×10−5

684.0 (0.011)

1591.4 (37.9)

4.5×10−2

5.0×10−6

1186.1 (0.009)

1089.2 (100)

0.15

1.3×10−5

1208.4 (0.008)

1067.1 (100)

0.14

1.2×10−5

1220.9 (0.27)

1067.1 (100)

0.13

3.6×10−4

1198.7 (0.016)

1089.2 (100)

0.14

2.2×10−5

286.2 (0.0058)

2002.1 (88)

0.22

1.2×10−5

13

Effect of neutron capture background for 0νββ in 150

124

Sn

The main motivation for the present study was to assess the neutron-induced background in the ROI close to Qββ (124 Sn), namely, 2250 to 2300 keV. For this purpose, the spectrum of the irradiated Sn sample was measured in a close counting geometry after tc ∼ 1 d and 39 d (∼ 4T 1/2 ) as well as after a prolonged

2242.0 keV

tc = 1.19 d (Tcounting = 0.68 d) tc = 38.9 d (Tcounting=1.97 d) tc = 1.5 y (Tcounting = 1.97 d)

1000

2240.7 keV

2201.0 keV

Counts / 0.7 keV

1250

750

500 2150

2175

2200

2275.7 keV

cool-down time of 1.5 y (see Fig. 7).

2225

2250

227

Energy (keV) Figure 7: Comparison of energy spectra in the vicinity of region of interest after different cool-down times. The left scale corresponds to tc = 1.19 d, while that for tc = 38.9 d and 1.5 y is indicated on the right side.

155

The energy windows chosen were: 2282–2300 keV (w1: Qββ ± 9 keV), 2269– 2281 keV (w2: photopeak region of the highest-energy gamma rays in the 125 Sn decay cascade), and 2166–2265 keV (w3: as a measure of the integral background level). It is important to note that there is no direct gamma transition of 2288.2 keV and the observed enhancement in window w1 results from the coincident 14

160

summing as depicted in Table 3. For energy windows w1 and w2, the excess counts above the background are obtained using a Gaussian peak shape together with a linear background fit function. In the lower-energy window (w3), integral counts are taken into consideration. The ambient background was measured before and after the counting of the irradiated sample at tc = 1.5 y, and is shown

165

in Table 4 for the energy windows of interest. In the background and tc = 1.5 y spectra, no peaks are visible in either window, and hence the integral counts are listed. In the w3 region, initially the background is dominated by the Compton tail of 24 Na gamma rays, but shows a significant reduction after tc ∼39 d. Thus, in the spectrum at tc ∼ 39 d, a weak coincident summing peak corresponding

170

to E = 2240.7 keV is also visible. It is evident that the counts in energy windows w1 and w2, in both data sets at tc ∼ 1 d and 39 d, show a reduction consistent with T1/2 (125 Sn). The measured yields are also compared with the expected yield inclusive of the coincident summing contribution obtained in the simulation. The errors in the simulated spectra were estimated by considering

175

1 mm error in l during the placement of the source (i.e., l = 9–10 mm). It can be seen that the measured yield for the 2275.7 keV photopeak agrees very well with the corresponding simulated value. For the crucial Qββ region (i.e., w1) the measured and simulated yields seem to agree within errors, although the experimentally measured mean yield appears to be ≈ 30% higher than the

180

corresponding simulated value.

15

Table 4: Background in ROI close to Qββ (124 Sn) for energy windows of interest: w1 (2282– 2300 keV), w2 (2269–2281 keV), and w3 (2166–2265 keV) after different cool-down times (125 Sn, Ns0 = (1.36 ± 0.2) × 109 ). The shielding configuration of TiLES for each case is also listed.

tc

1.19 d (without muon veto)

38.9 d (without muon veto)

Window

Yield (experiment)

Yield (simulation)

(/d)

(/d)

479(168)a

w1

a

357(30)

w2

4213(191)

3825(559)

w3

175784(419)

–

a

w1

36(12)

a

23(2)

w2

275(12)

250(36)

w3

972(31)

–

1.5 y

w1

36(6)

(with muon veto and

w2

28(5)

passive shield )

w3

204(14)

Ambient background

w1

38(6)

(with muon veto and

w2

24(5)

passive shield ) w3 193(14) a Area of the peak with Gaussian plus linear background fit.

From the measured yields in Table 4, the activity due to

124

Sn(n,γ) in the

2282–2300 keV energy window, immediately after the irradiation, is estimated to be 14.4 mBq/g/keV for an incident neutron fluence of 3 × 1015 /cm2 . It is evident that after the prolonged cool-down period of 1.5 y, the integral counts 185

in all three energy windows are similar to the ambient background. However, 125

Sb (T1/2 = 2.76 y) gamma rays are still present in the spectra below 1 MeV.

For energy windows w1 and w2, the minimum detectable limit for detection (LD ) can be estimated by Currie’s method [13]: LD = 4.65σB + 2.7,

190

(3)

where σB is the standard deviation in the ambient background level (NB ). Using √ σB = NB , we obtain LD ∼ 25 µBq/keV. Hence, the activity of the irradiated sample after tc = 1.5 y is found to be lower than 1 mBq/g/keV. 16

It is important to note that the prolonged exposure of 124 Sn to thermal neutrons will lead to a build-up of 125 Sn and the expected equilibrium concentration will be ≈ K/λ, where K is the production rate and λ is the decay constant. 195

In a Sn detector module, the background contribution in the ROI will arise both from the high-energy electrons originating from the beta decay of to the ground state of

125

after the beta decay of

125

Sn

Sb (I = 81%) and from the gamma rays originating

125

Sn to

125

+

Sb( 11 2 ) with feeding fraction I = 0.29%.

In the former case, the expected fraction of electrons in this energy window, 200

which is close to Qβ (see Fig. 1), is ≈ 1 × 10−3 . The beta decay of

125

Sn to

states other than the ground state of 125 Sb has an effective Q value of 1290 keV or less and hence is not relevant for the background consideration in the ROI. +

Assuming that the decay cascade of 125 Sb( 11 2 ) is contained within the detector module, the maximum contribution to the background from gamma rays will 205

be ≈ 3 × 10−3 (of the order of the feeding fraction). To estimate the tolerable neutron dose, based on the observed background in the w1 energy range (see Table 4), a fraction of ≈ 5 × 10−3 spread over a 10 keV window is assumed to contribute to the ROI. Therefore, to achieve a desired background rate of 10−2 counts/(kg y keV) in an energy window of 2282–2300 keV, the Sn detector

210

material should not be exposed to a neutron flux larger than 3 × 10−5 n/cm2 /s during the handling or processing stages.

Summary The thermal neutron capture–induced gamma ray background in studied in connection with 0νββ studies in 215

124

Sn. An enriched

124

124

Sn was

Sn sample

was irradiated with thermal neutrons in the Dhruva reactor at BARC, Mumbai. Gamma-ray spectra were measured in the low background counting setup TiLES. No additional short-lived activities were observed in the sample besides 125

+

Sn*( 32 ). The measured half-life values of

125

+

Sn∗ ( 32 ) and

125

−

Sn( 11 2 ) are

10.01(8) min and 9.63(2) d, respectively. The intensities of high-energy gamma −

220

rays populated in the decay of 125 Sn( 11 2 ) were found to be consistent with those

17

published previously. The contribution to the background due to

124

Sn(n,γ) in

the energy window close to the ROI of 0νββ (124 Sn), namely, 2282–2300 keV, is estimated to be 14.4 mBq/g/keV for an incident neutron fluence of 3× 1015 /cm2 immediately after irradiation. After a cool-down time of 1.5 y, the background 225

due to neutron capture in the ROI is less than 1 mBq/g/keV. From the measured yield in the ROI near Qββ (124 Sn), it is estimated that prolonged exposure to neutron flux exceeding 3 × 10−5 n/cm2 /s can be a significant concern for the background.

Acknowledgments 230

We thank the Dhruva reactor irradiation facility staff, Dr. Suparna Sodaye, and Dr. R. Acharya for their help during irradiation and initial counting at BARC. We are grateful to Dr. K.G. Bhushan for SIMS measurements, and to Mr. M.S. Pose, Mr. S. Mallikarjunachary, and Mr. K.V. Divekar for their help during measurements at TIFR.

235

[1] S. Dell’Oro et al., 2016. Neutrinoless Double Beta Decay: 2015 Review. AHEP 2016, 2162659. [2] L. Roszkowski et al., 2018. WIMP dark matter candidates and searchescurrent status and future prospects. Rep. Prog. Phys. 81, 066201. [3] D. M. Mei et al., 2006. Muon-induced background study for underground

240

laboratories. Phys. Rev. D 73, 053004. [4] V. Nanal, 2014. Search for neutrinoless double beta decay in

124

Sn. EPJ

Web of Conferences 66, 08005. [5] N. Dokania et al., 2014. Study of neutron-induced background and its effect on the search of 0νββ decay in 245

124

Sn. JINST 9, P11002.

[6] I. Tomandl et al., 2011. Nuclear structure study of semi-magic (n,γ) and (d,p) reactions. Phys. Rev. C 83, 044326.

18

125

Sn via

[7] M. Wang et al., 2017. The AME2016 atomic mass evaluation (II). Tables, graphs and references. Chin. Phys. C 41, 030003. [8] H. R. Verma et al., 1980. The decay of 250

125g

Sn. J. Phys. Soc. Jpn. 48,

1415–1422. [9] J. Katakura, 2011, Nuclear Data Sheets for A = 125. Nucl. Data Sheets 112, 495. [10] N. Dokania et al., 2014. Characterization and modeling of a low background HPGe detector. Nucl. Instrum. Methods A 745, 119–127.

255

[11] http://www.tifr.res.in/~pell/lamps.html [12] S. Agostinelli et al., 2003. GEANT4—a simulation toolkit, Nucl. Instrum. Methods A 506, 250–303. [13] L.A. Currie, 1968. Limits for qualitative detection and quantitative determination. Anal. Chem. 40, 586–593.

19

• The impact of gamma-ray background due to thermal neutron

capture in 124Sn for the reason of interest around Qββ- (124Sn) ~ 2.291 MeV is discussed. • The measured background due to gamma rays from 124Sn (n, γ) in the Qββ- (124Sn) region, is found to be consitent with simulation within errors. • The present data give an independent measurment of the halflives of 125Sn*( +) and 125Sn( -) as 10.01(8) min and 9.63(2) d, respectively.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: