We developed a CANDLES-III system to study the neutrinoless double beta () decay of . The proposed system employs 96 scintillation crystals (305 kg) with natural Ca () isotope which corresponds 350 g of . External backgrounds were rejected using a active shield of a liquid scintillator surrounding the crystals. The internal backgrounds caused by the radioactive impurities within the crystals can be reduced effectively through analysis of the signal pulse shape. We analyzed the data obtained in the Kamioka underground for a live-time of 130.4 days to evaluate the feasibility of the low background measurement with the CANDLES-III detector. Using Monte Carlo simulations, we estimated the background rate from the radioactive impurities in the crystals and the rate of high energy -rays caused by the reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately events/keV/yr/(kg of ), in the window. In conclusion, the background candidates were estimated properly by comparing the measured energy spectrum with the background simulations. With this measurement method, we performed the first search for decay in a low background condition using a detector on the scale of hundreds of kg of nonenriched Ca. Deploying scintillators enriched in will increase the sensitivity strongly. has a high potential for use in decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.
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"titles": [
{
"source": "APS",
"title": "Low background measurement in CANDLES-III for studying the neutrinoless double beta decay of <math><mrow><mmultiscripts><mrow><mi>Ca</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>48</mn></mrow></mmultiscripts></mrow></math>"
}
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"dois": [
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"value": "10.1103/PhysRevD.103.092008"
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"publication_info": [
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"journal_volume": "103",
"journal_title": "Physical Review D",
"material": "article",
"journal_issue": "9",
"year": 2021
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"source": "APS",
"value": "We developed a CANDLES-III system to study the neutrinoless double beta (<math><mrow><mn>0</mn><mi>\u03bd</mi><mi>\u03b2</mi><mi>\u03b2</mi></mrow></math>) decay of <math><mrow><mmultiscripts><mrow><mi>Ca</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>48</mn></mrow></mmultiscripts></mrow></math>. The proposed system employs 96 <math><mrow><msub><mrow><mi>CaF</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></math> scintillation crystals (305 kg) with natural Ca (<math><mrow><mmultiscripts><mrow><mi>Ca</mi></mrow><mprescripts></mprescripts><none></none><mrow><mi>nat</mi></mrow></mmultiscripts></mrow></math>) isotope which corresponds 350 g of <math><mrow><mmultiscripts><mrow><mi>Ca</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>48</mn></mrow></mmultiscripts></mrow></math>. External backgrounds were rejected using a <math><mrow><mn>4</mn><mi>\u03c0</mi></mrow></math> active shield of a liquid scintillator surrounding the <math><mrow><msub><mrow><mi>CaF</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></math> crystals. The internal backgrounds caused by the radioactive impurities within the <math><mrow><msub><mrow><mi>CaF</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></math> crystals can be reduced effectively through analysis of the signal pulse shape. We analyzed the data obtained in the Kamioka underground for a live-time of 130.4 days to evaluate the feasibility of the low background measurement with the CANDLES-III detector. Using Monte Carlo simulations, we estimated the background rate from the radioactive impurities in the <math><mrow><msub><mrow><mi>CaF</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></math> crystals and the rate of high energy <math><mi>\u03b3</mi></math>-rays caused by the <math><mrow><mo>(</mo><mrow><mi>n</mi><mo>,</mo><mi>\u03b3</mi></mrow><mo>)</mo></mrow></math> reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately <math><mrow><msup><mrow><mn>10</mn></mrow><mrow><mo>\u2212</mo><mn>3</mn></mrow></msup></mrow></math> events/keV/yr/(kg of <math><mrow><mmultiscripts><mrow><mi>Ca</mi></mrow><mprescripts></mprescripts><none></none><mrow><mi>nat</mi></mrow></mmultiscripts></mrow></math>), in the <math><mrow><mn>0</mn><mi>\u03bd</mi><mi>\u03b2</mi><mi>\u03b2</mi></mrow></math> window. In conclusion, the background candidates were estimated properly by comparing the measured energy spectrum with the background simulations. With this measurement method, we performed the first search for <math><mrow><mn>0</mn><mi>\u03bd</mi><mi>\u03b2</mi><mi>\u03b2</mi></mrow></math> decay in a low background condition using a detector on the scale of hundreds of kg of nonenriched Ca. Deploying scintillators enriched in <math><mrow><mmultiscripts><mrow><mi>Ca</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>48</mn></mrow></mmultiscripts></mrow></math> will increase the sensitivity strongly. <math><mrow><mmultiscripts><mrow><mi>Ca</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>48</mn></mrow></mmultiscripts></mrow></math> has a high potential for use in <math><mrow><mn>0</mn><mi>\u03bd</mi><mi>\u03b2</mi><mi>\u03b2</mi></mrow></math> decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements."
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"date": "2021-05-26",
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