탄화규소 중성자 검출기 제작 및 특성 평가

Abstract

Neutron detectors used in nuclear reactors, spent fuel facilities, high-energy physics experiments or outer space often face harsh environmental conditions such as high temperature and high radiation dose. It has been a continuing challenge to develop a neutron detector which is durable and provides reliable outputs under such conditions. One of the most promising materials is SiC which can be operated as a semiconductor detector at elevated temperatures by virtue of its wide band-gap energy. In this study, neutron detectors based on silicon carbide (SiC) were fabricated and evaluated under various conditions. The detectors were fabricated using the metal mask technique instead of a lithography. 6LiF was selected as the converter to detect slow neutrons and metal contacts were deposited by a thermal evaporation procedure in high vacuum. Using a conducting epoxy, the SiC chips were mounted onto PCB, Teflon, and Ceramic substrates after the metallization process. The fundamental performance of the devices as radiation detectors was tested using an alpha source (241Am). Afterwards, the thermal neutron response of the detectors was measured at ex-core neutron irradiation facility in the Korea Atomic Energy Research Institute. To confirm a linear response of the detectors, the neutron intensity was modified using a Cd aperture while the equivalent thermal neutron flux was obtained using a gold wire activation measurement. The neutron response of the detectors has been shown to be highly linear (Adjusted R-square: 0.997) in the thermal neutron flux range from 1.6×106 to 1.9×107 n/cm2s. A gamma irradiation test and alpha response measurements with respect to cable length and temperature were carried out as preliminary tests. In the gamma irradiation experiment, the Schottky barrier height of the non-irradiated SiC sample and two samples irradiated by 40 and 120 kGy gamma dose were determined as 0.905, 0.936 and 0.928 eV, respectively, to confirm that the SiC detector is resistant to gamma radiation. In the signal measurement with respect to cable length between detector and preamplifier, the shapes of the spectra were similar at lengths from 50 to 700 cm and noise increased at greater lengths. The total counts of each spectrum were calculated through Gaussian fitting and showed about 3.5 % variation. In the last preliminary test, alpha spectra of the detector were measured under high temperature conditions from 300K to 523K. Even though the temperature increased, the centroid positions of the alpha peaks were almost the same. Also the energy resolution as a function of temperature for the 5.486 MeV alpha-ray was almost uniform within 1.0% of the standard deviation. This is evidence that the SiC detector has thermal properties independent of heat effects.|원자로, 사용후핵연료, 고에너지 실험 또는 우주 분야에서 사용되는 중성자 검출기들은 고온, 고방사선의 극환경에 자주 노출된다. 이러한 환경에서 내구성과 안정적인 신호 측정이 가능한 중성자 검출기 개발이 끊임없이 시도되고 있다. 가장 주목받는 물질들 중 하나는 광대역밴드갭 에너지에 의해 고온에서 반도체검출기로 동작할 수 있는 탄화규소이다. 본 연구에서는 탄화규소 반도체를 기반으로 하는 중성자 검출기를 제작하고 평가하였다. 검출기는 리소그래피 공정 대신 메탈마스크 기법을 사용하여 제작하였다. 6LiF를 속중성자 검출를 위한 변환박막으로 선택하였고, 금속박막은 고진공 열증착방식을 통해 제작하였다. 금속박막 공정 이후에 전도성 에폭시를 사용하여 탄화규소 시편을 PCB, 테프론, 세라믹 기판에 고정하였다. 방사선 검출기로써의 기본성능은 241Am 알파 선원을 사용하여 평가하였고, 이후에 검출기의 열중성자 반응성을 한국원자력연구원의 중성자 조사 시설에서 측정하였다. 검출기의 선형성을 평가하기 위하여 카드뮴 어퍼처를 이용하여 중성자 강도를 조절하고 gold wire를 이용한 방사화분석을 통하여 열중성자속을 구하였다. 검출기의 중성자 반응성은 열중성자속 1.6×106 에서 1.9×107 n/cm2s 범위에서 상관관계 0.997의 높은 선형성을 나타내었다. 감마조사실험과 케이블 길이 및 온도에 따른 알파 반응 측정을 예비연구로 수행하였다. 감마조사 실험에서는 조사되지 않은 시편과 40 kGy, 120 kGy가 조사된 검출기들의 쇼트키 장벽 높이가 각각 0.905, 0.936, 0.928 eV로 탄화규소 검출기가 감마선에 대한 저항성을 보여주었다. 검출기와 전치증폭기 사이의 케이블 길에 따른 신호 측정에서는 50 cm 에서 700 cm 길이에서는 측정된 스펙트럼의 형태가 유사하지만, 이 이상의 길이에서는 노이즈가 증가하였다. 가우시안 피팅을 통해 계산된 각 스펙트럼의 총 개수는 약3.5% 차이를 나타내었다. 마지막 예비평가는 300K에서 523K까지의 고온 조건에서 알파 스펙트럼을 측정하였다. 온도가 증가하여도 알파 피크의 중심지점은 거의 동일하였고, 온도에 따른 5.486 MeV 알파선에 대한 온도에 따른 에너지 분해능은 약 1.0% 편차 내에서 일정하였다. 이것은 탄화규소 검출기가 온도 변화와 무관한 특성을 가지고 있는 것을 나타낸다.; Neutron detectors used in nuclear reactors, spent fuel facilities, high-energy physics experiments or outer space often face harsh environmental conditions such as high temperature and high radiation dose. It has been a continuing challenge to develop a neutron detector which is durable and provides reliable outputs under such conditions. One of the most promising materials is SiC which can be operated as a semiconductor detector at elevated temperatures by virtue of its wide band-gap energy. In this study, neutron detectors based on silicon carbide (SiC) were fabricated and evaluated under various conditions. The detectors were fabricated using the metal mask technique instead of a lithography. 6LiF was selected as the converter to detect slow neutrons and metal contacts were deposited by a thermal evaporation procedure in high vacuum. Using a conducting epoxy, the SiC chips were mounted onto PCB, Teflon, and Ceramic substrates after the metallization process. The fundamental performance of the devices as radiation detectors was tested using an alpha source (241Am). Afterwards, the thermal neutron response of the detectors was measured at ex-core neutron irradiation facility in the Korea Atomic Energy Research Institute. To confirm a linear response of the detectors, the neutron intensity was modified using a Cd aperture while the equivalent thermal neutron flux was obtained using a gold wire activation measurement. The neutron response of the detectors has been shown to be highly linear (Adjusted R-square: 0.997) in the thermal neutron flux range from 1.6×106 to 1.9×107 n/cm2s. A gamma irradiation test and alpha response measurements with respect to cable length and temperature were carried out as preliminary tests. In the gamma irradiation experiment, the Schottky barrier height of the non-irradiated SiC sample and two samples irradiated by 40 and 120 kGy gamma dose were determined as 0.905, 0.936 and 0.928 eV, respectively, to confirm that the SiC detector is resistant to gamma radiation. In the signal measurement with respect to cable length between detector and preamplifier, the shapes of the spectra were similar at lengths from 50 to 700 cm and noise increased at greater lengths. The total counts of each spectrum were calculated through Gaussian fitting and showed about 3.5 % variation. In the last preliminary test, alpha spectra of the detector were measured under high temperature conditions from 300K to 523K. Even though the temperature increased, the centroid positions of the alpha peaks were almost the same. Also the energy resolution as a function of temperature for the 5.486 MeV alpha-ray was almost uniform within 1.0% of the standard deviation. This is evidence that the SiC detector has thermal properties independent of heat effects.