The Neutron Science Facility (NSF) would be constructed to measure the cross section data of fast neutrons. The NSF is separated into two halls that target hall and Time of Flight (TOF) hall by 4 m of thick concrete wall. The collimator is built into the wall to define a well collimated neutron beam. To design collimator for RAON n_TOF, several parameters that beam channel diameter, beam channel shape, and collimator type were optimized through the MCNPX simulations. The design and optimization results of the collimator are summarized hereafter.
First of all, the diameter of the collimator channel was optimized. The diameter was set to 2 cm to deliver all neutrons passing the collimator into the detector that have 12 cm radius. Through the diameter, all neutrons passing the collimators could be focused on the detector so that avoid neutrons spread over to the TOF hall. Consequently, the background in the TOF hall is highly reduced.
For the beam channel shape, a conical-opened shape was selected. A narrow entrance of the collimator shields the neutrons causing scatters inside the collimator. Through the design, almost neutrons emitted from the collimator exit were passing the collimator without scatters. Through the simulation, entry radius of the conical-opened shape collimator was set to 0.5 cm. With 0.5 cm radius, the background was reduced about 94.1 % compared to the cylindrical shape collimator while maintain the neutron intensity.
Materials to be used for collimator were identified. There are two categories of materials forming collimator; one for scattering material that slow down the high energy neutrons, another for moderator/absorber which moderate and absorb the low energy neutrons. Through the material survey, iron was selected as a scattering material, 5 % borated polyethylene was suitable for moderator/absorber.
Several design layouts for collimator types were studied. Among the ‘Ring’ type and ‘Sandwich’ type, the Ring type was verified much suitable for the collimator type. Through the optimizing process, the specification of the ring type was determined. Iron was set to 1 cm thickness at inner part, 20 cm 5 % borated polyethylene is arranged at outer part of the collimator. As a result, cylindrical ring type collimator reduces the background about 28.1 % versus simple cylindrical collimator only made of concrete.
All things considered, the final design of for RAON NSF has 4 m of length, channel of 0.5/2 cm entry/exit radius. Materials are arranged in a ring type that putting 1 cm iron inside, 20 cm 5 % borated polyethylene outside. With the final design of collimator, background reduction about 95.7 % was achieved.
For the safety evaluation in the facility, neutron activation analysis was carried out through the MCNPX code and the SP-FISPACT 2010 code. The activation of the collimator was evaluated in two operation conditions. First, assume the maximum operation time at once for 1week. Then the simulation results were checked at 1 second, 10 minutes, 30 minutes, 1 hour, 8 hours and 1day after 1 week operation. In second condition, total operation period of neutron science facility was set to 30 years. In a year, the facility was assumed to operate for 30 days and cool downed for 335 days. The simulation results were checked every 1 year during the operation period. As a result, dose rate at the target room is much higher than the occupational dose limit of 5 μSv/hr that recommended by the ICRP 103. The dose rate at the TOF room is much lower than the dose limit.
중이온가속기 RAON(라온) 내 중성자 측정시설은 고 에너지 중성자의 단면적 데이터(cross section)를 측정하기 위해 건설될 예정이다. 중성자의 단면적 데이터는 TOF (Time-of-Flight) 방법을 이용하여 측정된다. 이 때 표적으로부터 발생하는 중성자 빔은 모든 방향으로 발생하기 때문에 계측에 불필요한 빔들은 차폐하고 실험에 필요한 빔들만을 계측기로 전달하기 위해 콜리메이터가 필요하다. 콜리메이터는 표적실과 계측실을 구분하는 4m 두께의 콘크리트 벽 안에 삽입되게 된다. 콜리메이터 디자인에서 결정해야 할 요소들은 빔 채널의 반지름, 빔 채널의 형상, 콜리메이를 구성하는 물질 배열에 관련된 콜리메이터 타입이 있으며, 이 요소들은 MCNPX 2.7코드를 이용하여 최적화 되었다. 그 결과 콜리메이터 빔 채널의 형상은 콘 모양으로 설계되었고 빔이 들어가는 쪽 반지름은 0.5cm, 빔이 나가는 쪽 반지름은 2cm로 결정되었다. 콜리메이터를 구성하는 물질로는 철이 산란물질로, 5% borated polyethylene이 흡수물질로 최적의 조건을 갖추어 선택되었다. 두 물질의 배열방식은 링 타입으로, 안쪽에 1cm 두께의 철이, 철 바깥쪽에 20cm 두께의 5% borated polyethylene이 배열되었다. 결과적으로 최적화된 콜리메이터를 이용했을 경우 발생하는 백그라운드는 최적화되지 않은 단순한 콘크리트 구멍을 통해서 빔이 전달되는 경우에 비해서 95.7% 감소했다. 콜리메이터의 설계에 이어서 안전성과 디자인의 유효성을 판단하기 위하여 방사화 평가가 진행되었다. 방사화 평가는 MCNPX 코드와 SP-FSIPACT 2010코드를 이용하여 진행되었으며 중성자에 의해서 발생하는 방사성 핵종들을 확인하였다. 또한 각 실험시설에서의 선량을 확인함으로써 작업자의 출입가능여부를 판단하는 기준을 제공하였다. 그 결과 표적실의 경우 ICRP 103에서 권고하는 작업자 선량제한 5 μSv/hr을 초과하는 선량을 보여주었으며 TOF룸에서는 선량제한보다 낮은 선량 확인할 수 있었다.
