Patent Number: 
Section: claims

1. A radioactive gas monitor comprising:a detection unit that detects radiation emitted from radioactive nuclides contained in sample gas; anda measurement unit that processes a signal transmitted from the detection unit and outputs an engineering value corresponding to radioactive concentration, whereinthe detection unit has a detection tube through which the sample gas flows, a first detector to which a first concentration measurement range within all required measurement ranges of radiation is allocated, a second detector to which a second concentration measurement range within all required measurement ranges of radiation is allocated, wherein concentrations in the second concentration measurement range are higher than concentrations in the first concentration measurement range, and a shield which shields the detection tube, the first detector, and the second detector from environmental radiation,the measurement unit has a first measurement unit which processes a signal transmitted from the first detector and a second measurement unit which processes a signal transmitted from the second detector,the shield has at least one detector installing hole having a central axis orthogonal to a central axis of the detection tube, andat least the first detector between the first detector and the second detector is arranged inside the detector installing hole, and an inner diameter of the detection tube and a relative position between the detection tube and the first detector are determined so that the measurement range of the first detector and the measurement range of the second detector are overlapped with each other. 2. The radioactive gas monitor according to claim 1, whereinthe shield has one detector installing hole, anda radiation sensor of a columnar scintillation detector used as the first detector is arranged inside the detector installing hole, and a cylindrical ionization chamber used as the second detector is arranged side by side in parallel with the detection tube. 3. The radioactive gas monitor according to claim 2, whereinthe second measurement unit includes a logarithmic amplifier which inputs a DC current output from the cylindrical ionization chamber and outputs a voltage by converting the DC current into the voltage proportional to a logarithm of the DC current. 4. The radioactive gas monitor according to claim 2, whereinthe columnar scintillation detector has a preamplifier which converts a current pulse output from the radiation sensor into an analog voltage pulse, and the preamplifier is installed outside the shield. 5. The radioactive gas monitor according to claim 2, whereina collimator which narrows down radiation incident on the radiation sensor arranged inside the detector installing hole is disposed inside the detector installing hole. 6. The radioactive gas monitor according to claim 2, whereina detector temperature sensor is attached to a radiation sensor of the columnar scintillation detector, andbased on a temperature signal input from the detector temperature sensor, the first measurement unit performs temperature compensation on an engineering value corresponding to radioactive concentration. 7. The radioactive gas monitor according to claim 1, whereinthe shield has two detector installing holes arranged so as to oppose each other across the detection tube, anda radiation sensor of a columnar scintillation detector used as the first detector is arranged inside one detector installing hole, and a radiation sensor of a fibrous scintillation detector used as the second detector is arranged inside the other detector installing hole. 8. The radioactive gas monitor according to claim 7, whereinthe columnar scintillation detector has a preamplifier which converts a current pulse output from the radiation sensor into an analog voltage pulse, and the preamplifier is installed outside the shield. 9. The radioactive gas monitor according to claim 7, whereinthe radiation sensor of the fibrous scintillation detector has a light guide which is optically bonded to a scintillation fiber which is cut to have a predetermined length, and a photomultiplier tube which is optically bonded to the light guide. 10. The radioactive gas monitor according to claim 7, whereinthe fibrous scintillation detector has a preamplifier which converts a current pulse output from the radiation sensor into an analog voltage pulse, and the preamplifier is installed outside the shield. 11. The radioactive gas monitor according to claim 7, whereina collimator which narrows down radiation incident on the radiation sensor arranged inside the detector installing hole is disposed inside the detector installing hole. 12. The radioactive gas monitor according to claim 7, whereina detector temperature sensor is attached to a radiation sensor of the columnar scintillation detector, andbased on a temperature signal input from the detector temperature sensor, the first measurement unit performs temperature compensation on an engineering value corresponding to radioactive concentration. 13. The radioactive gas monitor according to claim 7, whereina detector temperature sensor is attached to a radiation sensor of the fibrous scintillation detector, andbased on a temperature signal input from the detector temperature sensor, the second measurement unit performs temperature compensation on an engineering value corresponding to radioactive concentration. 14. The radioactive gas monitor according to claim 1, whereinthe detection unit includes a sample gas temperature sensor which detects a temperature of sample gas flowing through the detection tube, and a sample gas pressure sensor which detects a pressure of the sample gas flowing through the detection tube, andthe measurement unit obtains a sample gas temperature and a sample gas pressure, based on a temperature signal input from the sample gas temperature sensor and a pressure signal input from the sample gas pressure sensor. 15. The radioactive gas monitor according to claim 14, wherein based on the sample gas temperature and the sample gas pressure, the measurement unit outputs an engineering value corresponding to radioactive concentration by converting the engineering value into a predetermined temperature and pressure.