Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 42, No. 8, p. 673–677 (August 2005)
ORIGINAL PAPER
Development of High Dose Rate Sensing Method Based on Cavity Ring-Down Laser Spectroscopy Hideki TOMITA1; , Kenichi WATANABE1 , Jun KAWARABAYASHI1 , Tetsuo IGUCHI1 and Nobuteru NARIYAMA2 2
1 Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 Beamline Division, Japan Synchrotron Radiation Research Institute, Mikazuki, Sayo-gun, Hyogo 679-5198
(Received November 26, 2004 and accepted in revised form May 24, 2005) We have developed a new method for high radiation dose rate monitoring based on cavity ring-down laser absorption spectroscopy and have measured 60 Co gamma-ray induced chemical species in stationary and flowing air at the wavelength of 532 nm to check the basic performance of the new method. In the stationary air measurement, we confirmed linearity between the absorbed dose and the measured light absorbance by the radiation induced chemical species. Furthermore, in the flowing air measurement, we found the response time and the magnitude of the light absorbance largely depend on the air flow rate. In addition, a preliminary experiment was done to demonstrate the applicability to an intensity monitor for the intense undulator X-rays of BL47XU beamline at SPring-8. Results showed that the measured ring-down rate is proportional to the photon intensity and the detectable range is from 21011 to 31015 photons/s with time resolution around 3102 s. KEYWORDS: high dose rate, cavity ring-down, laser spectroscopy, cobalt 60, synchrotron radiation, SPring-8
I. Introduction We have developed a novel dose rate monitoring method based on cavity ring-down (CRD) laser absorption spectroscopy.1) In this method, the dose rate is derived from concentrations of radiation induced chemical species in air, corresponding to the light absorbance measured by CRD spectroscopy. It has a large potential as a reliable high dose rate monitoring system with almost no radiation damage because only the irradiated air (or gas) is remotely sampled and measured with the CRD spectroscopy system located outside the intense radiation fields, such as industrial irradiation plants, intense synchrotron radiation beamlines and also next generation facilities like high energy and intense beam accelerators and nuclear fusion devices. So far we have developed a calculation model to estimate the yield of radiation induced species by simultaneously solving chemical reaction rate equations and also verified the validity of the calculation model through comparison of results from the off-line basic experiment using a UV laser source.2) In addition, we have experimentally confirmed the operational principle of this method under air flow and using a 532 nm laser source.3) However, detailed experimental checks have not been made yet on the relation between the light absorbance and the absorbed dose, and also the time response. In this paper, we present the results of two kinds of experiments on the measurement of 60 Co gamma-ray induced chemical species in stationary air and flowing air with CRD spectroscopy to discuss the linearity between the light absorbance and the absorbed dose, and also the dependence of the response time and the magnitude of the light absorb-
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ance on the air flow rate. As one of the applications of the new method, preliminary experiments are also introduced for beam intensity monitoring of intense synchrotron radiation at SPring-8, an 8-GeV synchrotron radiation facility in Japan.
II. Principle of Measurement 1. Cavity Ring-down (CRD) Spectroscopy CRD spectroscopy, which was developed by O’Keefe and Deacon, is a highly sensitive laser absorption spectroscopy technique using ultra long light path in an optical cavity.4) This technique is based on the measurement of a decay time of pulsed laser light stored in the optical cavity, which consists of a pair of high-reflectivity (>0:9995) mirrors. The intensity of the stored light in the optical cavity IðtÞ decays exponentially as a function of time t. Thus, IðtÞ is given by IðtÞ ¼ I0 ð1 RÞ expðtÞ ¼ I0 ð1 RÞ exp 0 t c 0 ¼ c
X
X
! i Ni t ;
ð1Þ
i
i N i ;
ð2Þ
i
where I0 is the intensity of an incident laser pulse, R is the reflectivity of cavity mirrors, is the ring-down rate which is the inverse of a decay time constant of light intensity stored in the optical cavity, 0 is the background ring-down rate without absorbing species between cavity mirrors, i is the absorption cross section of absorbing species i, Ni is the concentration of absorbing species i and c is the velocity of light in air. IðtÞ is measured as the light intensity leaking out from the optical cavity, for which the decay signal is called a ‘ring-down’ signal. The absorbance of absorbing species
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yields of initial radiation induced species are estimated from G-values and the absorbed dose rate in air (atmospheric pressure). From calculated results and photo-absorption cross sections, we found that the detectable species are ozone (O3 ) and nitrogen dioxide (NO2 ), and also the yield of NO2 is larger than that of O3 by 2–3 orders of magnitude. Additionally, the production rate of the main products is nonnegative. We have, therefore, adopted NO2 as the target molecule for the measurement at fixed wavelength (532 nm). For this wavelength, we can neglect the O3 contribution to the ring-down signal because the absorption cross section of NO2 is higher than that of O3 by two orders of magnitude.5,6)
III. Experimental Setup Fig. 1 A conceptual drawing of the principle of the dose rate measurement based on CRD spectroscopy In this method, the dose rate can be derived from the steady state ring-down signal which is related to the concentrations of the radiation induced chemical species.
P
i i Ni can be derived from a difference in the ring-down rates 0 .
2. Dose Rate Measurement Figure 1 shows a conceptual drawing of the principle of the dose rate measurement based on CRD spectroscopy.1) Since various chemical species are induced through interactions between ionizing radiations and the air molecules around the radiation source, the dose rate can be derived from the steady state concentration of radiation induced chemical species, which depends on the balance between their generation due to irradiation and loss on the wall and/or by air flow. It is necessary to place the gas cell for irradiation in the radiation field, but we can choose optimum materials and shapes of this cell to fit the irradiation conditions (radiation species, energy, intensity etc.). According to Eq. (2), the light absorbance ci Ni Ni must be larger than the detectable level of which was estimated as approximately 5103 s1 in Ref. 3). We have made a rough estimate on the yields of radiation induced chemical species in air by solving material balance equations numerically, i.e. chemical reaction rate equations under the assumption that the radiation energy is deposited on the air uniformly.2) Therefore, the equations of material balance are given as X d G jk ðNi Þ Nj ¼ Sj þ dt k ð3Þ X Nj ; l jk ðNi ÞN j k where N j is the concentration of chemical species j (molecules/cm3 ), S is the yields of initial radiation induced species, G is the generation rate with chemical reactions (molecules/cm3 s), lN is the loss rate with chemical reactions (molecules/cm3 s), is the time constant of residence of gas and subscript k is the chemical reaction number. The
We did two kinds of experiments to measure radiation induced NO2 in air using CRD spectroscopy; one is for stationary air contained in the irradiation gas cell and the other is for flowing air through the cell. The concentration of NO2 accumulated in the stationary irradiated air was directly monitored by injecting the laser light into the CRD gas cell placed near the radiation source. Under the flowing air condition, the irradiated air was continuously sampled from the irradiation gas cell and flowed to the CRD spectroscopy system located outside the radiation field for remote measurement. The CRD system consists of the CRD gas cell, pulse laser, photo detector and data acquisition system. These experiments were carried out in the 60 Co gamma-ray irradiation facility at Nagoya University and the BL47XU undulator beamline at SPring-8. 1. Stationary Air Experiment Figure 2 shows a schematic view of the experimental arrangement to measure the gamma-ray induced NO2 in the stationary air irradiated in the 60 Co gamma-ray irradiation facility at Nagoya University. In this experiment, the CRD gas cell functioned as the irradiation gas cell. The CRD gas cell consisted of a stainless steel tube (24 mm diameter, 2.0 mm thick and 0.80 m length) and two ultra-high reflectivity cavity mirrors (Los Gatos Research, Inc.; R>99:95%@
Fig. 2 A schematic view of the experimental arrangement to measure the gamma-ray induced NO2 in stationary air irradiated in the 60 Co gamma-ray irradiation facility at Nagoya University
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Development of High Dose Rate Sensing Method Based on Cavity Ring-Down Laser Spectroscopy
2. Flowing Air Experiment Figure 3 shows a schematic view of the experimental apparatus for the flowing air measurement of 60 Co gamma-ray/ synchrotron radiation induced NO2 . The air flowing through the irradiation gas cell was irradiated by 60 Co gamma-ray/ synchrotron radiation and then flowed to the CRD gas cell placed outside the radiation field. The CRD gas cell was 34 mm in diameter and 0.46 m in length. Moreover, the polyacrylonitrile irradiation gas cells were 34 mm39 mm 49 mm for the 60 Co gamma-ray radiation and 30 mm 30 mm80 mm for the synchrotron radiation, respectively. The irradiated air was continuously sampled into the CRD gas cell, where the air flow rate Fair was measured and controlled with a flow meter and/or a mass flow controller. The NO2 concentration of the irradiated air was measured by DPSS laser based CRD spectroscopy.
IV. Results and Discussion 1.
60
Co Gamma-ray Induced NO2 Measurement for Stationary Air Figure 4 shows a typical ring-down signal measured from the stationary air under 60 Co gamma-ray irradiation as the average value during 256 signals. The ring-down rates were determined with least squares fitting of the decay signals to an exponential function. Figure 5 shows the time evolution of the ring-down rate 0 . Beta was measured as the average value during 1:6103 laser pulses at the absorbed dose rate of 0.09 Gy/s up to the irradiation time of 6103 s and background 0 was measured under no irradiation, showing the fluctuation indicated as the hatched region. Although the variation of measured data was not so small, the ring-down rate linearly increased with the irradiation time, given by the simple relation: 0 ¼A¼At (A: constant, t: irradiation time, : absorbed dose rate and : absorbed dose) in the range up to 5103 Gy at least. We also confirmed the repeatability of the present result through two measurements under the same condition. No radiation damage of mirror reflectivity was found in the experiments, where the absorbed dose
PMT output voltage (mV)
532 nm). One mirror was placed at each end of the tube at a distance of 0.96 m apart. The CRD gas cell filled with air at atmospheric pressure was placed above the center of 60 Co gamma-ray source (129 TBq) at a distance of 0.15 m. The incident light source was a compact/portable diode pumped solid state (DPSS) pulse laser (Intelite Inc.; UVSQAOM266-1), which had a wavelength of 532 nm, pulse width of
