THE CONCEPT OF BUILDING A HIGH-SENSITIVE LASER
SENSOR FOR DETECTION OF ISOTOPES OF IODINE
V.V. Elizarov, A. S. Grishkanich, S. V. Kascheev, L.A. Konopelko, A. P. Zhevlakov
NATIONAL RESEARCH UNIVERSITY OF INFORMATION TECHNOLOGIES, MECHANICS AND OPTICS
Introduction
Among the various environmental disasters leading position on the severity of the consequences is an emergency at nuclear facilities. In all
cases, the radioactive isotopes of iodine in the first days after the accident to determine the level of internal irradiation of people. The most
effective use of the Raman method for remote detection of a wide range
of isotopes radionuclides. Spectroscopy of spontaneous Raman scattering (SRS), especially spectroscopy coherent anti-Stokes Raman scattering
(CARS), offer significant advantages relative to other methods of spectral
analysis.
These include:
• high signal intensity CARS, on 6 orders of magnitude higher than the intensity of the SRS signal
• performance of the echo signals of the order of the duration of the ex-12
-8
citing laser pulse (in the range 10 -10 ),
• higher frequency anti-Stokes signals relative to fluorescence interference.
a)
Figure 3. Raman shifts of the isotopes of CO2
DESIGN OF CARS -LIDAR
Functional diagram CARS laser sensor was created and its technical characteristics are defined. (Monostatically scheme in CARS-sensor used. The
sensing is carried out using Ti:Al2O3 and YAG:Nd lasers at wavelengths
230-323;345-485; 690-970 nm and 266; 355; 532; 1064 nm, respectively.Total reciprocal linear dispersion of polychromator obtained is equal to
dλ/dl = 0,063 nm/mm at a wavelength of λ=266нм. This ultraspectral
resolution allows you to confidently separate the lines of the isotopes of
radioiodine.PMT (Hamamatsu R7154) are used as detectors. The photon
counting module (Hamamatsu C9744) in combination with the measuring unit (Hamamatsu С8555-01) is used to implement the photon
counting mode.
b)
Figure 1. Energy diagrams spectroscopy transitions: a) Spontaneous
Raman scattering ; b) Anti-Stokes Raman scattering
THE ISOTOPE SHIFTS OF IODINE MOLECULES.
Using the RS selection method features wide range of substances can be
achieved by performing a probing laser any time of day. Analytical information is extracted from the results of measurement of parameters of
spectral lines, such as the intensity of I and Δν wave number characterizing the shift of the lines relative to Raman scattering of the exciting radiation(ν0). The wave number allows to correlate the measured signal with
a specific form of scattering particles. We have calculated the wave numbers of Stokes frequency shifts characterizing the set of isotopes of iodine, including indicators of radioactive contamination.
Table 1. isotopes shifts of iodine
Isotope
127
I2
129
I2
132
I2
127/129
I2
129/131
I2
127/131
I2
∆ν, см
-1
212,89
211,44
221,11
211,78
210,31
211,27
Figure 4. 1-control block of laser output settings, 2-module processing, 3-deflector 4-polychromator, 5-lens, 6-scanner 7 – receiver
Digital recording devices with a bandwidth from 1GHz to register a wide
range of isotopes of radioiodine concentration in the background level
(200-500ppb) are used . This equipment allows to accurately reconstruct
the pulse that characterizes the spectral response of the substances. As
for the bandwidth defined minimum data transfer speed is 400 Mbps.
Software using LabView, C++ and allowing to solve problems of interaction
with the hardware of the lidar system in real time was created. Calculation
of the minimum concentrations apparently detected, with the parameters
of the lidar , using the formula
where r is the length of the test object; ∆σ=σ_1-σ_2 - differential cross
section of the gas absorption for wavelengths λ1 and λ2; L – the distance
to the object; Ps-the power of the echo signal; Pt-threshold power in the
absence of background noise in the conditions of the reference atmosphere; Calculated that, KARS-lidar allows the sensitivity of the concentration measurement at the level of 3÷10 molecules.
Parameter
Spectral shifts of the various isotopes of iodine are close to each other.
Therefore, the laser sensor must have ultraspectral resolution (λ/
12
13
Δλ>1000). We selected isotopes of carbon dioxide СО2 and СО2as reference tools in the control in it ultraspectral resolution necessary for
identification of the isotopic modifications of the molecules of radioiodine. The measured values characterized by frequency shifts, using the
Raman spectrometer OPTEC-785-H and mass spectrometric analyzer
PICARRO G2131-i
Application distance, m
Speed range , m/s (km/h)
Spatial resolution, m
Spectral resolution (λ/Δλ)
The instantaneous field of view for the laser channel, mrad
Value
up to 50- 100
up to 6 to (220)
up to 0.03
>1000
≤0,1
Conclusion
Laser sensor with remote sensing, based on the following scheme CARS,
allows to solve the problem of remote detection of radioactive isotopes of
iodine. Determined that the spectra of Raman scattering of different iodine isotopes are close to each other and for their registration with the lidar system it is necessary to provide ultraspectral resolution (λ/Δλ>1000).
Allows you to create a system capable of recording signals with a concentration of about 3÷10 molecules using a gigahertz frequency range. The
optimal principles of organization of system software, based on the use of
modular architecture. Created program complex, capable of recording,
processing and storage of lidar signals in real time.
a)
b)
Figure 2. a) Analyzer isotopic composition PICARRO G2131-i; b)
Raman spectrometer OPTEC-785-H
ACKNOWLEDGMENT
This study was supported by "Research and development on priority directions of
scientific-technological complex of Russia for 2014-2020", project №
14.578.21.0090. The project was implemented and is supported by the Noncommercial partnership" Global Energy "
Contacts: S.V. Kascheev, e-mail: [email protected]