Fluence measurements employing iodide/iodate chemical actinometry as applied to upper-room germicidal radiation.

To measure the fluence distribution in the upper room of a facility equipped with germicidal UV lamps a method has been developed utilizing iodide/iodate chemical actinometry together with spherical (1 cm) quartz irradiation chambers. The use of spherical vessels allows radiation from essentially all directions to be measured. Such a measurement allows an estimate of the radiation flux at a given point in space, i.e. the fluence rate. When a battery of spheres located at various points in a room are simultaneously irradiated, a measure of the fluence distribution can be obtained. The use of the iodide/iodate chemical actinometer is uniquely qualified to measure germicidal UV radiation. The purpose of this report is to provide details on how this system can be used to measure fluence rates. In particular, it describes how a hand-held colorimeter can be used to measure the absorbance changes in irradiated spheres.

Iodide/iodate chemical actinometry using spherical vessels for radiation exposure as well as for monitoring absorbance changes.

Spherical quartz vessels containing an iodide/iodate chemical actinometer are useful for measuring the fluence of omnidirectional germicidal radiation. It is shown here that such vessels can be used not only for exposure purposes but also for measuring the absorbance of the resulting triiodide endpoint. A hand-held commercially available colorimeter using a 420 nm LED light source was adapted to hold the spheres in the optical light path. The absorbance at 420 nm obeyed Beer's Law and the dose response showed linear kinetics following irradiation at 254 nm. The determination of the fluence obtained in this manner was consistent with that obtained following transfer of the contents of the spheres to a 1 cm cuvette and measuring the absorbance at 352 nm in a conventional spectrophotometer. Hence, dose response data can be obtained making absorbance measurements on the same sample following sequential or continuous irradiation. Furthermore, because the expression for the fluence is independent of the radius of the sphere, there is no need to measure and keep track of the volume of each sphere, thus simplifying the experimental procedure. Furthermore, the use of an inexpensive colorimeter circumvents the need for a more expensive spectrophotometer and allows measurements to be made in the field.

The iodide/iodate actinometer in UV disinfection: determination of the fluence rate distribution in UV reactors.

Thirty-seven Suprasil quartz spheres, each approximately 1 cm in diameter and containing an iodide-iodate actinometric solution, were attached to a metal rack and inserted into a bench-scale UV reactor filled with water. The spheres were located at various distances and heights around a 12.4 W low-pressure Hg lamp housed inside a 3.2 cm-radius quartz sleeve in the middle of an annular batch reactor. UV light exposure at 254 nm was performed with the percent transmittance of the water present in the reactor at either 73% or 100% defined over a 1 cm path length. The spheres were simultaneously exposed to the UV light for a given period of time, after which the solutions were removed from the spheres and the yield of triiodide determined from the increase in absorbance at 352 nm. The resulting fluence rate at each site was then calculated on basis of the yield of triiodide. These results were compared with the predictions of a mathematical model based on the multiple point source summation approximation, including reflection and refraction at the air-quartz-water interface. Initially, the agreement was not satisfactory, especially in regions at an oblique angle to the lamp. The model was modified from a multiple point source model to a multiple cylindrical segment model by incorporating a cosine factor. The agreement between the new model and the experimental data was excellent and these experiments provide a strong validation of the model, even under conditions in which the fluence rate varied by >1000-fold between extreme sites in the reactor.

Spatial distribution of upper-room germicidal UV radiation as measured with tubular actinometry as compared with spherical actinometry.

Chemical actinometry of UV germicidal irradiation using sections of quartz tubing as compared with quartz spheres as irradiation vessels has been investigated . Vessels were either 3 mm inner diameter quartz tubing, 46 cm in length (tubular actinometry), or 1 cm quartz spheres (spherical actinometry). The vessels containing an iodide/iodate actinometric solution were suspended from the ceiling at 24 positions in a room (6 x 6 m) containing five germicidal lamp fixtures in the corners and in the center of the room. The lamp fixtures were louvered collimating the radiation in the horizontal (x, y) plane. Hence, the tubes, which span the depth of the radiation field, essentially integrate the radiation along the z-axis for a given x, y position. The pseudospatial average fluence rate obtained using tubular actinometry was 18 mW/cm(2) for the volume contained in the upper 46 cm (18 inch) of the room. Spherical actinometry, which measured the fluence rate in the center of the beam, provided an average value of 32 mW/cm(2) over the volume of the beam. A comparison of the fluence rates obtained by these two methods allowed the average depth of the beam to be estimated as 26 cm. It is concluded that tubular actinometry is more advantageous than spherical actinometry for this application.

Quantum yield of the iodide-iodate chemical actinometer: dependence on wavelength and concentrations.

The quantum yield (QY) of the iodide-iodate chemical actinometer (0.6 M KI-0.1 M KIO3) was determined for irradiation between 214 and 330 nm. The photoproduct, triiodide, was determined from the increase in absorbance at 352 nm, which together with a concomitant measurement of the UV fluence enabled the QY to be calculated. The QY at 254 nm was determined to be 0.73 +/- 0.02 when calibration was carried out against a National Institute of Standards and Technology traceable radiometer or photometric device. At wavelengths below 254 nm the QY increased slightly, leveling off at -0.80 +/- 0.05, whereas above 254 nm the QY decreases linearly with wavelength, reaching a value of 0.30 at 284 nm. In addition, the QY was measured at different iodide concentrations. There is a slight decrease in QY going from 0.6 to 0.15 M KI, whereas below 0.15 M KI the QY drops off sharply, decreasing to 0.23 by 0.006 M KI. Calibration of the QY was also done using potassium ferrioxalate actinometry to measure the irradiance. These results showed a 20% reduction in QY between 240 and 280 nm as compared with radiometry. This discrepancy suggests that the QY of the ferrioxalate actinometer in this region of the spectrum needs reexamination.

Chemical dosimetry using an iodide/iodate aqueous solution: application to the gamma irradiation of blood.

A method is presented for measuring and verifying the radiation dose in gamma irradiators used for treating blood prior to transfusion. This method employs the iodide/iodate dosimeter (0.6M iodide, 0.1M iodate, and 0.01 borate at pH 9.25) which forms triiodide upon exposure to ionizing radiation; for Cs-137 radiation the G value is 14.1. Samples were placed in a canister and irradiated in a conventional blood bank irradiator containing several Cs-137 sources. The following were exposed: (a) nine 1.5 ml plastic tubes containing dosimetry solution taped inside a 250 ml blood bag, which, in turn, was filled with either water or blood, (b) 50 ml plastic syringes containing varying amounts of dosimetry solution, (c) a whole blood bag containing 250 ml of the dosimetry solution. A water phantom was not used during exposure. The absorbance changes at 352 nm due to triiodide formation were used to determine a dose rate, which was on the order of 10 Gy/min (+/-5%) for all samples measured. This value is consistent with an average time-decayed dose rate for the irradiation volume as determined from the manufacturers calibration of the unit taking into account the heterogeneous nature of the radiation field inside the irradiator and the absence of a water phantom. Because of its sensitivity, ease of operation, and reproducibility, it is suggested that the iodide/iodate dosimetry system be considered for on-site periodic conformation/verification of the radiation dose as part of a quality assurance requirement for blood irradiators.

Dosimetry of ionizing radiation using an iodide/iodate aqueous solution.

Details of a new aqueous chemical dosimeter consisting of a mixture of KI (0.6 M) and the electron scavenger KIO3 (0.1 M) in 0.01 M borate buffer (pH 9.25) are presented. Exposure was either to gamma sources (137Cs, 60Co) or to linear accelerator (LINAC) radiations (18-MeV electrons, bremsstrahlung X-ray spectrum). G values were obtained for the formation of triiodide; the absorbance at its maximum was then measured at 352 nm. The dose response was linear up to 6000 Gy, the lower limit of detection being approximately 0.25 Gy. G values calculated from the initial slopes of the dose-response curves were 14.1 +/- 0.8 for 137Cs radiations and 13.8 +/- 0.4 and 13.9 +/- 0.8 for the NIST and AFRRI 60Co radiations, respectively. G values obtained for the electron and bremsstrahlung radiations were 12.2 +/- 1.9 and 11.9 +/- 1.8, respectively. The iodide/iodate dosimeter extends the range of detection an order of magnitude both above and below the accepted detection limits of the Fricke dosimeter.