Abstract— We have investigated the ultraviolet photochemistry of thymine in aqueous solution. Four isomeric dimers are produced, and the yield of each has been measured as a function of thymine concentration, oxygen concentration, and temperature. At low thymine concentration, dimerization proceeds via the triplet state, while at high concentration it arises mainly from aggregates, probably from a singlet precursor. We have determined the ratios of rate constants for the triplet state mechanism and the quantum yield for dimerization from aggregates. The quantum yield for dimerization from the triplet state in thymine is smaller by a factor of about 10 than that in uracil, which in turn is smaller by another factor of 10 than that in orotic acid. It increases with the energy of the exciting radiation in a manner similar to the behaviour of uracil and orotic acid. On the other hand, dimer formation from aggregates is independent of photon energy. Dimerization from aggregates decreases with increasing temperature, while the total production of dimers from the triplet state is independent of temperature.

Abstract— Cytidylyl (3′‐5′) cytidine (CpC) was irradiated with ultraviolet light (u.v.) to produce the single hydrate (a mixture of C*pC and CpC*) and the double hydrate C*pC* which were separated by electrophoresis. These photoproducts rapidly dehydrate to CpC and deaminate to a mixture of U*pC and CpU*. The rate constants for dehydration and deamination of the hydrates were evaluated for a range of pH values from 3 to 8 at 0°C. It is observed that the rate constants for decay of C*pC* lie between those for C*pC and CpC* for all pH values studied. Both single and double hydrates show minimum stability around pH 4·5 and maximum stability around pH 8. The maximum rate constants for dehydration of C*pC*, C*pC and CpC* are 0·26, 0·145 and 0.35 hr‐1 respectively and the minimum values are 0.024, 0.011 and 0.091 hr‐1 respectively all at 0°C. The rate constants for deamination of C*pC to U*pC for a range of pH values at 0°C were measured. The amount of deamination product varies from about 2 to 10 per cent of the hydrate depending on pH with the maximum amount being produced around pH 8.

Ultraviolet irradiation of cytidylyl-(3′-5′)-cytidine (CpC) results in the production of only one dimer, [Formula: see text], and for ultraviolet exposures in the range of biological interest, only the single hydrate (CpC)*. A second dimer of CpC, [Formula: see text], arises from [Formula: see text] through a dark reaction. Both these dimers deaminate in the dark to dimers of uridylyl-(3′-5′)-cytidine (UpC) and then to [Formula: see text]. The cross sections and quantum yields for the production of [Formula: see text] and (CpC)* have been determined at a number of wavelengths. In the wavelength range 240–289 mμ, the cross sections for dimer production are at least two times that for hydrate production. Approximate molar extinction coefficients have been determined for a mixture of [Formula: see text] and [Formula: see text], yielding values at ail wavelengths which are at least four times larger than for thymine dimers. The cross sections for dimer reversal of [Formula: see text] and [Formula: see text] are about the same and are larger than those of thymine dimers by at least a factor of 6.0. The quantum yield for reversal of CpC dimers is about 1.0 and independent of wavelength. These photochemical findings prove that in CpC, dimers are a much more important class of photoproduct than hydrates at low ultraviolet exposures. A similar result could be expected in polymers containing cytosine such as occur in DNA and RNA.

Photochemistry and PhotobiologyVolume 4, Issue 4 p. 673-692 THEORY AND DESIGN OF HIGH INTENSITY U.V MONOCHROMATORS FOR PHOTOBIOLOGY AND PHOTOCHEMISTRY H. E. Johns, H. E. Johns Department of Medical Biophysics, University of Toronto, Toronto, CanadaSearch for more papers by this authorA. M. Rauth, A. M. Rauth Department of Medical Biophysics, University of Toronto, Toronto, CanadaSearch for more papers by this author H. E. Johns, H. E. Johns Department of Medical Biophysics, University of Toronto, Toronto, CanadaSearch for more papers by this authorA. M. Rauth, A. M. Rauth Department of Medical Biophysics, University of Toronto, Toronto, CanadaSearch for more papers by this author First published: September 1965 https://doi.org/10.1111/j.1751-1097.1965.tb07910.xCitations: 71AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume4, Issue4September 1965Pages 673-692 RelatedInformation

Photochemistry and PhotobiologyVolume 4, Issue 4 p. 693-707 COMPARISON OF SPECTRAL PURITY AND INTENSITY OF DIFFERENT U.V. MONOCHROMATORS H. E. Johns, H. E. Johns Department of Medical Biophysics, University of Toronto, Toronto, Canada.Search for more papers by this authorA. M. Rauth, A. M. Rauth Department of Medical Biophysics, University of Toronto, Toronto, Canada.Search for more papers by this author H. E. Johns, H. E. Johns Department of Medical Biophysics, University of Toronto, Toronto, Canada.Search for more papers by this authorA. M. Rauth, A. M. Rauth Department of Medical Biophysics, University of Toronto, Toronto, Canada.Search for more papers by this author First published: September 1965 https://doi.org/10.1111/j.1751-1097.1965.tb07911.xCitations: 36AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume4, Issue4September 1965Pages 693-707 RelatedInformation

For many years the roentgen has been used as the unit of exposure dose, and doses in roentgens have been obtained from measurements with standard air chambers. The International Commission on Radiological Units (ICRU) has long been aware of the difficulties of making measurements in roentgens above 3 Mev, and at the present time no agreed-upon method has been selected for measurements of exposure doses at these energies. Even in the range from 1 to 3 Mev, many difficulties are encountered in determining an exposure dose. In this energy range, reliance is usually placed on thimble chambers of known volume and composition, and the exposure dose in roentgens is calculated from a measurement of the ionization produced in the chamber by use of the Bragg-Gray formula or recent extensions of it. In 1950 the ICRU defined the unit of absorbed dose, the rad, and in NBS Handbook 62 gave detailed methods showing how an absorbed dose in rads could be calculated from a measured exposure dose in roentgens. In the meantime a number of investigators have shown that absorbed dose may be measured directly with a calorimeter (1-3). However, no standardization laboratory has as yet set up the apparatus to measure absorbed dose directly on a routine basis as a standardization procedure. In the work to be reported in this paper, absorbed dose was measured calorimetrically, and the ionization in small Bragg-Gray cavities was measured. It was found that, in general, a more reliable measurement can be made of absorbed dose than of exposure dose. It is therefore suggested that the time has come when standardization laboratories should consider using as their primary standard a calorimetric measurement of absorbed dose for energies above 1 Mev.

A caesium 137 unit containing 60 c in a source of diameter 1·5 cm is described. It gives rectangular and circular fields up to 10 × 10 cm. The treatment distance is 15 cm. The dose-rate on the skin varies from 35 to 38 r/minute, according to the field size. The isodose distributions obtained with the unit are identical with those from a cobalt 60 unit at the same source to skin distance. Because of the longer half-life of caesium 137, the unit appears to offer some advantages over cobalt 60. It can be made much smaller than a corresponding unit using cobalt 60.

A caesium 137 unit containing 1300 curies in a source of diameter 1 1⅙ in. has been described. The unit gives rectangular fields from 5 × 5 to 20 × 20 cm at 35 cm. The absorbed dose-rate on the skin from the unit varies from 38 to 40 rads/minute as the field size is varied from 4 × 4 to 20 × 20 cm. The isodose distributions are similar to those produced from a 400 kV machine for large fields. For small fields, the caesium gives a higher depth dose. The unit appears to be a useful radiotherapeutic tool.

The dependence of depth dose on focal skin distance has been the subject of a number of investigations. Mayneord and Lamerton (1941) reduced the then available depth dose data from a variety of centres for a variety of focal skin distances to one reference focal skin distance using a suitable conversion factor. The basis for their conversion factor is the inverse square law factor which is given by Mayneord and Lamerton used combinations of F and (F + 1)/2 to reduce the published data to a standard focal skin distance. In the depth dose determinations by the group in Saskatoon (Johns, Fedoruk, Kornelsen, Epp and Darby, 1952; Supplement No. 5), careful measurements were made of the variation with focal skin distance. Their measurements showed that the conversion factor usually lay between (1 + F)/2 and F and varied from F to (1 + F)/2 as the area of the field increased from zero area to a large field. The factor was also shown to be a complicated function of the quality of the radiation. In calculations of tumour dose in rotation therapy, use has been made of the concept of tumour air ratio (Johns, Morrison and Whitmore, 1956; Johns, Whitmore, Watson and Umberg, 1953). This has been defined as the ratio of the dose at the point in tissue to the dose at the same point in space “in air”.

The spectral distribution of scattered radiation from a cobalt 60 therapy unit has been calculated. Spectra are given for once- and multiply-scattered radiation from the cobalt metal source and from the tapered sides of a lead collimator. Calculations of the attenuation of the cobalt beam including the scattered components are in good agreement with experimental data.

The polarity effect in ion chambers is caused by the interaction of the high-energy radiation with the collecting electrcde. This effect is most marked when conditions of electronic equilibrium are not established in the back wall of the ion chamber. Calculations of the polarity effect have been made which agree with measured values. Two sets of curves are presented which enable the polarity effect to be quickly calculated for parallel-plate ion chambers measuring cobalt- 60 radiation. (auth)

A scintillation counter employing a large sodium iodide crystal has been used to measure the spectral distribution of the scattered radiation in a water phantom irradiated with 400 kVp X rays. The spectrum was determined at various points along the central axis of the X-ray beam for various scattering angles. The complete energy spectrum at each point was found by integrating over all angles. The shape of the spectral distribution of the scattered radiation was found to be nearly independent of the depth below the surface of the phantom. The variation of spectral distribution with field size was also investigated and in this case also the shape of the distribution was very nearly constant but was somewhat shifted to lower energies as the area of the field was increased. The scattered spectra were also plotted as distributions of dose and then combined with the dose distributions of primary radiation to give the total dose distribution. As an illustration of the use of radiation spectra, these dose distributions were used to calculate the effective half-value layer in copper and the average linear energy transfer of the radiation at various depths and for various field sizes. In addition, the absorbed dose in bone compared to the dose in muscle was calculated from the spectral distributions.

The intensity flux of the scattered radiation from x-ray beams in low atomic number materials has been obtained by a combination of analytical and Monte Carlo methods. Analytical methods were used to calculate the intensity flux of first-order scatter, and Monte Carlo methods to estimate the spectrum resulting from scatterings of higher order. The electronic computer at the University of Toronto was employed to obtain case histories for a pencil-like beam of photons with initial energies of 50, 100, 200, 500 and 1,250 kev impinging at right angles on a water scatterer. From the characteristics of the penetration of a pencil of rays, the properties of the diverging beams used in radiotherapy were obtained. Energy spectra were obtained for depths up to 20 cm. for areas of 25, 50, 100, and 400 sq. cm. and an infinite area. From these data for monoenergetic radiation, methods were developed to enable one to estimate the spectrum of scattered radiation produced by a continuous spectrum of incident radiations. By this means it is possible to estimate the spectral distribution of the scattered radiation for depths of 0, 2, 5, 10, and 15 cm. and areas of 25, 100, and 400 sq. cm. and infinite area for radiations in the range 50 to 1,250 kev. The energy absorbed, expressed as depth dose, was obtained for a number of incident spectra and compared with experimentally determined depth dose curves. The results were in good agreement.

Two scintillation detectors are simultaneously carried over the surface of a sphere which is concentric with the skull. The collimators for the detectors are arranged so that they accept only those gamma rays which emerge nearly normal to the skull. At present, RISA is being used as the tracer. However, any radioactive element which emits suitable gamma rays and which is preferentially absorbed by the tumor could be used with the scanner. The number of counts received by the two detectors are continually subtracted, so that any asymmetry in the distribution of the tracer is immediately indicated. The difference in count rate from the left and right detector, as well as the total number of counts received from each, are recorded on a circular chart of a size comparable to that of the actual head. By applying this chart directly to the patient's head, the point at which abnormal activity is observed on the chart may be located on the patient. The patient is scanned while supine with his head immobilized in a sling, so designed that it does not interfere with the motion of the detectors. Two points on the record charts are marked to correspond to two points on the head. These points are sufficient for interpretation of the actual position on the patient of any abnormal activity indicated on the chart. While the scan is in progress, the operation of the machine is completely automatic. The procedure, including time for setting up the patient, takes about forty minutes. Suitable scans are obtained with a dose of 250 microcuries of RISA. Scans are taken twenty-four hours after injection of the radioactive tracer. While the device uses the same principle as is employed in manual scans with one detector, it incorporates all the features of an automatic scanning system. The scans are reproducible, since the angles of the detectors with respect to the patient are identical for each scan. The scanner is operating in the Saskatoon Cancer Clinic and a second unit is being constructed for use in the Ontario Cancer Institute. Tests are being carried out on phantom models to establish the limits of the technic from a purely physical point of view. A complete description of the apparatus is being published elsewhere.

It is generally recognized that the most satisfactory way of expressing radiation dose is a statement of the energy locally absorbed, that is, of the amount of energy imparted by ionizing particles to unit mass of the irradiated material at the point of interest. It is often impossible, however, to measure directly the energy absorbed, and therefore the dose must be determined indirectly, generally from measurements of the ionization in a cavity-type chamber. The energy absorbed, Em, can be calculated from the measured ionization, Jm, using the Bragg-Gray relation (1, 2)

Earlier measurements of the ratio Em/Jm for Co60 radiation have been made at the Saskatoon Cancer Clinic by J. P. Bernier and the present writers. A detailed report of this work is being published in Radiation Research. The method employed in the measurement of the ratio for betatron radiation is essentially the same, with minor modifications to facilitate integration of the dose. A small sample of carbon (20 gm.) was thermally isolated and supported in the path of the beam from the University of Saskatchewan 22-MEV betatron. The value of Em, the energy absorbed per unit mass by the sample on exposure to a given amount of this radiation, was measured. An ionization chamber having physical dimensions identical to those of the absorption sample was supported in the same position in the radiation beam. A measurement was made of Jm, the ionization produced per unit mass of air in the sensitive volume of the chamber by the same given amount of radiation. If we apply the Bragg-Gray relation, the ratio of these two quantities gives an experimental determination of the product Wρm, where W is the average energy required to produce an ion pair in air and ρm is the ratio of the mass-stopping power of carbon to that of air. If, then, a value for ρm is calculated from theoretical considerations, we may determine W. A tabulation of results appears above. It is interesting from a clinical point of view to compare the values of Em∕Jm> for Co60 and betatron radiation. It can be seen from the values in the table that 6 per cent less energy is absorbed per gram for betatron radiation than for Co60 radiation for the same ionization dose. This difference is due to the change in the stopping-power ratio, pm. ρm decreases with increasing photon energy because of the increasing importance of the polarization or density effect in carbon. The values of W for Co60 and betatron radiation differ by a negligible amount. After completion of the betatron experiments, the apparatus, in its modified form, was used to remeasure Em∕Jm for Co60 radiation. The results obtained checked very well with the original measurements made by Bernier et at. and are an indication of the reproducibility of these measurements.

When fixed field or rotation therapy is used in the treatment of tumours located in the thorax, dosage errors of 10 to 40 per cent will be made if standard depth dose tables or tumour-air ratios are used, unless corrections are made for the presence of air cavities and bony structures. Some indication of these errors may be obtained by transmission dose measurements and such measurements have been carried out by a number of workers. Kornelsen (1954) measured the exit dose for 250 kV therapy with an ionization chamber completely embedded in a scattering medium so that the exit dose with full scatter was measured. Robbins and Meszaros (1954) have measured the transmission dose for 250 kV radiation with an ionization chamber placed in a presdwood phantom. The distance from exit point to detector for the largest diameter phantom used in their experiment was 33 cm. Phalzner (1956) has carried out similar measurements with cobalt 60 with the detector 70 cm from the exit point. All of these measurements are complicated by the fact that the detector sees an unknown amount of scattered radiation. It would be advantageous if a geometry could be selected in which all scattered radiation is excluded. In the method to be described in this paper, an ionization chamber has been built which is designed to see only primary radiation and to exclude all scattered radiation.

When Xor y-rays are absorbed in a material, their energy is imparted to electrons which then lose energy through ionization and excitation of the atoms of the material. In 1953 the International Commission on Radiological Units defined the rad as the unit of absorbed dose; 1 rad is 100 ergs of energy absorbed per gram of irradiated material (1). A calorimetric determination of the absorbed dose is very attractive in principle because of its directness. It is necessary to assume that all the absorbed energy is ultimately degraded into the form of heat. In the past, calorimetric methods have not been very practicable because of the difficulty in measuring the small temperature changes involved. For this reason, absorbed dose has usually been determined indirectly from measurement of the ionization of the gas in an ionization chamber. In calculating absorbed dose in absolute units, such as the rad, from ionization measurements one must know the value of W, the average energy expended in producing one ion pair in the gas.) In the experiments reported here, the value of W for air has been determined by a comparison of the energy absorbed in the wall of an ionization chamber and the ionization produced in the air volume of the chamber. The energy absorbed per unit mass of the wall, Em , and the ionization per unit mass of air, Jm , are related by the Bragg-Gray formula (2, 3),