L. H. Luthjens

The lifetime of the fluorescent excited state of liquid cyclohexane

Abstract The average lifetime of the fluorescence of liquid cyclohexane was determined by means of pulse radiolysis with a time resolution of ca. 100 ps and found to be 0.95 ns.

Optically isolated electronic trigger system for experiments on a subnanosecond time scale with a pulsed Van de Graaff electron accelerator

An optically isolated electronic trigger system for a pulsed Van de Graaff electron accelerator, producing an external pretrigger pulse 75 ns before arrival of the electron pulse at the target, is described. The total time jitter between trigger signal and electron pulse is 50 ps. The measurement of optical and electrical transients on a subnanosecond time scale with a sequential sampling oscilloscope is demonstrated. The contribution of various parts of the equipment to the total jitter is discussed. Those contributions to the jitter due to the electron transit time fluctuations in the accelerator assuming a constant acceleration voltage gradient and to the shot noise in the photomultiplier detector of the trigger system are calculated to be 5 ps and 12 to 21 ps respectively. Comparison with the experimental results leads to the conclusion that a considerable part of the total jitter may be attributed to acceleration voltage gradient fluctuations, to accelerator vibrations and possibly to density fluctuations in the insulation gas. Possible improvements of the trigger system are discussed. The apparatus is used for pulse radiolysis experiments with subnanosecond time resolution down to 100 ps in combination with subnanosecond time duration electron pulses.

Subnanosecond pulsing of a 3-MV Van de Graaff electron accelerator by means of a passive coaxial pulse shaper

A passive coaxial pulse shaper has been developed which produces a subnanosecond duration pulse with short rise and decay time from a long pulse with short rise time. The mechanical construction of the pulse shaper is a modified coaxial air line T-section. The pulse shaper has been incorporated in the pulsing circuit of a 3-MV Van de Graaff accelerator. The form of the resulting electron beam pulses was monitored both as the charge collected by a coaxial target and as the Cerenkov light emitted by a quartz plate. In both cases sequential sampling techniques were used. The electron beam pulses were found to have rise and decay tiems of approxiamtely 100 ps and a half-width as short as approximately 200 ps could be obtained. An advantage of this method of producing subnanosecond beam pulses is that it does not interfere with normal nanosecond pulsed operation of the Van de Graaff.

High-energy radiation monitoring based on radio-fluorogenic co-polymerization II: fixed fluorescent images of collimated x-ray beams using an RFCP gel

We have produced an optically clear, close to water-equivalent gel that is radio-fluorogenic, i.e. fluoresces in UV light after exposure to high-energy radiation. Its potential as a 2D and 3D dosimetric medium is demonstrated by fixed fluorescent images of the cross-section, track and intersection of collimated (10 × 10 or 5 × 5 mm(2)) 205 kVp x-ray beams. The images, produced by doses on the order of 10 Gy, are formed instantaneously and can be digitally recorded and scanned with a spatial resolution on the order of 0.1 mm. No loss of spatial resolution occurs on standing under ambient conditions for at least 3 days.

Monitoring the radiation-induced bulk polymerisation of methyl methacrylate with N-(1-pyrene)maleimide

Abstract The fluorogenic probe N-(1-pyrene)maleimide, MPy, exhibits a fluorescence response directly correlated to the monomer conversion in the radiation-induced polymerisation of methyl methacrylate (MMA). In the low-dose region, the ratio between propagation rate constants for the cross-over reaction in which a MMA radical chain-end reacts with MPy relative to the homopolymerisation reaction of MMA is found to be 1.38±0.15.

A Radio‐Fluorogenic Organic Gel for Real‐Time, 3D Radiation Dosimetry

IO N We have previously shown how radio-fl uorogenic co-polymerization (RFCP) can be used to monitor the radiation dose in a small volume of a liquid radio-fl uorogenic solution. [ 1 , 2 ] More recently we have devised a method of preparing a quasi-rigid RFCP gel that has been shown to be capable of creating a fi xed fl uorescent image of an X-ray beam. [ 3 ] The radio-fl uorogenic property of the gel is based on the conversion of non-fl uorescent maleimido-pyrene into the fl uorescent succinimidoderivative on co-polymerization into radiation-initiated growing chains of a bulk acrylic monomer (illustrated in the table-of-contents fi gure). In this work we demonstrate the capability of such a gel to provide a visual image of the more complex radiation fi eld surrounding a small seed of the radio-isotope 192 Ir. Such a radio-fl uorogenic gel could form the basis of a new, chemical method of three-dimensional radiation dosimetry. The gel consists of partially (ca 15%) pre-polymerized tertiary-butyl acrylate (tBuA) containing ca 100 ppm maleimidopyrene (MPy). The gel preparation procedure has been fully described elsewhere. [ 3 ] A 2 cm square glass cell containing a 6 cm length of such a gel is shown in Figure 1 A . A 1.9 mm diameter catheter has been inserted into the gel via a septum seal. The catheter is attached to a Nucletronics afterloader, [ 4 ] for the rapid transfer of a 3.6 mm long 0.9 mm diameter seed of 192 Ir to and from the end of the catheter. The image in Figure 1 B is of the cell exactly as in 1A but with the room lighting turned off and illumination occurring only via two 160-watt UV bulbs 15 cm on either side. This illustrates the extremely low fl uorescence from the gel prior to exposure to the radioactive seed. The change from negligible emission to bright blue fl uorescence after a 3 minute exposure can be seen in Figure 1 C. The image in Figure 1 D was taken after exposure but with the room lighting turned back on. It shows that the turbidity that usually occurs due to phase separation in irradiated aqueous polymer-gel media, is absent in this all-organic recipe. The absorption spectrum of the gel is also unaffected by irradiation since this is controlled mainly by the aromatic pyrene moiety which remains unchanged on co-polymerization of MPy. The optical density of the gel at the main, 365 nm, emission line of the UV lamps was 0.4 cm − 1 .

In-situ radiation dosimetry based on Radio-Fluorogenic Co-Polymerization

A fluorimetric method of radiation dosimetry is presented for which the intensity of the fluorescence of a (tissue equivalent) medium is linearly dependent on accumulated dose from a few Gray up to kiloGrays. The method is based on radio-fluorogenic co-polymerization (RFCP) in which a normally very weakly fluorescent molecule becomes highly fluorescent when incorporated into a (radiation-initiated) growing polymer chain. The method is illustrated with results of in-situ measurements within the chamber of a cobalt-60 irradiator. It is proposed that RFCP could form the basis for fluorimetric multi-dimensional dose imaging.

Radiation-induced polymerization monitored in situ by time-resolved fluorescence of probe molecules in methyl methacrylate

A technique is presented for monitoring radiation-induced polymerizations in situ based on the measurement of the fluorescence lifetime of molecular probes dissolved in the polymerizing medium. This method is illustrated with results on methyl methacrylate (MMA) using two fluorogenic probe molecules; N-(2-anthracene)methacrylamide (AnMA) and maleimido-fluoroprobe (MFP), a molecule which has a highly dipolar excited state.

Singlet and triplet excited-state formation in high-energy electron tracks in liquid cis- and trans-decalin as studied by product formation in radiolysis and photolysis

The yields of products formed as a result of irradiation of liquid cis- and trans-decalin with γ-rays and 3 MeV electrons as well as with 7.6 eV photons have been determined by means of gas–liquid chromatography (GLC). The major products for both cis- and trans-decalin are hydrogen, unsaturated C10 products and dimers; in the case of cis-decalin, cis–trans isomerization takes place via a chain reaction. Singlet and triplet excited states are formed by direct excitation and by charge recombination in the tracks of high-energy electrons. The product formation resulting from the singlets is determined by the photolysis. From a knowledge of the total yield of singlets from the radiolysis, the contribution of the singlet excited states to product formation during radiolysis is calculated and the remaining products are assigned to the triplets.For the sum of the yields of singlets and triplets a value of 6.4 (100 eV)–1(= 6.6 × 10–7 mol J–1) is found for cis-decalin and 6.0 (100 eV)–1(= 6.2 × 10–7 mol J–1) for trans-decalin and for the singlet fractions 0.53 and 0.47 in cis- and trans-decalin, respectively. The probability of C—H bond breakage in the singlet and triplet states is 0.42 and 0.82, respectively, for cis-decalin and 0.50 and 0.94, respectively, for trans-decalin.

Feasibility of obtaining short electron‐beam pulses from a Van de Graaff 3‐MV accelerator using laser‐photoelectron emission from a cold trioxide cathode

Production of electron‐beam pulses with a 700‐ps FWHM duration, using laser‐photoelectron emission from a cold trioxide cathode in a 3‐MV Van de Graaff accelerator has been demonstrated. At 70‐kW laser power (λ=337.2 nm) a current of about 1 mA has been measured which means a cathode quantum efficiency of 10−7. An electron transit time jitter of about 40 ps has been measured, to be attributed to voltage fluctuations on the accelerator tube electrodes resulting from the charging belt system. The results are discussed with respect to the feasibility to produce short pulses with a 1‐nC total charge, taking into account the quantum efficiency of 10−4 measured in a test setup for the cold trioxide cathode. Larger quantum efficiency for the accelerator cathode and decrease of the transit time jitter are the main goals set for development of the Van de Graaff accelerator as a radiation source for very high time‐resolution experiments in radiation chemistry and physics.