Patent Number: 059050144
Section: description

The invention is illustrated by the following examples without however limiting it thereto. Important concerning image quality as reflected in S-SWR measuring methods will be described hereinafter in the examples. EXAMPLES Definitions and methods used. Measurement of sensitivity S and square wave response SWR for stimulable phosphor screens For the photostimulable phosphor screens coated with BaSrFBr:Eu.sup.2+ phosphor the measurement of S and SWR was carried out with an image scanner made up with a He-Ne laser. The beam of a 10 mW red He-Ne laser is focussed to a small spot of 140 .mu.m (FMWH) with an optic containing a beamexpander and a collimating lens. A mirror galvanometer is used to scan this small laserspot over the entire width of a phosphor sample. During this scanning procedure the phosphor is stimulated and the emission light is captured by an array of optical fibers which are sited on one line. At the other end of the optical fibers being mounted in a circle a photomultiplier is situated. To attenuate the stimulating light an optical filter, type BG3 from SCHOTT, is placed between the fiber and the photomultiplier. In this way only the light emitted by the phosphor is measured. The small current of the photomultiplier is first amplified with an I/V convertor and digitalised with an A/D convertor. The measuring set up is connected with a HP 9826 computer and a HP 6944 multiprogrammer to controll the measurement. Starting the procedure an electronic shutter is closed to shut down the laser. A phosphor sample measuring 50 mm.times.210 mm is excited with a 85 kV X-ray source provided with an aluminum filter having a thickness of 21 mm. The radiation dose is measured with a FARMER dosemeter. Between the X-ray source and the phosphor layer a thin lead-raster containing 6 different spatial frequencies is mounted to modulate the X-ray radiation. Frequencies used are 0.50, 1.00, 2.00 and 3.00 line pairs per mm. After exposure the sample is put into the laser scanner. To read out one line the shutter is opened and the galvanometer is moved linearly. During the scanning procedure the emitted light is measured continuously with the A/D convertor at a sampling rate frequency of 100 kHz and stored within a memory card in the multiprogrammer. One scan thus contains 100000 pixels. Once the scan is complete the shutter is closed again and the galvanometer is put on his original position again. The data of the scan line are transferred from the memory card in the multiprogrammer to the computer where said data are analysed. A first correction takes into account the sensitivity variation of the scan line with the distance. Therefore a calibration scan was measured previously for a phosphor sample that was exposed quite homogeneously. A second correction takes into account the amount of X-ray dose by dividing said values by the said dose amount. The different blocks are separated and the amplitude on each spatial frequency is calculated, making use of Fourier analysis. The amplitude of the first block having a spatial frequency of 0.025 line pairs per mm is taken as the sensitivity of the stimulable phosphor screen. The other values are the results for the curve of the Square Wave Response (SWR: SWRl referring to the response at 1 line pair per mm; SWR2 to the response at 2 line pairs per mm) which is representative for the resolution of the screen. Composition of the screens ______________________________________ Antihalation undercoat layer: Solution A: MOWILITH CT5 (from HOECHST AG) 300 g Ethanol 700 g CYMEL 300 60 g p-toluene sulphonic acid 12 g Solution B: Dye-1 0.750 g Ethanol 150 g Sodium hydroxide 0.08 g *16 hours after its preparation solution B is filtered off: a red-brown solution is obtained. Coating solution: Solution A 33.3 g Solution B 3.0 g Ethanol 63.6 g ______________________________________ The coating solution was coated by dipcoating techniques at a rate of 4 m per minute on a polyethylene terephthalate support (see further Table 1) having reflecting properties (containing BaSO.sub.4 particles) or absorbing properties (having carbon black particles). Thermal curing was performed over one night at 80.degree. C. after drying. Properties of the thus obtained antihalation layer. An absorption of 0.22 at a wavelength of 633 nm (HeNe laser emission wavelength). No substantial absorption is measured at the emission wavelength of the stimulable phosphor (having its maximum emission at 390 nm). ______________________________________ Phosphor layer composition: BAEROSTAB M36 (from Barlocher GmbH) 1.5 g DISPERSE AYD 9100 (from Daniel Produkts Company) 0.75 g KRATON FG19101X (from Shell Chemicals) 12.5 g BaSrFBr:Eu (mean particle size: 7 .mu.m) 270 g BaSrFBr:Eu (mean particle size: 3 .mu.m) 30 g ______________________________________ Preparation of the phosphor laquer composition: BAEROSTAB M36, DISPERSE AYD 9100 and KRATON FG19101X were dissolved while stirring in the prescribed amounts in 61.5 g of a solvent mixture from wash benzine 100-120, toluene and butyl acrylate in ratios by volume of 50:30:20. The phosphors were added thereafter and stirring was further proceeded for another 10 minutes at a rate of 1700 r.p.m.. The composition was doctor blade coated at a coating rate of 2.5 m per minute onto a subbed 175 .mu.m thick polyethylene terephthalate support and dried at room temperature during 30 minutes. In order to remove volatile solvents as much as possible the coated phosphor plate was dried at 90.degree. C. in a drying stove. It has been established that a layer composition was obtained having good curable properties. Moreover no diffusion of colorant was found from the intermediate antihalation layer between support and phosphor layer to the phosphor layer: this particular dye is advantageously soluble in ethanol but insoluble in solvents present in the coating composition of the phosphor layer. In Table I the coating composition is given for the stimulable phosphor. For each screen sample the following data referring to the composition are summarised in the respective Tables: number of the sample (Matl. No.) PA1 support: expressed as % reflectance: 0-10% corresponds to a support having carbon black dispersed in the support material; 85-100% to a support having BaSO.sup.4 dispersed therein; PA1 presence of antihalation undercoat layer (AHU): yes (Y) or no (N); PA1 phosphor coverage weight (PCW) (between 70 and 90 mg/cm.sup.2). PA1 presence (N=No) and, if present, amount of dye in the phosphor layer; PA1 screen speed (a higher figure refers to a more sensitive screen); PA1 SWR1 and SWR2 values respectively. TABLE I __________________________________________________________________________ Stimulable phosphor screens with BaSrFBr:Eu.sup.2+ as a stimulable phosphor. Matl. Support PCW No. (% refl.) AHU (mg/cm.sup.2) DYE SPEED SWR1 SWR2 __________________________________________________________________________ 1 85-100 Y 69 N 135 .70 .37 2 85-100 Y 78 N 150 .67 .34 3 0-10 N 79 N 132 .68 .36 4 0-10 N 90 N 146 .66 .33 5 0-10 N 85 N 143 .67 .34 6 0-10 N 86 0.006%* 138 .68 .36 7 0-10 N 87 0.06%* 108 .73 .43 __________________________________________________________________________ *solution B (in volume %) As is clear from the data related with speed and sharpness the highest (most preferred) values are attained for materials Nos. 1 and 2 (both having an antihalation undercoat and a reflective support): an increased amount of phosphor coated makes speed increase, without a remarkable decrease in sharpness. Addition of antihalation dye in minor amounts to the phosphor layer alone as in material No. 6 having no reflection layer also leads to a suitably good relationship between speed and sharpness.