Storage Phosphor Screens

Storage Phosphor Screens are used in conjunction with a storage phosphor system, such as Typhoon™ Variable Mode
Imagers, Storm™ Gel and Blot Imaging Systems, and PhosphorImager™ instruments. Storage Phosphor Screens retain energy from beta particles, X-rays, and gamma rays, and require exposure times that are 50–90% shorter than that of conventional film. Upon laser-induced stimulation, light is emitted from the storage phosphor screen in proportion to the amount of radioactivity in the sample.

The resulting digital image allows for quantitation of subtle signal intensity differences over the wide dynamic range of the system and for production of publication-quality images. Latent images are erased with the ImageEraser, a white light source. Screens are reusable and are not degraded by repeated exposure to laboratory levels of radioactivity.

Screen Types

General Purpose (GP)

General purpose screens are reliable for a wide variety of applications and can be used with 32P, 125I, 35S, 33P and 14C. The durable cellulose acetate coating and the phosphor layer formulation makes the GP screen ideal for 32P and 125I detection and quantitation. This is the screen of choice for 32P Northern blots, Southern blots, 125I Western blots, and gels.

Tritium (TR)

To detect the weak energy of the 3H signal, Tritium (TR) screens are constructed without the protective cellulose acetate overlay. The maximum energy range of 3H is 5 microns and that signal can be further attenuated by a thick sample or sample media.

A sample prepared with a fluorographic reagent is not recommended. Best results are obtained when the 3H signal is on the surface of the sample and available to penetrate the screen (for example, tissue sections on glass and whole body autoradiography). 

Screen styles

Two types of screen configurations are available. Mounted screens are permantly fixed to an aluminum backing plate, and they require a specially designed exposure cassette. Unmounted screens have no additional backing and can be used with a standard exposure cassette. PhosphorImager Systems (other than Storm and Typhoon) require the use of mounted screens. Storm and Typhoon Systems, with their lid pressing mechanism, can use mounted or unmounted screens.

Detection threshold

The lower limit of detection for a one-hour exposure is less than 2 dpm/mm² for ¹⁴C using the GP screen scanned at 200 microns.

Screen selection guide by instrument compatibility 

Excitation

Creation of excited (stored) electrons occurs when the screen is exposed to wavelengths shorter than 380 nm or to α, β, or γ radiation.

Erasure

Approximately 80% of stored information is released upon scanning with the Phosphor Imager. The remaining signal must be erased before the Storage Phosphor Screen is reused. The most efficient wavelengths of erasing light are between 475 and 650 nm.

For example, using a 500W photoflood tungsten light bulb and a yellow filter (Wratten 16), an exposure of 10 J/cm² will leave a residual signal level of less than 10^-5 of the original exposure level.

Coverings

To prevent contamination, radioactive samples are covered prior to their exposure to the Storage Phosphor Screens. There are a variety of materials used for this purpose. Mylar™ provides the best transmittance for ¹⁴C particles followed by Saran™ wrap and then a hybridization bag. The thinner Mylar allows more transmittance. However, as the thickness of the Mylar decreases, the chances of screen contamination increase. Therefore, unless it is pertinent to get as much signal as possible, a Mylar thickness of 6 ”m should be used to ensure that the screen does not get contaminated. Also, the 6-”m Mylar is easier to handle than the 1.5-”m and 3-”m versions.

Physical properties

Test conditions for storage stability: 25șC-49șC with 20%-90% relative humidity.

Results: Storage phosphor screens are highly resistant to the effects of high relative humidity.  Representative screens have been incubated at 32°C/90% relative humidity for up to one year with no detectable loss in sensitivity or change in physical properties.

Test conditions for scratch resistance: .08 mm stylus and 25-200 g load.

Results: Visible damage to the overcoat occurs when a load exceeding 50 g is applied to the stylus.  Damage to the phosphor layer, seen radiographically, occurs when the load applied to the stylus exceeds 100 g.  

Flatness

Screens are manufactured to be flat under conditions of 21°C and 50% relative humidity.  Actual flatness will vary with temperature, relative humidity, and screen size.  The aluminum mounting plate is flat to 0.005 inches.