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INFORMATION STORAGE BY F-XK-CENTER
CONVERSION
R. Voszka, Sz. Tökés, P. Kolin, K. Raksányi
To cite this version:
Abstract. — F-centers generated by ionizing radiation in extremely pure OH-free NaCl show high stability. These F-centers transform at higher temperatures into Xh-centers. The transformation can be accelerated by F-light absorption, therefore the process can be used for information storage. The maximum of the Xi<-band absorption lies at about 580 nm at RT and is sharply separated from that of the F-band at 458 nm. XK-centers can be retransformed into F centers by ionizing radiation. The Xi<-centers are optically stable and seem to be of colloidal nature. (Patent no. 163584.)
It is a well-known phenomenon that F-centers in 250 °C but it is closely preceded by a transformation additively or electrolitically colored NaCl mono- into a center of different type. The transformation crystals transform upon heating into X-centers at can be accelerated by bleaching in the F-band at tem-about 250 °C. The maximum of the absorption band peratures below 250 °C. The new band has a maximum of the latter is at 550 nm. These crystals are reddish- at 580 nm. The optical absorption of the crystal after purple in contrast to the yellow color of the crystals X-irradiation for 10 hours and after subsequent expo-containing F-centers which absorb in the 458 nm band, sure to F-light at 160 °C is shown on figure 1. Both
On the other hand the natural blue rock-salt is known _ to have its main absorption maximum at 640 nm. ""•
According to the literature [1] X-centers are either very small colloids or atomic centers, and the blue
color is caused by larger colloids. Different views „ regarding the role of O H " ions in the colloid gene- „„ i* ration process are quoted [I]. ' >
We produced OH-free NaCl monocrystals [2] con- I /"\ taining a concentration of less than 2 x 10 "7 mole/ " I F ' I
mole of divalent impurities. It was found that coloring I j i \ these crystals by ionizing radiation an extremely stable ~g I ' I coloration can be achieved, much more stable, than l f\ > I in the case of crystals containing OH ~ ions (air-grown I I I ' '
nominal pure crystals) or/and divalent metal impurities 1 1 1
even in small (10~6 mole/mole) concentrations. The / I 1 •
F-center concentration (A^F) as a function of irradiation n, / 1 1 1 '
time (t) can be given in the form [3] " / I I \
NF = A[\ - e x p ( - at)] + Bt - / I ' \ \ M
where the constants have the following values for the / L I 1 ' • X-ray apparatus working at 200 kV, 20 mA and 0.2- J j \ \A 3 000R/min. / / '\ \ X\
A = 1.85 x 10, 6crrT3 -• / , ' \ \ \ \
B = 3.85 x l O ^ m i n - ' c m -3 ^ s « r « r ^ - - - " ^ I ^ \ /
a = 0.076 min"1 L ~ I 1 _ ^
. . 300 WO ' 500' 600 700 nm Investigating our extremely pure X-irradiated „ _. . . , . . ^ , xr „, .
-.» „. , , r ,, . . FIG. 1. — The optical absorption of extremely pure NaCl after
NaCl crystals the following interesting effect was x-irradiation (full curve) and after subsequent bleaching (dashed noted : thermal bleaching occurs only at about curve).
INFORMATION STORAGE BY F-X
K-CENTER CONVERSION
R. VOSZKA, Sz. TOKES, P. KOLIN and K. RAKSANYI Research Laboratory for Crystal Physics of the Hungarian Academy
of Sciences, Budapest XI, Budaorsi ut 45, Hungary
C7-84 JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 12, Tome 37, Décembre 1976
Résumé. — Des centres F créés par quelque rayonnement ionisant en NaCl extrêmement pur
et exempt d'OH montrent une haute stabilité. Ces centres F se transforment à des températures élevées en centres XK. La transformation peut être accélérée par l'absorption de la lumière dans la bande F, c'est pourquoi le processus peut être utilisé pour l'enregistrement de l'information. A la température ambiante le maximum de la bande d'absorption XK est à 580 nm approximativement et se trouve bien séparé de celui de la bande F à 458 nm. Les centres X K peuvent être retransformés en centres F par quelque rayonnement ionisant. Optiquement les centres X K sont stables et parais-sent être d'une nature colloïdale.
INFORMATION STORAGE BY F-XK-CENTER CONVERSION C7-85 curves are taken at room temperature. One can clearly
see the disappearance of the initially present F- and M-centers and the increase of the new band, denoted X, as it appears between the X and the blue colloid band. The crystals are purple-blue. The X,-band is optically stable. The F-X, conversion takes place exponentially as illustrated on figure 2 where the dependence of the logarithm of the F-center concen- tration on bleaching time is shown. The used F-light intensity was 10-3 W/cm2. The defect responsible for the X,-band is unstable when the crystal is exposed to ionizing radiation. At room temperature it can be reconverted to F-centers by roughly the twentieth part of the dose necessary to the initial generation of
f I G . 2. - The dependence of the F-XK conversion on time.
the F-centers. The new band probably belongs to colloids of medium or eventually to colloids of varying size, but for its essential understanding further inves- tigations are necessary and are under way.
The experimental fact of F-X, conversion however may be used for optical information storage because of its several advantages.
These are :
1) The crystal may be homogeneously colored in either thin or thick layers by ionizing radiation.
2) By changing the irradiation time the F-center concentration and thus the optical density can be varied.
3) For storage at 1600C an energy density of
1 j o ~ l e / c m - ~ is needed. By increasing the temperature this can be diminished.
4) The F and
XK
absorption bands are well sepa- rated.5) F and X,-light is available by usual lasers. 6) Reading in the X,-band is free of destruction. 7) Information storage is unlimited in time at room temperature.
8) Information can be cleared by a moderate dose of ionizing radiation.
Relying upon these properties, we think that by further work a simple and cheap holographic memory can be developed on the basis of the observed pheno- menon.
References
[l] SCHULMAN, J. H., COMPTON, W. D., Color Centers in Solids
(Pergamon Press) 1963, Chapter IX.
[2] VOSZKA, R., RAKSANYI, K., BERKES, L., Krist. Tech. 5 (1970)
40941 2.
[3] COKRADI, G., GOMAA, I. M., VOSZKA, R., Phys. Status Solidi (a) 8 (1971) K81.
DISCUSSION N. ITOH. - May I refer the work made by Tazaki
of Sanujo Electric Company, who used M-center in X-irradiated NaF orientation, developed by Schneider, for information storage. The stability against the repetition of erasing and writing was improved greatly according to their results, by removing OH- from the crystal. They could repeat l 000 repetitions without much loss of capability of information storage.
R. VOSZKA. -Thank you for your comment. A. E. HUGHES. - HOW offen is it possible to cycle the crystal between the F- and X-band states ?
R. VOSZKA. - The X,-centers were destroyed by X-rays. T o say something on the mechanism of des- truction it is needed to know the structure of the center. But presently it is unknown. Thermodynamically the irradiation means introduction of energy which shifts the crystal in the direction of RT equilibrium. This means the presence of F-centers.
R. MONNIER. -The same type of phenomenon has been observed by Pellaux in Neuchltel (unpu- blished), in CsI : Eu doped with F-centres.
In particular be observed a practically infinite
storage time at room temperature. The stored infor- VOSZKA et al. - We made only one cycle, because
mation could be erased by heating up the sample, it was a new phenomenon. A lot of investigations are
which could then be reused. As far as I remember, to d o from point of view of practical application.
there was no limit on the number of possible cycles. P. THEVENARD. - HOW do you explain the des-
truction of X,-centres by the electron bombardment ? R. VOSZKA. - Thank you for your comment.
