James H. Schulman

Luminescence in Solids

Publisher Summary This chapter discusses luminescence in solids. Luminescence has been an active field of scientific investigation for a period that essentially encompasses the present century. During this period, there have been almost revolutionary changes in the materials, methods, and concepts that distinguish current research in the field from those of the earlier work. There has occurred, first, a realization of the extreme importance of small concentrations of impurities in luminescence. This has led to the development of careful preparative procedures for controlling phosphor compositions and for reducing the concentration of unwanted impurities to a concentration of 10 -6 or less. A second important advance has been the application of the investigative techniques and concepts of solid state physics to the analysis and interpretation of luminescence phenomena. This approach has accelerated rapidly during the last decade. Luminescent systems have been divided into three categories: (a) systems in which the absorption and emission of energy both take place in the same center; (b) systems in which absorption occurs in one center and luminescence is emitted by another center, the energy transfer taking place without accompanying movement of charge carriers; and (c) systems in which the transport of energy by charge carriers is the dominant feature.

On the Luminescence of Divalent Manganese in Solids

It is shown that, under certain conditions, measurements on the emission and absorption of luminescent centers allow the determination of the configurational coordinate curves for the ground and excited states of the centers. The method is used as a first approximation in obtaining configurational coordinate curves for divalent manganese in zinc silicate. These curves offer an explanation of the asymmetry of the manganese emission. Comparison of the energy levels for the manganese ion in the solid and gaseous state leads to an identification of the various levels and to the suggestion that emission in the solid state corresponds to a 4G→6S transition.

The sensitized luminescence of manganese-activated calcite

Synthetic manganese‐activated calcites are shown to be practically inert to ultraviolet excitation in the range 2000–3500A, while they are luminescent under cathode‐ray excitation. The incorporation of small amounts of an auxiliary impurity along with the manganese produces the strong response to ultraviolet radiation hitherto ascribed to CaCO3:Mn itself. Three such impurities have been studied: lead, thallium, and cerium. The first two induce excitation in the neighborhood of the mercury resonance line, while the cerium introduces a response principally to longer wave ultraviolet. The strong response to 2537A excitation shown by some natural calcites is likewise found to be due to the presence of lead along with the manganese, rather than to the manganese alone. The data do not warrant ascribing the longer wave‐length ultraviolet‐excited luminescence of all natural calcites to the action of an auxiliary impurity. The essential identity of the cathode‐ray excited luminescence spectra of CaCO3:Mn, CaCO3: (...

Luminescent sulfur centers in alkali halides and other inorganic solids

Abstract Luminescent centers are formed by heating alkali halides in sulfur vapor. The similarity between their emission spectrum and the spectrum attributed to 0 2 - in alkali halides suggests that the centers are S 2 - molecule-ions. From the structure in the emission spectrum the ground state vibrational frequency is found to be 1.74 × 10 13 sec -1 .

Thermoluminescence and Color Centers in LiF:Mg

A variety of optical absorption, emission and luminescence excitation spectra have been measured in an attempt to identify the centers involved in the thermoluminescence of commercial LiF:Mg. It is concluded that the principal trapping centers consist of a hole trapped near various groupings of Mg2+ ions and vacancies. The optical absorption bands of these centers occur in the 3100–3800 A region which contains several absorption bands corresponding to different geometries of the centers. It is suggested that the 2200‐A band arises from Mg2+ ion‐vacancy complexes which have captured two holes. During thermoluminescence, holes are transported from traps to emitting centers. The luminescent center appears to be the F center both in an isolated position and when adjacent to a complex involving Mg2+ ions.

New Thermoluminescent Dosimeter

A simple dosimeter design is described in which a thermoluminescent phosphor is mounted on an electrically heatable support in an evacuated or gas‐filled envelope. With CaF2:Mn as the phosphor, the device detects gamma‐ray doses in the milliroentgen range and is linear in response up to at least 2×105 r. Dose readings can be made in less than a minute with simple instrumentation requiring no darkroom facilities. The dosimeter may be reused many times. The response is independent of dose rate at least over the range 10 mr/min to 7000 r/min. With suitable tin shields the response is independent of energy over the range 40 kev to 1.25 Mev. The advantages of this device for monitoring of personnel in health physics operations are pointed out.

APPLICATION OF LUMINESCENCE CHANGES IN ORGANIC SOLIDS TO DOSIMETRY

The degradation of photoluminescence in anthracene and in naphthalene has been studied under gamma‐ray and electron irradiation over the range 3×105 to 2×108 rep. For both materials the variation of brightness with Co60 gamma‐ray dose D, in rep, may be represented by (I/I0)=(1+AD)−1, where A=8.5×10−7 for anthracene and A=14×10−7 for naphthalene. Irradiation also produces a radiophotoluminescence in naphthalene, with emission in the visible spectral region. Both the degradation and radiophotoluminescence are stable with time at room temperature. The employment of these effects for dosimetry is discussed.

Some Optical Properties of Lead-Activated Sodium Chloride Phosphors

Absorption and luminescence phenomena in NaCl:Pb phosphors have been found to be rather complex. Melt-grown NaCl:Pb has an asymmetrical, 2730A-peaked absorption band. The variation of emission spectrum with wave-length of excitation within this band shows that it consists of two poorly resolved absorption bands, one peaking at 2730A and the other at 2900A. At low Pb concentrations, irradiation into the first of these causes a near-ultraviolet emission peaking at 3200A; irradiation into the second causes a visible emission peaking at about 4500A. At high Pb concentrations, irradiation into the first band gives a second near-ultraviolet emission band peaking at 3850A in addition to the one peaking at 3200A. Precipitated NaCl:Pb shows all the above phenomena, and in addition, has an excitation band peaking at 2600A, producing simultaneously a 3300A-peaked emission and a visible emission. The NaCl:Pb phosphors are unstable, deteriorating after a few days at room temperature and more rapidly at 130°. X-irradiation of these phosphors destroys the above absorption and emissions, and gives a print-out effect due apparently to the formation of colloidal Pb. The x-rayed material can be excited by near ultraviolet to give a red emission, peaking at about 6100A. Some suggestions concerning the interpretation of the above phenomena are given.

The CaSiO 3 :(Pb+Mn) Phosphor

A calcium silicate phosphor, activated by lead and manganese, is described, and the conditions for its preparation are presented in detail. Under excitation by a low pressure mercury discharge, such as is employed in fluorescent lamps, this phosphor luminesces with an emission lying principally in the red end of the spectrum and with a high efficiency comparable to that of zinc beryllium silicate of the same luminescence color. Data are presented on the spectral emission of CaSiO3:Pb, CaSiO3:Mn, and CaSiO3:(Pb+Mn), both under cathode-ray and under 2537A excitation. It is shown that CaSiO3:Mn is excited to luminescence in the red end of the spectrum by cathode rays but not by ultraviolet light. The introduction of lead as an auxiliary impurity makes possible the excitation of this material by radiation in the neighborhood of the mercury resonance line. Qualitative absorption experiments indicate that the lack of response of CaSiO3:Mn to 2537A excitation is due to the absence of an absorption for this radiation; and that lead acts as a sensitizer for the manganese by introducing an absorption band in the vicinity of 2537A. The presence of a sensitizer is unnecessary under cathode-ray excitation, since in this case the energy is absorbed by the host crystal itself.

Anomalous fading of CaF2:Mn thermoluminescent dosimeters☆

Abstract A surprising effect encountered in the application of the thermoluminescence of CaF 2 :Mn to dosimetry is the initially high fading rate. This fading is much more rapid than would be expected considering the high-temperature, apparently uncomplicated glow curve of CaF 2 :Mn. This effect is present when rapid heating rates (∼20°C/min) are used to obtain the glow curve, but when slow heating rates (∼20°C/min) are employed no fading is observed. It was determined that the glow curve is a composite of several closely spaced components of varying stability. With rapid heating rates all the glow curve components are shifted upward in temperature, causing preferential thermal quenching of the stabler (higher temperature) components. The composite glow curve is thus distorted and the unstable components, being attenuated to a lesser degree, give a greater contribution to the thermoluminescence output.