Abstract
Radium salt has been generally used for radioactive self luminous materials. However, this type of self luminous materials has many drawbacks; 1) the phosphor crystal is damaged and deteriorated by a-rays and brightness decreases, 2) γ-rays from radium have high energy and very hazardous. Recently, self luminous materials using /3-emitter instead of radium have been studied. In this paper the emission characteristics of ZnS-phosphors excited by tri-tium gas is described.
As phosphors cubic-ZnS: Cu: Cl (φ=8μ) and hex-5 ZnS· 5 CdS: Ag: Cl (φ=6μ), which gave most intense emission, were used. Inner walls of a 9mmφ glass globe and a 5mmφ glass cylinder were coated with monolayer of phosphor crystals. After evacuating the globe or cylinder, tritium gas (>96%, 2 c) was introduced by a Toepler pump. Emission intensity was measured by RCA-931 A photomultiplier tube.
The relationship between emission intensity and pressure of tritium gas is shown in Fig.2. Emission intensity increased with increasing tritium gas pressure, but in high pressure region, the emission efficiency decreased gradually. The phenomena can be explained by the self-ab-sorption of tritium /3-rays, which is expressed by the following equation.
N=N∞(1-e-μx) (1)
N: apparent radioactivity
N∞: apparent radioactivity at infinite thickness
μ: self-absorption coefficient
χ: thickness of tritium gas
The emission intensity (L) is proportional to the apparent radioactivity (N) .
Thus eq. (2) is obtained from eq. (1) .
L=L∞(1-e-μχ) (2)
The self-absorption coefficient (μ) is proportional to the pressure of tritium gas. On the other hand, the factor (x) can be assumed to be constant in our systems.
Therefore,
L=L∞(1-e-KP) (3)
K=kχ, k=μ/P
From the experimental results, L. and K were calculated using eq. (3) (Table 1) .
L∞ depends on the kind of phosphors and the shape of glass tube, and K depends only on the shape of glass tube. Solid lines in Fig. 2 are calculated curves obtained from eq. (3) using K and L∞ values. It is clear that experimental results can be expressed by eq. (3) .
The absolute measurement of the emission intensity was made at 0.28 atm of tritium gas and found to be 44.3 microlamberts. Putting the values in eq. (3), L∞ is calculated to be about 160 microlamberts for 9 mmφ cubic-ZnS: Cu: Cl.
No decrease in brightness due to phosphor deterioration has been observed during these three months.
The self luminous lamp excited by tritium gas may be useful as a standard light source.
