Patent GB-1024932-A (GB)
IPC: G01N23/225
Abstract: 1,024,932. X-ray emission analysis. NATIONAL RESEARCH CORPORATION. Jan. 17, 1964 [Oct. 18, 1962], No. 39452/62. Heading G1A. [Also in Divisions H1 and H4]. The invention is concerned with analysing a sample by investigating the X-rays emitted when it is irradiated by an electron beam. It is of particular value in determining the quantity of light elements such as carbon and nitrogen present, these elements producing long wavelength X-rays which are readily absorbed. If a single line spectrum input is applied to a proportional counter a spread of pulse heights of a Gaussium nature occurs. Thus if a plurality of elements are present giving overlapping pulse height spreads it is impossible to determine the proportions of the elements directly by a measurement of the count rates of pulses of the heights associated with their spectral lines. The Specification explains how the amount of any element may be obtained in such circumstances by feeding the output of a proportional counter receiving the X-rays from a sample to a plurality of pulse-height discriminators and combining the discriminator outputs with the required polarity and weighting in an adding circuit. Figure 2 shows how the electron beam from a gun 3 may be focused and scanned over the face of the specimen 1. The X-rays produced are detected by proportional counter 7 and after amplification applied in parallel to the inputs of three pulse-height discriminators 9 feeding ratemeters 10. The outputs from the discriminators 10 are fed through individual polarity determining circuits 11 and multiplying coefficient devices 12 before combination in adding circuit 13. The signal from this circuit is applied to modulate the beam in display cathoderay tube 14 scanned in synchronism with the scanning of the specimen to show the distribution of the particular element. In a modification (Fig. 3 not shown), stated to be easier to adjust, difference circuits are included in channels 2 and 3 between the ratemeters and polarity determining circuits to give, in channel 2, the difference between inputs from channels 1 and 2, and, in channel 3, the difference between inputs from channels 2 and 3. As theoretical equations giving the factors used in the coefficient devices assume idealized distribution curve shapes which do not occur in practice it is preferred to set up the factors by calibration operations using specimens of know substances. The apparatus may be set up for investigating a plurality of elements with ganged switches changing the discriminator levels, polarity devices, and coefficient devices as necessary. Proportional counter output stabilization. Gates may be introduced in the proportional counter output circuits operative during scanning flyback to apply a proportion of an output from the pulse analyzing network to an integrator controlling the counter e.h.t. voltage. Thus if the electron beam is directed to a reference standard scanning flyback the output derived from X-rays from the known substance can be maintained constant. Another factor affecting the amplitude of the proportional counter pulses is the gas density within the counter. Due to the need to use a very-thin window a sealed counter is not possible and it is proposed to use a valve V 1  controlled by a diaphragm D 1  comparing the pressure in a sealed end chamber in the counter and that in the rest of the counter to control the admission of gas at pressure P 2  to the counter. The rate of flow of gas is determined by the outlet valve N. This allows for temperature variations. For higher stability the gas can be directed through a pre-regulating diaphragm stabilizer to the diaphragm valve of the counter (Fig. 6, not shown).