Abstract: This paper addresses the role of the cesium source receiver in the automatic control system of the stopper rod in the continuous casting machine of Xuanhua Steel Plant, identifying its weaknesses and problems, and proposing effective measures for rational modification. The molten steel level detection system of the three continuous casting machines in Xuanhua Steel Plant adopts the cesium source stopper rod digital control system from Hengyang Leimu Company, replacing the traditional manual pouring method, reducing labor intensity, and improving the continuous casting machine's operating rate. The cesium source receiver is a key component of the stopper rod digital control system and plays a crucial role. Keywords: Automatic control of the stopper rod; Cesium source receiver Abstract: In view of the steel mill conticaster filling the cesium source receiver in the automatic control system, the role of the cesium source receiver in the production movement is discussed. However, the existence of weak links and existing problems leads to effective action and rational transformation. The steel mill's three-conticaster crystallizer molten steel liquid level examination system utilizes the cesium source receiver from Hengyang Radium Item Company to improve the numerical control system, replacing traditional manual casting in the steel mill, reducing labor intensity, and enhancing the conticaster's work rate. The cesium source receiver is a key component of the improved numerical control system and plays a vital role. Keywords: Automatic control of the stopper rod; Cesium source receiver 0. Introduction Xuan Steel Plant currently has three continuous casting machines: No. 4, No. 5, and No. 6. The normal operation of these continuous casting machines is crucial to steel production and the development of steelmaking. The continuous casting machine employs an automatic stopper rod control system. A crucial component of this system is the acquisition of data on the molten steel level within the crystallizer. The cesium source receiver is a vital part of this system, and its quality directly impacts the continuous casting machine's operating rate. Therefore, to improve billet output and ensure billet quality, the cesium source receiver is modified. 1. Application of the Cesium Source Receiver in the Automatic Stopper Rod Control System of the Continuous Casting Machine The block diagram of the automatic stopper rod control system is as follows: A key aspect of the automatic stopper rod control in the continuous casting machine is the acquisition of data on the molten steel level within the crystallizer. To collect this data, a Cs-137 cesium source is installed on one side of each crystallizer in each flow, and a cesium source receiver is installed on the opposite side. The cesium source receiver transmits the acquired signal via a coaxial cable to a secondary detection instrument. After the detection instrument amplifies and analyzes the pulse data acquired by the receiver, this data is sent to the main unit (PLC) through the central processing unit to generate a pulse counter. It is evident that the cesium source receiver is a crucial component of the stopper rod control system, and its quality directly impacts the normal operation of the continuous casting machine. [b]2. Modification of the Cesium Source Receiver[/b] Due to the high ambient temperature, significant vibration, and abundant dust, the photomultiplier tube, preamplifier, and high-voltage circuit board of the cesium source receiver are prone to damage, leading to data acquisition failure. Previously, damaged cesium source receivers had to be returned to the manufacturer for repair and replacement, which was not only time-consuming and disruptive to production but also expensive, significantly increasing spare parts costs. To save costs, the team members carefully analyzed the internal structure and principles based on available information and, after experimental verification, decided to repair the cesium source receiver themselves. The cesium source receiver plays an indispensable role in data acquisition and is crucial in the automatic stopper rod control system. The cesium source receiver consists of a crystal, photomultiplier tube, preamplifier, and high-voltage circuit board. The rays emitted by the cesium-137 radioactive source strike the crystal, causing it to fluoresce. This fluorescence then strikes the photocathode of the photomultiplier tube (PMT), where the photocathode emits electrons and amplifies them. The PMT converts this fluorescence into electrical pulses. These pulses are amplified by a preamplifier and sent to a secondary instrument. A high-voltage circuit board generates a DC high voltage to provide the operating voltage for the PMT. 2.1 Modification of the Photomultiplier Tube The PMT is a highly sensitive and fast-responding photodetector. A PMT is an electron tube whose photosensitive material is primarily cesium oxide, modified by doping with oxides of other active metals to improve sensitivity and correct the spectral curve. Photocathodes made from this material emit electrons under illumination, called photoelectrons. These photoelectrons are accelerated and amplified by a grid before striking the anode, ultimately forming a current. Figure 1 shows a window-type PMT: A PMT is a vacuum device. It consists of a photocathode, a focusing electrode, an electron multiplier electrode, and an electron collecting electrode (anode). When light shines on the photocathode, the photocathode excites photoelectrons into the vacuum. These photoelectrons enter the multiplication system according to the focusing electrode electric field and are amplified through further secondary emission. The amplified electrons are then collected by the anode as a signal output. Typical photomultiplier tubes can be divided into two types according to the incident light receiving method: end-window type and side-window type. Figure 2 shows a cross-sectional view of the end-window type photomultiplier tube. Its main working process is as follows: Analysis and inspection revealed that the photomultiplier tube used in the cesium source receiver is a GDB-443 type photomultiplier tube, which uses an end-window type semi-transparent antimony potassium cesium photocathode with a spectral response range of 300-670nm and a peak wavelength of 400nm. The effective surface diameter of the cathode is 23nm, and it has an 11-stage box-type multiplication system. Once the photomultiplier tube is damaged, the spectral response will not reach the required range, the current amplification will not reach the required amplification factor, and the cathode illumination sensitivity, anode illumination sensitivity, and dark current will not reach the required values. Replace the photomultiplier tube with the same model and add waterproof and shockproof measures. Wrap the photomultiplier tube with adhesive plastic tape to prevent water ingress and vibration from breaking it. Apply silicone grease between the crystal and the photomultiplier tube to prevent poor contact. The modified photomultiplier tube can accurately convert extremely weak light signals into electrical signals, ensuring that its main parameters reach the required values. 2.2 Modification of the preamplifier and high-voltage circuit board: The pulse is amplified by the preamplifier and sent to the secondary instrument. The high-voltage circuit board generates DC high voltage to provide the operating voltage for the photomultiplier tube. The preamplifier and high-voltage circuit board play a crucial role in the cesium source receiver. Due to the high dust levels in the field, the circuit board is prone to short circuits, and high voltage and high current may be generated during operation, burning out resistors, capacitors, and breaking down transistors. Inspect the circuit board, use a multimeter to measure the resistors, capacitors, and transistors, replace them with the same model components, and measure the circuit board again with a multimeter. Install after confirming everything is correct. 3. Key Technologies 3.1 Spectral Response The photomultiplier tube (PMT) receives the energy of incident photons at the cathode and converts it into photons. Its conversion efficiency (cathode sensitivity) varies with the wavelength of the incident light. This relationship between photocathode sensitivity and incident light wavelength is called the spectral response characteristic. Figure 3 shows the spectral response curve. 3.2 Current Amplification (Gain) After the photoelectrons emitted from the photocathode are accelerated by the electric field, they collide with the first dynode, generating secondary electron emission. This produces an electron stream exceeding the number of photoelectrons. These secondary electron streams are then accelerated and collide with the next dynode, generating another secondary electron emission. This process is repeated continuously until the secondary electron emission from the final dynode is collected by the anode, thus achieving current amplification. At this point, the small photoelectron current generated by the photomultiplier tube cathode is amplified into a larger anode output current. 3.3 Anode Dark Current The photomultiplier tube still outputs a small current even in complete darkness. This small current is called the anode dark current. It is one of the important factors determining the photomultiplier tube's ability to detect weak light signals. 3.4 Light Sensitivity Cathode light sensitivity refers to the cathode photoelectron current generated per unit flux of incident light measured using 2856K color temperature light generated by a tungsten lamp. Anode light sensitivity is the anode output current generated per unit of incident light energy on the cathode (i.e., the output current multiplied by the secondary emitter). Once the photomultiplier tube is damaged, the photoelectron current emitted by the photocathode cannot be amplified into a larger anode current, and the weak light signal cannot be detected. After the modification, vibration was reduced, ensuring that the current amplification, anode dark current, and light sensitivity of the photomultiplier tube reached the required values, ensuring that the photomultiplier tube converted the fluorescence generated on the crystal into electrical pulses, ensuring data acquisition of the liquid surface, and ensuring the operation of the stopper rod automatic control system. This ensured the normal production of the continuous casting machine. [b]4. Conclusion[/b] After more than two years of operation, the cesium source receiver has been operating stably, improving the operating rate of the continuous casting machine and increasing output. References [1] Li Fahai and Wang Yan, Fundamentals of Electric Machines and Drives, Tsinghua University Press [2] Shen Anjun, Principles of Automatic Control and Regulation, Machinery Industry Press