Spark Chamber - Eyes for Microparticles

Foreword

Throughout history, man has never been lack of curiosity about the unknown. The ancients relied on a pair of eyes to observe the sky. With the development of science and technology, people gradually learned to use tools instead of "eyes" to watch everything, such as telescopes and cameras. As objects became smaller and smaller, more sophisticated instruments were invented, such as microscopes. The above "eyes" can not do the task when we need to observe the subatomic particles. Thus scientists create special "eyes" to observe these particles, which are particle detectors. The spark chamber is just one of many "eyes" for subatomic particles.

1. Introduction of spark chamber

1.1 What is Spark Chamber ？

A spark chamber is a device for detecting charged particles. It was one of the detectors used by early elementary particle physicists to explore the subatomic particles that make up the universe and one of the few detectors that can show the visible motion of cosmic rays. Therefore, spark chambers are widely used in particle physics experiments.
The main purpose of a spark chamber is to observe the motion trajectory of high-energy charged cosmic rays. It enables people to have a visual and intuitive sense of the microscopic particles that cannot be directly observed by our eyes and helps people to have a preliminary understanding of particle physics.
Figure 1 is the spark chamber of Tsinghua University：

Fig1: Spark Chamber

1.2 Brief history of Spark Chamber

Spark-chamber detector has a long history and is one of the particle detectors used in the early scientific community. It can be traced back to the late 1950s and was widely used until the early 1960s. Many scientists have made important contributions to the development of spark chambers. The following is a brief list of important historical nodes in the development of the spark chamber. Some references can be found in the appendix:

1. In 1928, Hans Geiger and Walter Mueller demonstrated the first particle detector at Kiel University.
2. In 1955, Henning made the first spark chamber.
3. In 1959, The Japanese Fukui and Miyamoto made a major contribution to the improvement of the spark chamber. The spark chamber was the first widely used trajectory visualization device to allow triggering. This means that they can be used in conjunction with a separate logic circuit to trigger the detector's activation when certain conditions are met. In most studies, only a small fraction of particle interactions are of interest, so it is necessary to have a detector that can be triggered to select these.
4. In 1962, large spark Chambers were used to detect muon neutrinos.
5. In 1988, Leon Lederman, Melvin Schwartz, and Jack Steinberger received the Nobel Physics Prize for discovering the muon-neutrino in 1962. Figure 2 is Melvin Schwartz and the spark chamber.

Fig 2：Melvin Schwartz standing next to spark chamber

2. How Spark Chamber Works ？

Before the detailed introduction of Tsinghua's spark chamber, it is necessary to have a brief understanding of the overall working process and principle of the spark chamber:

2.1 Working Process

A spark chamber consists of a pile of conductive plates; the spacing between plates is about 1 cm, and all are immersed in the inert gas. Suppose a charged particle travels through the chamber with a few kV high-voltage applied, the spark chamber can produce bright sparks, showing traces of the charged particle and making certain sounds, like a smaller version of the lightning phenomena in nature, as shown in figure 3.

Fig 3: Sparks of spark chamber

2.2 Operating Principle

The working of a spark chamber lies in how to generate sparks. The method was first proposed by Gremacher in 1935. As shown in figure 4.

1. High voltage is loaded between two plates. When charged particles pass through the two plates, the gas between plates is ionized, generating electron-ion pairs.
2. In the electric field, electrons and ions accelerate in opposite directions parallel to the electric field. When the electron ions accelerate to a certain energy, they ionize the gas and produce more electron-ion pairs. This process is called an avalanche process.
3. When the electron ions accumulate to a certain amount, an inverse electric field is generated and superimposed on the original electric field, which weakens the intensity of the intermediate electric field. Some electron-ion pairs combine and emit high-energy photons.
4. The high-energy photons travel to other regions, ionizing the gas and then passing through an avalanche to form more electron-ion bands.
5. Finally, the areas of the electron-ion bands overlap, forming a band of electrons with minimal resistance that connects the positive and negative plates. A discharge occurs between the electrodes. The deexcitations after electron-ion recombinations produce visible photons, which usually come with an electrical pulse. This information can be used to record the particles.

Fig 4：Operating principle

3. Spark Chamber of Tsinghua University

3.1 How it works ?

The working process of the Tsinghua spark chamber is shown in Figure 5.
Charged particles passing through the spark chamber will first pass the scintillator at the top of the spark chamber and produce a signal. The particles then go through the spark chamber shell in nanoseconds and ionize the internal gas. Finally, the particles pass through the scintillator at the bottom of the spark chamber. The output signals of the upper and lower scintillator detector are a double coincidence, generating a coincidence signal indicating that a charged particle incident has been detected. The coincidence signal is the input of a high voltage switch which triggers the high voltage to the electrodes. So the sparking process starts, generating deexcitation light, showing the charged particle movement track.

Fig 5：Working scheme of Tsinghua spark chamber

3.2 Mechanical design of Tsinghua Spark Chamber

The whole spark chamber of Tsinghua University adopts modular design. Each spark chamber module is mainly composed of three parts. The module structure of the spark room is shown in Figure 6 below:

Fig 6：Structure of the spark chamber module

1. Shell：The shell of the spark cahmber is made by acrylic which is a kind of transparent insulation material , it facilitates the observation of sparks.
2. Electrode system: The electrode plates are parallel stacked at regular intervals (~1.5 cm). The plates are embedded into the spark chamber through bonding, and the chamber is bonded and polished to a certain degree. One adjacent plate is connected to the high-voltage pulse (~2-10 kV), and the other is grounded (0V) to form the electrode system of the spark chamber.
   Three electrodes are encapsulated in a chamber so that the number of electrode layers in the spark chamber can be changed by simply stacking the chambers to adjust the whole spark chamber size.
   
3. Ventilation system: To ensure noble gas fills in the spark chamber and meets the experimental requirement, a gas route is specially designed for the spark chamber.
   For the sealed spark chamber, there are two layers of electrode space. Neon gas enters the upper space through a plastic hose (6mm in diameter) on one side of the chamber, flows through a hose on the other side to the lower space, and then out through an outlet in the lower space. This design ensures maximum exhaling other gases from the chamber while keeping nitrogen circulating inside the shell. The specific ventilation situation is shown in Figure. 7.





Fig 7：Gas route of spark chamber

3.3 Electronic System of Spark Chamber

The power supply system of the spark chamber needs to meet certain requirements to make the spark chamber work normally. Therefore, a more detailed introduction is made to the electronics part of the spark chamber. The figure 8 below is the overall power supply circuit diagram of the spark chamber:

Fig 8: Power supply circuit diagram of the spark chamber

3.3.1 Trigger and High Voltage Circuit

1. Since the gas ionized by charged particles will recombine in the absence of an electric field, the high voltage needs to be loaded before the recombination of ionized gas, and the delay time between the particle entering and the high voltage pulse loading should be small enough (less than 500ns). If the high voltage is loaded too slow, electrons and ions will recombine and prevent the avalanche effect from occurring, then no visible light.
2. To enable the avalanche effect to occur, the voltage applied to the plate should be within an appropriate range (2 kV-10 kV), which will directly affect the visual effect of spark discharge. If the voltage is too low, the luminescent phenomenon will not be observed; if the voltage is too high, the gas in other positions in or out of the container may break down, and spark discharges will occur.
3. As the process of spark discharge takes a certain time, the high-voltage pulse needs to last for a period of about 100 to 500 nanoseconds, which makes the whole process of spark discharge complete. The spark discharge cannot occur if the duration is too small, and the spark discharge is likely to occur in all parts of the spark chamber if the duration is too long. However, it does not correspond to the flight time of initial charged particles.
4. High voltage pulse rise time should be small enough, according to our experience, around 10ns is good enough, long pulse rise time will make the spark discharge effect worse.

Figure 9 below is the equivalent power supply circuit diagram of the spark chamber. The above requirements can be met through this circuit design and the use of certain electronic components.

Fig 9：Equivalent power supply circuit diagram

The whole high-voltage trigger circuit logic of the spark chamber is as follows: When particles pass through the top and bottom scintillator detector of the spark chamber, electronic signals are generated through the discriminator. These two electronic models are input into the coincidence module, and the high-voltage switch of electronics is turned on, and the high voltage is output to the metal plates so that the spark discharge process occurs. Because of the small size of the spark chamber, the time for particles to pass through the spark chamber is less than the width of the output waveform of the discriminator, so the upper and lower discriminator signals can be directly matched. But for a large spark chamber, it is necessary to delay the signal of the upper discriminator to ensure the coincidence. The trigger logic also shows that the spark chamber can only detect charged particles passing through the upper and lower scintillators simultaneously, as shown in figure 10.

Fig 10：Schematic diagram of spark room power supply time

3.3.2 Electronic Components

Due to the special working requirements of the spark chamber circuit, there are certain requirements for the high-voltage power supply circuit and electronic components of each part of the spark chamber:

1. High voltage power supply: In order to meet the demand of spark discharge, the high voltage power supply needs to be able to provide up to 10 kV voltage. In addition, as there are frequent electrical discharges in the circuit, the high-voltage source needs to be able to provide stable power when the load voltage changes frequently, that is, to handle with capacitive load. When the circuit has frequent pulses, the power supply should not shut down due to self-protection. The spark chamber of Tsinghua University adopts a high-voltage power supply, as shown in Figure 11 below.



Fig 11：High voltage power

2. Capacitance and resistance: In order to protect the high-voltage power supply, a 10-megohm voltage protection resistor should be connected in series with the high-voltage source, and 1.6 nF capacitors should be connected at the end of each set of plates, and a 10-kilo-ohm resistor should be connected in parallel. In this way, when the high voltage switch is off, the voltage at both ends of the 1.6-nF capacitor is the same as the supply voltage, and about 1/100,000 coulomb charge is accumulated on the plate; when charged particles going through the chamber, the high voltage switch is closed, in a few nanoseconds the charge on 1.6 nF capacitance discharges quickly, the pulse is passed on to the chamber plates with equivalent capacitance for pF scale, rapid accumulation of about one over one hundred million of the coulomb of charge, make the plate voltage rise rapidly to a certain height, produce the spark discharge.

3. Since the time interval between high voltage loading and particle penetration is required to be small enough (100-500ns), the high voltage switch needs to respond quickly. Tsinghua university chamber adopted the fast high voltage switch HTS 101-03 made by BEHLKE as shown in figure 11。



Fig 11：Fast high voltage switch

3.3.3Time Characteristic Check

An oscilloscope and a special high voltage probe (As shown in FIG. 12) are used to measure the high voltage trigger line. The measurement results of circuit time characteristics are shown in Figure. 13 below.The high voltage probe is used to measure the circuit time characteristics when adding 8 kV high voltage. In the figure, yellow signals represent the generation of coincident signal, that is, charged particles passing through the spark chamber, and green signals represent high voltage changes at the two metal plates. It can be seen from the measurement results that the total time delay of the spark chamber in Tsinghua University is within 200 ns, and the pulse rise time is within 30 ns, which meets the requirements of the circuit characteristics of the spark chamber.

Fig 12：High voltage probe

Fig 13：Measured Time Characteristic

4. Tsinghua University Spark Chamber Videos

The videos linked below illustrate the operation of the spark chamber at Tsinghua University.

5. Application and Promotion of Spark Chamber

Reference

1. Brief hstory of spark chamber
2. Birmingham spark chamber
3. Harvard spark chamber
4. Cambridge spark chamber
5. Princeton spark chamber
6. 刘皇风. 火花室[J]. 原子能科学技术,1964,(01):49-62.

Contact for the equipment and web: Zhe Wang (wangzhe-hep@mail.tsinghua.edu.cn)