Detectors, Particle

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DETECTORS, PARTICLE

When a particle, such as a proton or an electron, goes through a gas, liquid, or solid, it can interact in various ways and leave evidence that it passed through or stopped. The interactions that take place can be recorded by building a particle detector that is designed to observe a specific process. The most common method involves using ionizations caused by a charged particle going through a material.

Of the many types of particle detectors, seven common detectors will be described. Some are in large-scale use. Some, such as the cloud chamber, were very important in the earlier history of the study of particle physics. An important distinction between them is whether they can be triggered to record an event by the particles passing through them or can only take a snapshot of what is in the detector at a specific time.

Cloud Chamber

The cloud chamber, invented by Charles T. R. Wilson in 1912, played a vital role in the early development of particle physics. This detector produces visible trajectories (tracks) when charged particles traverse the apparatus. It was used to discover the positron in 1933 and was used extensively in the early studies of muons, which are particles produced by cosmic rays interacting high in the Earth's atmosphere. A cloud chamber works by suddenly decompressing a vapor-filled container, cooling the vapor rapidly so that it becomes supersaturated. When a gas is supersaturated, any areas of nonuniformity can form regions of condensation that can be observed or photographed. These areas are referred to as nucleation centers.

Ionized gas molecules along the path of a charged particle will be the nucleation centers on which droplets will form, giving rise to the visible track seen in the cloud chamber. Since the ionization is velocity-dependent, a measurement of the amount of ionization can determine the velocity of the particle, allowing one to calculate its mass if the momentum of the particle is also known. The momenta of the particles can be measured by placing the chamber in a magnetic field and measuring the curvature of the photographed tracks. It is important that the vapor in the cloud chamber be dust-free, or the vapor will condense around the dust particles rather than the ionized gas regions.

A variation on the cloud chamber is known as the diffusion cloud chamber, which was originally suggested by Alexander Langsdorf Jr. in 1939. This type of detector remains sensitive continuously by establishing a temperature gradient so that vapor will diffuse from the heated top portion of the chamber to a cooled bottom portion.

The cloud chamber was replaced by the bubble chamber in particle physics because of the bubble chamber's higher density liquid media and its higher repetition rates. Although the diffusion chamber has reasonable cycle times, they have only a relatively thin sensitive area, and the interaction rate is significantly lower in a gas as compared to a liquid. The one advantage of the cloud chamber over the bubble chamber is the fact that the cloud chamber could be triggered by the particle that traverses the detector because of its long sensitivity time. Thus, while the cloud chamber is rarely used today, it was an important device in the early development of the field of particle physics and is still one of the simplest ways to visually verify the presence of cosmic rays.

Bubble Chamber

The bubble chamber played an important role in the development and understanding of particle physics in what is sometimes termed the "golden age" of the field, from the 1950s to the 1970s. During this time large accelerators were developed to produce beams of protons and electrons at ever-increasing energies and intensities. The invention of the bubble chamber is attributed to Donald A. Glaser, who in 1950 began work on a detector that would record rare events, referred to as pothooks, V particles, and strange particles, better than a cloud chamber. His concept was based on the behavior of a liquid heated past its boiling point. Glaser recognized that the passage of a charged particle through a superheated liquid would produce a trail of bubbles that could be photographed.

Glaser's first success was with diethyl ether in glass containers holding a few cubic centimeters of liquid. The expansion and recompression was done manually using a crank-and-piston. After the expansion cycle, bubbles formed in the superheated liquid along the path of ionization left by a charged particle. Following Glaser's initial work in the years of 1950 to 1952, other liquids, such as liquid hydrogen, were used.

After expansion, the bubbles expand to 0.1 millimeter in about 1 millisecond. They then can be photographed. By placing the chamber in a magnetic field, the momenta of the charged particles can be found by measuring the curvature of the particle trajectories. The particle interactions can be easily visualized and thus verify the existence of a new particle. The discovery of the omega particle was an example of a single photograph producing one of the most famous bubble chamber results.

The beauty of the hydrogen bubble chamber is that the liquid serves as both the interaction target and the detector. Thus, one can record a particle interacting with a proton as long as charged particles are eventually produced somewhere in the interaction. Rare interactions require searching through thousands of photographs. This required automatic scanners, which were developed from 1960 to 1980. Since the bubbles must be produced within nanoseconds of decompressing the liquid to make it superheated, a bubble chamber cannot be triggered by the interacting particle itself, which is a major disadvantage.

Drift Chamber

A drift chamber (DC) has many similarities to the Multiwire Proportional Counter (MWPC) that was invented by Georges Charpack in the 1960s. Both consist of thin wires strung through a gas volume and will produce an electrical pulse indicating that a particle passed through the detector, ionizing gas molecules. The primary difference is that the time the particles are detected is recorded in a DC, allowing a more precise determination of the location of the particle track.

The electrons produced will be attracted to positively charged anode wires, and the positive ions to the cathodes. The cathodes may be wires, conducting planes, or most commonly, a combination of both. The ionization electrons are used to produce the detector signal because they drift much faster in a gas than the positive ions. A DC must have an arrangement of electric conductors to produce a uniform electric field. The uniformity is important so that the electrons will have a constant drift time. The anode wires have a very small diameter, typically 10 to 20 μm, so that the electric field within 50 to 150 μm of the wire is very large, accelerating the electrons. This produces large secondary ionization near the anode wire, referred to as an avalanche.

The avalanche electrons are collected in less than a nanosecond and produce a very small electrical pulse. The main signal seen in a DC is an induced electrical pulse, via the Faraday effect, from the drifting of the avalanche ions toward the cathodes. The electrical pulse produced is amplified and sent to a discriminator. This simple electric circuit produces a digital output pulse only when the analog electronic pulse exceeds a set voltage.

The discriminator sends the digital pulse to a Time-to-Digital Converter (TDC). The TDC measures when the pulse arrives relative to some time signal generated by fast response detectors, such as scintillators. This provides a fast and accurate measurement of the drift time, giving how far from the anode wire the particle passed. Combining signals from several anode wires, it is possible to determine which side of the wire that the particle passed. With the ability to measure accurately times of less than a nanosecond, accuracies of less than 0.15 mm are routine for a DC.

Geiger-Mueller Tube

The Geiger-Mueller tube probably is the particle detector most familiar to the public. These small hand-held devices, developed in the 1920s, are commonly used to check for radiation. They are referred to as Geiger counters and use a Geiger-Mueller (G-M) tube to detect particles produced by radioactive materials. This gaseous detector depends on a particle passing through a gas and causing ionization. In order to detect the electrons produced, the G-M tube has a central wire, referred to as the anode, that is kept at a high positive voltage and is surrounded by a conductive cylinder kept at ground (zero voltage). Since opposite charges attract, the electrons produced will move toward the anode wire.

The anode wire is very small, which produces a high electric field that will accelerate the electrons where they can produce ionization. This process builds on itself to produce a large number of electrons and positive ions near the wire and is referred to as an avalanche. A G-M tube is designed for this avalanche to be large. The discharge will usually involve the entire length of the wire, limiting how fast a G-M tube can respond. As many as 10¹⁰ electrons, and the corresponding number of ions, can be produced. The electrical pulse produced is amplified and is used to count the number of ionizing particles passing through the Geiger counter.

Scintillation Counter

In addition to ionization processes that take place when a particle passes through a material, a particle can produce ultraviolet and visible light that is easily detected. One such process is referred to as luminescence or scintillation. The first scintillator was built by Sir William Crookes in 1903. It was made out of a ZnS screen, and a microscope was used to view it. The most common materials used for scintillation counters are clear plastics, organic liquids, and inorganic crystals. Plastics that normally do not scintillate can be made to do so by adding materials such as anthracene or tolulene. Examples of inorganic crystals are sodium iodide doped with thallium [NaI(Tl)], and pure crystals such as CsI. Each of these has different response times, light output, and other properties (and cost!) that help the experimenter decide which material should be used.

Once the light is emitted, it must be detected. This is most commonly done by photomultiplier tubes (PMTs). These devices have a photosensitive surface that emits an electron when hit by light. This electron will be collected by the next stage of the PMT. Each PMT stage has an electrode with an applied voltage that will attract the electrons from the previous stage. Each electron absorbed will cause multiple electrons to be emitted. This can result in amplifications as high as 10¹², depending on the voltage and number of stages inside the PMT (usually referred to as anodes or dynodes, depending on the PMT type).

Since the 1990s, more choices have emerged as alternatives to the PMT. These include such devices as PIN diodes, avalanche diodes, and Visible Light Photon Counters (VLPCs). Each of these has different response times, amplification, timing accuracy, maximum rate, and cost. Scintillators see their greatest use in generating fast signals, or triggers, to indicate that a charged particle has passed through the detector and that the other systems should be recorded.

Materials that are excited so they emit light exhibit a wide range of response times. Plastic scintillators typically can respond in a few nanoseconds, and the resulting emission of light goes away in nanoseconds. Materials such as NaI(Tl) respond on the order of 100 nanoseconds and have secondary emission modes that cause the pulse to last on the order of a microsecond.

Silicon Vertex Detector

At high energies, interacting particles can produce large numbers of charged particles originating from the interaction point, that is, the vertex. Near the vertex, the particles are packed tightly together and require a high degree of precision to resolve them into their individual tracks. The silicon vertex detector is able to do this and is based on the ability to produce very narrow strips of silicon used in common devices such as computer processors. The silicon vertex detector is also useful when the experiment being done requires a very precise position measurement of a particle's passage. Silicon detectors can be used inside a magnetic field, where many other types of detectors will not work because the ions and electrons are deflected or curled up into a small spiral path, leaving them undetected.

Semiconductors are made by taking silicon and adding very small numbers of impurities that either have an extra electron in their outer shell, producing an n-type material, or have only three electrons in their outer shell, resulting in a hole or a p-type material. When a charged particle passes through the semiconductor, the charges produced can be recorded. In addition to strips of silicon, it is possible to make pads that can be read out like a CCD camera. This has the advantage of being able to record at very high intensities but the disadvantage of having much more data and electronic readouts.

Spark Chamber

The spark chamber, like most of the other detectors discussed, makes use of the trail of ionized particles left behind by the passage of a charged particle through a gas. A charged particle typically will produce thirty to fifty ions per centimeter as it goes through a gas. The specific number depends on the energy of the incoming particle and on the gas used. With a path of ions produced in its wake, the passage of the charged particle is seen by suddenly applying a high voltage between metal electrodes and observing the resulting spark that follows the path of ions. Since the spark can be seen, the event easily can be recorded with a camera.

Usually, a spark chamber will have a stack of metal plates insulated from each other, and be placed inside a sealed container filled with a gas. An inert gas such as neon or argon is used to maximize the lifetime of the detector. As in the case of a cloud chamber, it is possible to trigger a spark chamber by a charged particle that goes through it, as opposed to the bubble chamber that cannot be triggered by the particles entering it. In the case of the spark chamber, the ions produced by the passing particle will remain along the path of the particle long enough for a high voltage to be applied. With the use of modern electronics, the trajectories of particles can be photographed, digitized, and analyzed on a computer rather than using a film camera. This allows many more events to be studied than visually inspecting every photograph.