IONIZATION DETECTORS “High Energy Physics Phenomenology” class of “High Energy Physics Phenomenology” Mikhail Yurov Kyungpook National University October 10th

Introduction Ionization detectors were first electrical devices developed for radiation detection. During the first half of the century, three basic types of detector were developed: ionization chamber, the proportional counter and Geiger-Muller counter. During the late 1960’s, a renewed interest in gas ionization instrument was stimulated in the particle physics domain by invention of the multi-wire proportional chamber. These devices were capable of localizing particle trajectory to less than a millimeter and were quickly adopted in high-energy experiments. Stimulated by this success, the following years saw the development of the drift chamber and, somewhat later, the time projection chamber. They are now used extensively in high energy particle physics experiments and require more sophisticated electronics as well as data acquisition by computer.

GM tube of unknown manufacture Early end-window GM tubes GM tube of unknown manufacture Rossi Tissue Equivalent Proportional Counter (1960)

Gaseous Detectors When a charged particle passes through a gas, free electrons and positive ions are produced along its track. If no electric field is applied, ion pairs will recombine, and no signal is produced. By applying different strength fields, different types of detector can be realized. If a small field is applied, the electrons drift towards the electrodes.  The signal collected is just the original ionization produced.  This regime is exploited in an ionization chamber. When a larger field is present, the electrons are soon accelerated until they become ionizing themselves.  In this way amplification is produced due to secondary ionization.  At low gain, the resultant signal remains proportional to the original ionization, and this is known as operation in "proportional mode".

As the field is increased, so does the gain, and eventually the amplification becomes non-linear.  This regime is known as the "semi-proportional mode". As the field increases still further, recombination of ion pairs in the centre of secondary showers leads to the production of photons which can ionize other gas molecules anywhere in the detector volume. This is the Geiger-Muller effect, and produces very large saturated signals. If the field is increased any further, then the gas is likely to break down even in the absence of primary ionization.

Proportional Counters The simplest propor-tional counter is generally used for detecting low energy X-rays on the order of a few KeV and very low energy electrons. The basic feature of the proportional counter is the proportional gas multiplication which occurs. In an increased electric field, the electrons are accelerated and produce secondary ionization in an avalanche process. A gain, or gas multiplication, of 103 to 105 is then possible with a produced signal proportional to the original ionization.

The simplest way of producing a local region of high electrical field is to utilize a cylindrical geometry with a very fine central “sense" wire acting as anode. The maximum of the avalanche occurs very close to the wire. The electrons therefore have a very short drift distance before being collected at the anode, and so contribute little to the induced signal. Choice of Filling Gas The choice of a filling gas for proportional counters is governed by several factors: low working voltage, high gain, good proportionality and high rate capability. For a minimum working voltage, noble gases are usually chosen. Because of its higher specific ionization and lower cost, argon is usually preferred.

Multiwire Proportional Chambers (MWPCs) One of the basic requirements of particle physics is the determination of particle trajectories. Up until about 1970, all tracking devices were optical in nature and required the recording of the track information on film. An all-electronic device, therefore, was greatly desired as it would allow more events to be treated more accurately. The breakthrough occurred in 1968 with the invention of the multiwire proportional chambers by Charpak. Charpak showed, in effect, that an array of many closely spaced anode wires in the same chamber could each act as independent proportional chambers. The basic MWPC consists of a plane of equally spaced anode wires centered between two cathode planes. Typical wire spacing are 2 mm with an anode-cathode gap width of 7 or 8 mm.

Except for region very close to the anode wires, the field lines are essentially parallel and almost constant. If electrons and ions are now liberated in the constant field region they will drift along the field lines towards the nearest anode wire and opposing cathode.

Upon reaching the high field region, the electrons will be quickly accelerated to produced an avalanche. The neighboring wires are also affected; however, the signals induced here are of small amplitude. The signal from one anode plane only gives information on one coordinate of the ionizing event.

The second coordinate may be obtained by using a second detector whose anode wires are oriented perpendicularly to the first. Usually both detectors are integrated into the same chamber to form X-Y MWPC. To measure the trajectory of a particle, two or more aligned MWPCs may now be used to form a telescope. Reading the position of the signaling wires then allows a reconstruction of the track. The spatial resolution of a MWPC depends on the anode wire spacing and is typically one-half this value. In a MWPC with typical 2 mm wire spacing, therefore, the spatial resolution is ≈ ±1 mm. The information from MWPC may be extracted in a number of different ways. The standard method is to consider each wire in the chamber as a separate detector connected to its own electronics. The signal is first amplified, then discriminated and shaped to a standard logic level.

In addition to the separate wire readout method, a number of analog methods have been developed for obtaining one and two-dimensional information from one plane of anode wires only. These methods make use of the fact that the avalanche at the anode is also highly localized along the length of the wire. If the cathodes are arranged as a series of strips with one plane oriented parallel to the anode wires and other orthogonally, then we can see next situation.

The induced signals are largest on the strip closest to the avalanche and diminish proportionately with distance from avalanche point. So, we can estimate Y and X coordinates of the avalanche point by measuring charge on that strip. MPWC Efficiency The intrinsic efficiency of the MWPC depends on the number of electron pairs produced and collected in the chamber. As such it is dependent on the dE/dx of the fill gas, the width of the gap, the pressure of the gas, amount of electronegative gases, the high voltage applied, the threshold set on the electronics, the gate width on the readout, etc. The photograph shows one of the multiwire proportional chambers, PC12. The beam comes from the right side of this photograph.

Drift Chambers Early in the development of the MWPC, it was realized that spatial information could also be obtained by measuring the drift time of the electrons coming from ionizing event. In practice it is highly desirable to have a constant drift velocity and hence a constant electric field, so as to have a linear relationship between time and distance. Drift chambers exploiting this aspect of electron transport were built shortly after the MWPC and have been used extensively in particle physics ever since.

In order to cover large areas with MWPCs, very many wires, and therefore many channels of amplification and readout, are required.  With drift chambers, the wires are much more widely spaced. The drift cell is defined at one end by a high voltage electrode and at the other end by the anode of a simple proportional counter. To signal the arrival of a particle, a scintillation counter covering the entire sensitive area is placed before or after the chamber. A particle traversing the chamber and scintillator, now, liberates electrons in the gas which then begin drifting towards the anode. At the same time, the signal from the scintillator starts a timer. The signal created at the anode as the drifting electrons arrive then stops the timer to yield the drift time. If the trigger is available to signal the arrival of a particle and the drift velocity is known, then the distance from sensing wire to the origin of the electrons is x=∫udt

To cover a wider surface area, many adjacent drift cells can be used To cover a wider surface area, many adjacent drift cells can be used. Drift chambers several meters long have been constructed in this manner. To obtain several points on a track, several drift chambers with different wire orientations may also be stacked together. In principle, the chamber structure used for a MWPC may also be employed for drift chambers as well. The advantage of drift chambers is the small amount of wires and electronics required and the large surface area which can be covered. They are generally easier to operate, however, much more attention must be given to the fill gas and the field uniformity if good resolution is desired.

Drift Gases Spatial Resolution The purity of the gas is very important. In particular, if electronegative gases are present, electrons will be captured as they drift to the anode. The allowable level for these impurities depends on the length of the drift path: the longer this path, the higher the required purity level. To maximize operational stability, a gas exhibiting drift velocity saturation at not too high electric fields should be chosen. The magnitude of the drift velocity must also be considered. If the chamber is to operate to high count rate, then drift velocity must be high so as to minimize dead time. If, instead, high spatial resolution is desired, a slower drift velocity is required to minimize timing errors. Spatial Resolution The spatial resolution of a drift chamber depends on how well the relation between drift time and space coordinate is known and the amount of diffusion suffered by the electrons as they drift. To obtain higher spatial resolution a smaller drift length is necessary. In more recent years, much efforts has been put into designing very high resolution chambers with spatial resolution of 50µm or less fro the next generation of High-energy experiments. These efforts have included searches for new low-diffusion, low drift velocity gases and new chamber design and concepts.

Time Projection Chamber (TPC) The most sophisticated of the current ionization detectors is the time projection chamber or TCP. This device is essentially a three-dimensional tracking detector capable of providing information on many points of a particle track along with information on the specific energy loss, dE/dx, of the particle. The TPC makes use of ideas from both the MWPC and drift chamber. The detector is a essentially a large gas-filled cylinder with thin high voltage electrode at the center. When voltage is applied, a uniform electric field directed along the axis is created. A parallel magnetic field is also applied. The ends of the cylinder are covered by sector arrays of proportional anode wires. Parallel to each wire is a cathode strip cut up into rectangular segments. These segments are also know as cathode pads.

At a collider machine, the detector is positioned so that its center is at the interaction point. Particle emanating from this point pass through the cylinder volume producing free electrons which drift towards the endcaps where they are detected by the anode wires as in a MWPC.

One coordinate is given by the position of the firing anode wire while the second is obtained from the signal induced on the row of cathode pads along the anode wire. The third coordinate, along the cylinder axis, is now given by the drift time of the ionization electrons. Because of the relatively long drift distance, diffusion, particularly in the lateral direction, becomes a problem. This is remedied by parallel magnetic field which confines the electrons to helical trajectories about drift direction. Because of the very large amount of data produced for each event, an important consideration is the readout and data acquisition system for a TPC.

A Time Projection Chamber (TPC) has been constructed as a 4π detector for hadron photoproduction experiment. The TPC has the active volume of 700 mm in length and 350 mm in diameter. It will be installed in a superconducting solenoid magnet. The signals are read out with 1055 cathode pads.