TechniquesNukeAndParticleExperiments

Techinques for Nuclear and Particle Physics Experiments (W. R. Leo)

Very good! Should have read earlier in my career!
The art of experimental physics is learned through a type of "apprenticeship" with a more experienced physicist or physicists, not unlike medieval artisans. It is to these "apprentices" that I address these chapters.
All alpha-sources are generally limited to the range ~4 - 6 MeV with the higher energy sources having the higher transmission probability and thus the shorter half-life. For this reason also, most alpha-decays are directly to the ground state of the daughter nucleus since this involves the highest energy change.
Because of its double charge, +2e, alpha particles have a very high rate of energy loss in matter. The range of a 5 MeV alpha-particle in air is only a few centimeters, for example. For this reason it is necessary to make a sources as thin as possible in order to minimize energy loss and particle absorption. Most alpha-sources are made, in fact, by depositing the isotope on the surface of a suitable backing material and protecting it with an extremely thin layer of metal foil.
For most beta sources, this maximum energy ranges from a few tens of keV to a few MeV. Pure , beta-emitters exist but the list is astonishingly short
Since electrons lose their energy relatively easily in matter, it is important that, beta-sources be thin in order to allow the, beta's to escape with a minimum of energy loss and absorption. This is particularly important for positron sources since the positron can annihilate with the electrons in the source material or surrounding container.
Like the electron shell structure of the atom, the nucleus is also characterized by discrete energy levels. The energies of these photons, from a few hundred keV to a few MeV, characterize the high binding energy of nuclei. These high-energy photons like atoms, show spectral lines characteristic of the emitting nucleus.
Most gamma-sources are "placed" in their excited states as the result of a beta-disintegration, although excited nuclear states are often created in nuclear reactions also. Since electrons and positrons are more easily absorbed in matter, the beta-particles in such sources can be "filtered" out by enveloping them with sufficient absorbing material, leaving only the more penetrating gamma-ray.
The gamma spectrum from a thick positron source will thus show a peak at 511 keV (corresponding to the detection of one of the annihilation photons) in addition to the peaks characteristic of transitions in the daughter nucleus.
Unlike beta-decay, therefore, internal conversion electrons are monoenergetic having approximately the same energy as the competing gamma's, i.e., a few hundred keY to a few MeV. Internal conversion sources are one of the few nuclear sources of monoenergetic electrons and are thus very useful for calibration purposes.
A unit very often used for expressing thicknesses of absorbers is the surface density or mass thickness. This is given by the mass density of the material times its thickness in normal units of length, For discussing the interaction of radiation in matter, mass thickness units are more convenient than normal length units because they are more closely related to the density of interaction centers. They thus have the effect of normalizing materials of differing mass densities. Equal mass thicknesses of different materials will have roughly the same effect on the same radiation.
Of the two electromagnetic processes, (inelastic collisions with the atomic electrons of the material and elastic scattering from nuclei) the inelastic collisions are almost solely responsible for the energy loss of heavy particles in matter. In these collisions (sigma ~ 10e-17 10e-16 cm2 !), energy is transferred from the particle to the atom causing an ionization or excitation of the latter. The amount transferred in each collision is generally a very small fraction of the particle's total kinetic energy; however, in normally dense matter, the number of collisions per unit path length is so large, that a substantial cumulative energy loss is observed even in relatively thin layers of material. A 10 MeV proton, for example, already loses all of its energy in only 0.25 mm of copper!
The inelastic collisions are, of course, statistical in nature, occurring with a certain quantum mechanical probability. However, because their number per macroscopic pathlength is generally large, the fluctuations in the total energy loss are small and one can meaningfully work with the average energy loss per unit path length. This quantity, often called the stopping power or simply dE/dx
that the minimum value of dE/dx is almost the same for all particles of the same charge ~MeV - cm²/g
dE/depsilon, therefore, is almost independent of material type. A 10 MeV proton, for example, will lose about the same amount of energy in 1 g/cm2 of copper as it will in 1 g/cm2 of aluminium or iron, etc.
The behavior of photons in matter (in our case, x-rays and y-rays) is dramatically different from that of charged particles. In particular, the photon's lack of an electric charge makes impossible the many inelastic collisions with atomic electrons so characteristic of charged particles.

(1) x-rays and y-rays are many times more penetrating in matter than charged particles, and (2) a beam of photons is not degraded in energy as it passes through a thickness of matter, only attenuated in intensity. The first feature is, of course, due to the much smaller cross section of the three processes (PE, compton, pair-produciton) relative to the inelastic electron collision cross section. The second characteristic, however, is due to the fact the three processes above remove the photon from the beam entirely, either by absorption or scattering. The photons which pass straight through, therefore, are those which have not suffered any interactions at all. They therefore retain their original energy. The total number of photons is, however, reduced by the number which have interacted.
Photoelectric effect: For photon energies above the K-shell, however, it is almost always the K electrons which are involved.
It is interesting to note the dependence of the cross section on the atomic number Z. This varies somewhat depending on the energy of the photon, however, at MeV energies, this dependence goes as Z to the 4th or 5th power. Clearly, then, the higher Z materials are the most favored for photoelectric absorption, and, as will be seen in later chapters, are an important consideration when choosing y-ray detectors.
Rayleigh scattering, on the other hand, is the scattering of photons by atoms as a whole. In this process, all the electrons in the atom participate in a coherent manner. For this reason it is also called coherent scattering.
Like the photon, the neutron lacks an electric charge, so that it is not subject to Coulomb interactions with the electrons and nuclei in matter. Instead, its principal means of interaction is through the strong force with nuclei. These reactions are, of course, much rarer in comparison because of the short range of this force. Neutrons must come within ~1e-15 m of the nucleus before anything can happen, and since normal matter is mainly empty space, it is not surprising that the neutron is observed to be a very penetrating particle.
Radiative neutron capture, i.e., n+(Z,A) -> y+(Z, A+ 1). In general, the cross-section for neutron capture goes approximately as ~1/v where v is the velocity of the neutron. Absorption is most likely, therefore, at low energies.
Because of the strong energy dependence of neutron interactions, it has become customary to classify neutrons according to their energy. In general, high energy neutrons are considered to be those with energies above ~100 MeV or so, whereas those between a few ten's of MeV and a few hundred keV are known as fast neutrons. Between ~100 keV and ~0.1 eV, where nuclear resonance reactions occur, neutrons are referred to as epithermal. At lower energies comparable to the thermal agitation energy at room temperature, (i.e., E ~ kT ~ 1/40 eV), neutrons are known as thermal or slow.
The slowing down of fast neutrons is known as moderation and is an important process in nuclear physics and engineering. Elastic scattering is the principal mechanism of energy loss for fast neutrons. At energies of several MeV, the problem may be treated nonrelativistically and very simply with conservation laws. The lighter the nucleus the more recoil energy it absorbs from the neutron. This implies of course that the slowing down of neutrons is most efficient when protons or light nuclei are used. This explains the use of hydrogenous materials such as water or paraffin (CH2) in connection with neutron moderators and shielding.
An absorbed dose of alpha-particles produces more damage than an equal dose of protons and this, more damage than a similar dose of electrons or y-rays.
While the history of nuclear and elementary particle physics has seen the development of many different types of detectors, all are based on the same fundamental principle: the transfer of part or all of the radiation energy to the detector mass where it is converted into some other form more accessible to human perception. Charged particles transfer their energy to matter through direct collisions with the atomic electrons, thus inducing excitation or ionization of the atoms. Neutral radiation, on the other hand, must first undergo some sort of reaction in the detector producing charged particles, which in turn ionize and excite the detector atoms.
Assuming that the shape of the pulse does not change from one event to the other, however, this integral is directly proportional to the amplitude or pulse height of the signal, so that this characteristic may be used instead. The relation between the radiation energy and the total charge or pulse height of the output signal is referred to as the response of the detector.
organic scintillator: the response is linear for electrons down to a very low energies but is nonlinear for heavier particles such as the proton, deuteron, etc. This is due to the different reaction mechanisms which are triggered in the medium by the different particles.
Ionization detectors were the first electrical devices developed for radiation detection. These instruments are based on the direct collection of the ionization electrons and ions produced in a gas by passing radiation. During the first half of the century, three basic types of detector were developed: the ionization chamber, the proportional counter and the Geiger-Muller counter. Except for specific applications, these particular devices are not in widespread use in modern nuclear and particle physics experiments today. Their basic design and structure, in fact, have changed little since the late 1940's when the then newly-developed scintillation counter began taking the place of these instruments in nuclear research.
During the late 1960's, a renewed interest in gas ionization instruments was stimulated in the particle physics domain by the invention of the multi-wire proportional chamber. These devices were capable of localizing particle trajectories 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.
the ionization chamber, the proportional counter and the Geiger-Muller counter: These detectors are actually the same device working under different operating parameters,
- At some point, of course, all created pairs will be collected and further increases in voltage show no effect. This corresponds to the first flat region in Fig. 6.2.
- if we now increase the voltage beyond region II we find that the current increases again with the voltage. At this point, the electric field is strong enough to accelerate freed electrons to an energy where they are also capable of ionizing gas molecules in the cylinder. The number of electron-ion pairs in the avalanche, however, is directly proportional to the number of primary electrons. What results then is a proportional amplification of the current, with a multiplication factor depending on the working voltage V.
- If the voltage is now increased beyond point III, the total amount of ionization created through multiplication becomes sufficiently large that the space charge created distorts the electric field about the anode. Proportionality thus begins to be lost. This is known as the region of limited proportionality. Increasing V still higher, the energy becomes so large that a discharge occurs in the gas.

For gases, Mean Number of Electron-Ion Pairs Created turns out to be on the order of 1 ion-electron pair per 30 eV of energy lost, that is, for a 3 keY particle, an average of 3000/30 = 100 ion-electron pairs will be created. Moreover, what is surprising is that this average value does not depend very strongly on particle type and only weakly on the type of gas.
the presence of any electronegative gases in the detector will severely diminish the efficiency of electron-ion collection by trapping the electrons before they can reach the electrodes. Some well known electronegative gases are O2 H2O, CO2 , CCl4 and SF6. The noble gases He, Ne, Ar, in contrast, have negative electron affinities.
the pulse signal on the electrodes of ionization devices is formed by induction due to the movement of the ions and electrons as they drift towards the cathode and anode, rather than by the actual collection of the charges itself.

Up until about 1970, all tracking devices were, for the most part, optical in nature. Photographic emulsions, the cloud chamber, the bubble chamber, the spark chamber, etc. all required the recording of track information on film which was then analyzed frame by frame for events of interest. Charpak showed, in effect, that an array of many closely spaced anode wires in the same chamber could each act as independent proportional counters.
The most sophisticated of the current ionization detectors is the time projection chamber or TPC. 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. For this reason, the TPC has been referred to as an electronic bubble chamber. It has played an essential role in physics experiments at high-energy electron-positron colliders and more recently has been proposed for use in many other types of experiments.
Charge-coupled devices (CCDs) are essentially analog shift registers capable of storing time and pulse height information.
Liquid Ionizing Detectors: The principal problem, however, is technical: the presence of electronegative impurities such as oxygen which can attach drifting electrons. This requires purification to very high levels which is not always realizable for a given liquid. This situation has essentially restricted the choice of liquid ionization media to the noble elements, particularly argon and xenon,
A good detector scintillator should satisfy the following requirements:
1. high efficiency for conversion of exciting energy to fluorescent radiation
2. transparency to its fluorescent radiation so as to allow transmission of the light
3. emission in a spectral range consistent with the spectral response of existing photomultipliers
4. a short decay constant, T.
Currently, six types of scintillator materials are in use: organic crystals, organic liq- uids, plastics, inorganic crystals, gases and glasses.
The fact that S* decays to excited vibrational states of So, with emission of radiation energy less than that required for the transition So -+ S * also explains the transparency of the scintillators to their own radiation.
In nuclear and particle physics, plastic scintillators are probably the most widely used of the organic detectors today. Like the organic liquids, plastic scintillators are also so- lutions of organic scintillators but in a solid plastic solvent.
Inorganic Crystals: By far, the most commonly used material is NaI(Tl), where Thallium (Tl) is the impurity activator. In general, inorganic scintillators are 2 - 3 orders of magnitude slow- er ( - 500 ns) in response than organic scintillators due to phosphorescence. The advantage of inorganic crystals lies in their greater stopping power due to their higher density and higher atomic number. Among all the scintillators, they also have some of the highest light outputs, which results in better energy resolution. This makes them extremely suitable for the detection of gamma-rays and high-energy electrons and positrons.

Whereas the scintillation mechanism in organic materials is molecular in nature, that in inorganic scintillators is clearly characteristic of the electronic band structure found in crystals
In the photoelectric effect and pair production processes, the y-ray is completely absorbed, being transformed into a charged particle or particles. In the Compton process, however, the y retains its identity transferring only a part of its energy to a recoil electron. If the scattered y-ray does not suffer another interaction in the scintillator material, it is possible for it to escape so that only a part of its energy is deposited. To make an efficient y-ray detector, therefore, one must use a material in which the photoelectric and pair production cross sections are large compared to the Compton scattering cross section. Fortunately, the former two cross sections are much more strongly dependent on Z, going roughly as Z5 and Z2 respectively, while the Compton process only varies linearly with Z.
PMTs: the output signal at the anode is a current or charge pulse whose total charge is proportional to the initial number of electrons emitted by the photocathode. In fact, more than any other device, the photomultiplier satisfies the requirements of an ideal current generator.

While work on crystal detectors was performed as early as the 1930's [10.1], real development of these instruments first began in the late 1950's. The first prototypes quickly progressed to working status and commercial availability in the 1960's
The basic operating principle of semiconductor detectors is analogous to gas ionization devices. Instead of a gas, however, the medium is now a solid semiconductor material. The passage of ionizing radiation creates electron-hole pairs (instead of electron-ion pairs) which are then collected by an electric field. The advantage of the semiconductor device is that the average energy required to create an electron-hole pair is some 10 times smaller than that required for gas ionization. Thus the amount of ionization produced for a given energy is an order of magnitude greater resulting in increased energy resolution.

Unlike recombination and trapping states, this level is extremely close to the conduction band being separated by only 0.01 eV in germanium and 0.05 eV in silicon. At normal temperatures, therefore, the extra electron is easily excited into the conduction band where it will enhance the conductivity of the semiconductor.
The amount of dopant used is gen- erally very small with typical concentrations being on the order of a few times 1e13 atoms/cm3. Since the densities of germanium and silicon are on the order of 1e22 atoms/cm3, this implies impurity concentrations of only a few parts per billion!
Great use is also made of heavily doped semiconductors particularly as the electrical contacts for semiconductors. Impurity concentrations in these materials can be as high as 1e20 atoms/cm3, so that they are highly conductive.
This characteristic of the depletion zone is particularly attractive for radiation detection. Ionizing radiation entering this zone will liberate electron-hole pairs which are then swept out by the electric field. If electrical contacts are placed on either end of the junction device, a current signal proportional to the ionization will then be detected. The analogy to an ionization chamber thus becomes apparent.
While the pn-junction described above will work as a detector, it does not present the best operating characteristics. In general, the intrinsic electric field will not be intense enough to provide efficient charge collection and the thickness of the depletion zone will be sufficient for stopping only the lowest energy particles. As we will see later, this small thickness also presents a large capacitance to the electronics and increases noise in the signal output. Better results can be obtained by applying a reverse-bias voltage to the junction, i.e., a negative voltage to the p-side, as shown in Fig. 10.7.

The net effect is to enlarge the depletion zone and thus the sensitive volume for radiation detection - the higher the external voltage, the wider the depletion zone. Moreover, the higher external voltage will also provide a more efficient charge collection. The maximum voltage which can be applied, however, is limited by the resistance of the semiconductor. At some point, the junction will breakdown and begin conducting.
For the same radiation energy, the number of charge carriers created will therefore be almost an order of magnitude greater in these materials than in gases. Compared to the number of photoelectrons created in a scintillation counter, this increase approaches two orders of magnitude. Not surprisingly, semiconductors should provide a greatly improved energy resolution.
Since the gaps are only on the order 1 eV wide, it is clear that less than a third of the energy deposited by passing radiation is actually spent on the production of electron-hole pairs. The other two thirds, in fact, go into exciting lattice vibrations.
The third and by far the largest source of leakage current is through surface channels. This component is complex and depends on very many factors including the surface chemistry, the existence of contaminants, the surrounding atmosphere, the type of mounting, etc. Clean encapsulation is generally required here to minimize this component.
The obvious disadvantage of discrete detectors is the quantity of electronics required. Since each electrode is a separate detector, preamplifiers and other units must be provided for each strip.
Charged-coupled devices (CCD) are silicon devices consisting of a two-dimensional array of tiny potential wells each covering a surface area of a few square microns. One chip contains tens of thousands of elements. When struck by radiation, electrons are released which are then trapped in these wells. This charge information is then readout by successively shifting the charge from one well to the next until it reaches the output electronics. CCD's have been mainly used for light imaging purposes and are extremely sensitive, low noise devices. As a detector, spatial resolutions for these instruments are expected to be better than 2 mum. However, they are very limited in counting rate.
Higher voltages thus reduce the noise by increasing the depletion thickness; however, a greater risk of breakdown is also incurred.
For silicon detectors, increasing temperature will also result in higher leakage currents and greater noise. For each 10°C rise in temperature, there is roughly a three-fold increase in the leakage current.
As we have mentioned, semiconductor detectors are relatively sensitive to radiation damage. Incident particles colliding with lattice atoms cause point defects by "knocking" them out of their normal positions. These structural defects then give rise to discrete trapping levels in the forbidden band gap which reduce the number of charge carriers in the semiconductor. There also appears to be changes in the resistivity of the base material as well.

The main effects of radiation damage on detector performance are an increased leakage current and a degradation of energy resolution. If the damage is not too great, however, an increase in the bias voltage can compensate resolution loss to some extent by decreasing the collection time.
Distortion from reflections in the interconnecting cables can be a problem. This arises because of the short duration of fast pulses relative to their time of transit in the interconnection. Processing fast signals, therefore, not only requires special attention in the circuit design but also in the interconnections between modules. For this reason, essentially two standardized NIM systems have arisen: one designed for fast nanosecond pulses and the other for slower pulses. Within each system the electronic modules are compatible with each other; however, mixing the two without proper adaptation will invariably lead to problems. Special attention must be paid to how the interconnections are made in fast systems. This is one of the most important points to keep in mind when setting up nu- clear electronics.
The first (and simplest) standard established for nuclear and high energy physics is a modular system called NIM (Nuclear Instrument Module). The corresponding voltage levels are thus 0 V and - 0.8 V for logic 0 and 1 respectively.
Reflections, of course, occur when a traveling wave encounters a new medium in which the speed of propagation is different. In optical media this corresponds to a change in the index of refraction. By analogy, in transmission lines, reflections occur when the characteristic impedance of the line is abruptly changed.
occasions very often arise where an impedance mismatch cannot be avoided. One very common instance is when a fast signal must be viewed on an oscilloscope. Since oscilloscopes are high impedance devices (~1MOhm), direct entry of a fast NIM signal will result in an impedance mismatch and a false signal reading. In such cases, the cable can be terminated with an additional impedance of the appropriate value so as to adjust the total load impedance seen by the cable. For high impedance devices such as our oscilloscope, this is done by placing a resistance of 50Ohm in parallel with the device. The signal seen by the oscilloscope is then reflection free and appears as it would to a standard fast NIM module. Since the need to terminate cables arises fairly often, special 50Ohm terminators made so as to easily fit onto the cables are manufactured commercially.
The basic function of a preamplifier is to amplify weak signals from a detector and to drive it through the cable that connects the preamplifier with the rest of the equipment. At the same time, it must add the least amount of noise possible. Since the input signal at the preamplifier is generally weak, preamplifiers are normally mounted as close as possible to the detector so as to minimize cable length.
A solution to the problem of baseline shift at high count rates is to use bipolar pulses. When passing through a coupling capacitor, such a pulse, then, leaves no residual charge, (as a unipolar pulse would), so that baseline shifts are avoided. At high count rates, in fact, bipolar pulses generally give better resolution than corrected unipolar pulses.
The most common use of the discriminator is for blocking out low amplitude noise pulses from photomultipliers or other detectors. the discriminator is essentially a simple analog-to-digi-tal converter.
The time required for the digitization process is called the conversion time which, for most ADC's, is on the order of several tens of microseconds or more depending on the number of digits desired. Thus, they are slow devices when compared to other NIM modules. The advent of modern integrated circuits has also made possible a number of very high-speed ADC's with conversion times less than 1 micro sec. Among these are the flash or parallel ADC which determines each bit of the digital number simultaneously rather than sequentially
Multichannel analyzers (MCA) are sophisticated devices which sort out incoming pulses according to pulse height and keep count of the number at each height in a multichannel memory.
The flip-flop is a basic element in digital electronics as it is the only device which essentially memorizes the input signal. It may thus be considered as a 1-bit memory or latch. By combining several flip-flops in parallel, a multi-bit memory for a digital word can then be formed. This is the basis for the register described below. Other uses of flip-flops are illustrated in the following chapters.
In high counting rate measurements, it is sometimes desired to shorten the duration of the pulse to avoid pile-up. A simple method for fast pulses is to use a well-timed cable reflection to destructively interfere with the pulse. Figure 14.29 illustrates this method. A well-chosen length of shorted cable is connected in parallel to the output. After splitting at the junction, one-half the pulse travels down this length of cable where it is reflected back in an inverted state. If T is the delay of this cable, this reflection rejoins the other half at a time 2T later, where it interferes with the pulse tail. This leaves a pulse of width ~ 2T as shown.
A practice to be recommended, in fact, is to examine the input and output signals at each point in the system as it is being set-up. This not only leads to a better understanding of the system, but will also facilitate trouble-shooting later on.
A basic measurement in nuclear or particle physics experiments is a simple counting of the number of signals from the detector. For example, measuring the activity of a source, or "plateauing" a counter.
Depending on the ADC and the detectors used, it may be necessary to use a pulse stretcher to shape the signals before entry into the MCA. This is usually the case with fast signals from an organic scintillator.

TechniquesNukeAndParticleExperiments

Techinques for Nuclear and Particle Physics Experiments (W. R. Leo)

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