Outline Radiation Detectors Energy Resolution Detection Efficiency Dead Time Scintillation Detector Solid State Detectors Germanium Gamma Ray Detectors 2 Energy Resolution Energy Resolution For many applications it is important to measure the energy distribution of the incident radiation. These efforts are generally called radiation spectroscopy. One important property for spectroscopy can be seen in the detector response to a monoenergetic source of radiation. The differential pulse height distribution produced by a detector under these conditions is called the response function of the detector. 4 Radiation Detectors 1
Energy Resolution Energy Resolution Let s assume that in two cases the same number of pulses have been recorded. That means the area under the distribution should be the same. Both distributions are centered around H 0, but the width of one of the distributions is much greater. The width reflects the fact that a larger amount of fluctuations was recorded. If the amount of fluctuations is smaller, the width of the distribution will become smaller. 5 Examples of response functions for detectors with relatively good resolution and relatively poor resolution. 6 Energy Resolution As the width of the response function becomes smaller, the measurement will resolve more details in the incident energy region. The ability of a detector to distinguish between radiations of similar energy is called energy resolution. Full Width at Half Maximum (FWHM) The Full Width at Half Maximum (FWHM) is defined as the width of the distribution at a level that is just half the maximum ordinate of the peak. This definition assumes that any background is either negligible or has been subtracted. 7 8 Radiation Detectors 2
Full Width at Half Maximum (FWHM) Full Width at Half Maximum (FWHM) The energy resolution R is conventionally defined as the FWHM divided by the location of the peak. FWHM R E kev kev 100% It is a dimensionless quantity in percent. Definition of detector resolution. For peaks whose shape is Gaussian with standard deviation σ, the FWHM is given by 2.35σ. 9 10 Detection Efficiency Detection Efficiency A radiation detector will give an output for every radiation quantum that interacts within its active volume. Many particles will interact immediately upon entering the active volume, e.g. alphas & betas. In this case the detector sees every particle that enters its active volume. The detector has a counting efficiency of 100%. 12 Radiation Detectors 3
Detection Efficiency On the other hand some radiations, e.g. photons and neutrons, must undergo a significant interaction before detection becomes possible. These types of radiation can travel large distances between interactions. As a result the detector may not see every particle that enters its active volume. In this case the detector efficiency will be less than 100%. Detection Efficiency It is important to distinguish between two classes of counting efficiencies. The absolute efficiency is defined as: Number of pulses recorded ε Number of radiation quanta emittedby source The intrinsic efficiency is defined as: Number of pulses recorded ε Number of radiation quanta entering the detector 13 14 Dead Time Dead Time In most detector systems there is a minimum amount of time necessary so that two separate events are recorded as two separate pulses. This limit is either due to processes in the detector or due to the response time of the electronics. The minimum time separation required is called the dead time. Because of the random nature of radioactive decay, there is always a chance that a true event is lost because it occurs to quickly after another. 16 Radiation Detectors 4
Dead Time Dead time losses can become especially severe at high count rates. Any measurement made under these conditions must therefore include some correction for these losses. Two models for the dead time behavior in counting systems are commonly used: Paralyzable response Nonparalyzable response Paralyzable vs. Nonparalyzable Detector 17 Illustration of two assumed models of dead time behavior for radiation detectors. 18 Paralyzable vs. Nonparalyzable Detector Events that occur during the dead time period are lost for both types of detector models. However, events that are coming in during this time have no influence on the duration of the dead time in a nonparalyzable detector. In contrast such events extend the dead time period of a paralyzable detector. Scintillation Detectors 19 Radiation Detectors 5
Introduction The detection of ionizing radiation by scintillation light is one of the oldest techniques known. It still remains one of the most useful methods for radiation detection and spectroscopy. It is widely used in gamma ray spectroscopy (NaI, LaBr 3, BGO) and in beta, alpha and neutron counting (LSC). It can even be used for particle identification based on pulse shape analysis of the light pulse emitted. Introduction Inorganic scintillators (high Z) are useful for gamma ray detection and spectroscopy as well as for detection of alpha and charged particles. They have the best light output and linearity, but they have a slow response time. Examples of inorganic scintillators are: NaI CsI BaF ZnS BGO many, many more 21 22 Introduction Organic scintillators (low Z) are useful for beta spectroscopy and fast neutron detection. They have a fast response time but yield less light. Examples of organic scintillators are: Plastics Liquid scintillators Emission Processes Fluorescence is the prompt emission of visible radiation following excitation. Phosphorescence corresponds to emission at a longer wavelength over a longer period of time. Delayed fluorescence has the same emission as prompt fluorescence, but occurs over a much longer time period as well. 23 24 Radiation Detectors 6
Introduction A good scintillator should maximize the conversion of radiation to fluorescence. At the same time it should minimize the occurring phosphorescence and delayed fluorescence. Organic Scintillators Scintillation Mechanism 25 Organic Scintillators The fluorescence of organic scintillators is based on electron transitions between different energy levels. It is a molecular property and not a bulk property. It can therefore be observed independent of the physical state. Many organic scintillators are based on molecules with π bonds. Organic Scintillators These are covalent chemical bonds involving p orbitals (double bonds, aromatic rings). Energy can be absorbed by exciting an electron into a number of different excited energy states. 27 28 Radiation Detectors 7
Electronic States Electronic States Electronic states can be divided into singlet states S (spin 0) and triplet states T (spin 1). A series of singlet states is labeled as S 0, S 1, S 2, The typical spacing between S 0 and S 1 is 3 4 ev. Each singlet state is further divided into a number of vibrational states. S 00 represents the lowest vibrational state. At room temperature nearly all electrons are in the S 00 state. 29 Energy levels of an organic molecule with π electron structure. 30 Fluorescence Higher singlet states de excite quickly to S 1 through radiationless internal conversion. After a short amount of time, a population of excited molecules in the S 1 state is produced as result of the excitation through ionizing radiation. Scintillation light is emitted in transitions between S 1 and one of the vibrational states of the ground state. This transition is the basis for fluorescence. Phosphorescence Electrons from an excited singlet state can transfer to a triplet state through intersystem crossing. The lifetime for the first triplet state is much longer then that of the singlet state. Emission of light from a triplet state is therefore delayed. This is the basis for phosphorescence. T 1 lies below S 1, therefore the wavelength for phosphorescence is longer than for fluorescence. 31 32 Radiation Detectors 8
Delayed Fluorescence Wavelength & Absorption Some molecules in the T 1 state may be thermally excited back into the S 1 state and decay through fluorescence. This process gives rise to delayed fluorescence. 33 The optical absorption and emission spectra for a typical organic scintillator with the level structure shown in the previous figure. 34 Multi Component Scintillators In most organic materials, the excitation energy can be transferred to other molecules through collisions before de excitation occurs. This transfer is especially important for organic scintillators that involve more than one species of molecules. Let s assume that a small concentration of an efficient scintillator is added to a bulk solvent. Then energy taken up by the solvent molecules can be transferred to the scintillator molecules through collisions between the molecules. Multi Component Scintillators Once the energy has been transferred to a scintillator molecule, it can be emitted as light. Such binary organic scintillators are widely used both as liquid and plastic solutions. A third component is sometimes added to these mixtures to serve as a waveshifter. It works by absorbing the light from the primary scintillant and reradiating at a longer wavelength. This can be useful to match the emission spectrum closer to a photomultiplier tube. 35 36 Radiation Detectors 9
Quenching Quenching Scintillation efficiency is defined as the fraction of all incident particle energy that is converted to light. It should be as large as possible. Any process that reduces the efficiency of the conversion of particle energy into light is called quenching. There are two types of quenching: Chemical quenching Color quenching Source: Thomson, J., Use and Preparation of Quench Curves, Liquid Scintillation Counting Application Note P11399, PerkinElmer (2002) 37 38 Quenching Chemical quenching is caused if the excited molecules have alternate de excitation modes available that do not involve the emission of light. In these cases the excitation energy is mainly converted to heat. Color quenching is caused if the photons emitted are prevented from being detected by the PMT. The photon may be absorbed or its wavelength could be changed so that the PMT is not sensitive to it anymore. Inorganic Scintillators Scintillation Mechanism 39 Radiation Detectors 10
Scintillation Mechanism Scintillation in inorganic materials is based on transitions between energy levels determined by the crystal lattice of the material. This is not a molecular process, it is instead based on the properties of the bulk material. In insulators, only discrete bands of energy are available to electrons. Scintillation Mechanism The lower band is called the valence band. It represents those electrons that are bound at lattice sites. The upper band is called the conduction band. It represents the electrons that have enough energy to move through the crystal. The region between these to bands is called the forbidden band. In pure crystals electrons can not exist in this band. 41 42 Scintillation Mechanism Scintillation Mechanism Absorption of energy results in the transfer of an electron from the valance to the conductive band. As a result a hole is created in the valence band. In a pure crystal, return of the electron with emission of a photon is an inefficient process. Furthermore, the photon emitted would not be in the visible range, because the width of the gap is generally to wide. Energy band structure of a pure crystalline scintillator. 43 44 Radiation Detectors 11
Scintillation Mechanism Scintillation Mechanism To enhance the probability of visible photon emission during de excitation, small amounts of impurities can be added to the scintillator material. Such impurities are called activators. They generate special sites in the lattice where the normal energy band structure is modified. As a result energy states within the forbidden band will be created. The excited electron can de excite through these states to the valence band. 45 Energy band structure of an activiated crystalline scintillator. 46 Scintillation Mechanism Because the energy of these states is less than that of the full gap, the emitted photons can have wavelengths in the visible range. The de excitation centers created by the impurities are called luminescence centers or recombination centers. Their energy structure in the host crystal lattice determines the emission spectrum of the scintillator. Scintillation Mechanism A charged particle passing through the material will elevate a large number of electrons from the valance to the conductive band. As a result a large number of electron hole pairs will be created. The positive hole will drift to the location of an activator site and ionize it. This is due to the lower ionization energy of the impurity. The electron can wander freely through the crystal until it encounters an ionized activator. 47 48 Radiation Detectors 12
Scintillation Mechanism Scintillation Mechanism At this point the electron can drop into the activator site and form a neutral atom again. The electron is now in an excited state of an atom with its own set of excited energy states. If the excited state has an allowed transition to the ground state, de excitation will occur quickly through emission of a photon. If the activator is chosen properly, this transition can be in the visible range. 49 The emission spectra of several common inorganic scintillators. Also shown are the response curves for two widely used photocathodes. 50 Introduction Semiconductor Diode Detectors The use of a solid detection medium is very advantageous for radiation spectroscopy. The higher density of the material allows the use of smaller detectors for gamma ray spectroscopy compared to gas filled detectors. Spectroscopy with scintillation detectors is one example of the use of a solid medium. The major limitation of scintillation counting is the relatively poor energy resolution. 52 Radiation Detectors 13
Introduction A series of events needs to occur to convert the incident radiation energy into light and finally into an electrical signal. This chain of events involves many inefficient steps. The energy to produce one information carrier is on the order of 100 ev or more. Not more than a thousand carriers are created for a typical interaction. Introduction The statistical fluctuation in such a small number of carriers limits the energy resolution that can be achieved. It is impossible to improve the resolution beyond the limits set by statistics. The only way to improve the statistics limit is to increase the number of information carriers. The use of semiconductor materials as radiation detectors can result in a much larger number of information carriers for a given incident radiation. 53 54 Introduction Consequently, semiconductor detectors can achieve the best energy resolution of all detectors used for radiation spectroscopy. The information carriers created in the detector are electron hole pairs that are formed along the path of the incident radiation. The electron hole pair fulfills a purpose similar to an ion pair in gas filled detectors. The motion of the electron hole pairs in an electric field generates the basic electric signal. Introduction Semiconductor detectors have also several disadvantages: Need to operate at low temperatures (liquid N 2 ) True for GeLi and Ge detectors, but not for Si Lower intrinsic efficiency compared to NaI(Tl) This is due to the lower Z of the detector material Bigger crystals need to be fabricated Higher cost Rough cost estimate: Efficiency x $1,000 Susceptible to radiation damage 55 56 Radiation Detectors 14
Band Structure Semiconductor Properties The lattice structure of crystalline materials creates allowed energy bands for the electrons in the solid. The energy of an electron in the pure material must be confined to one of these bands. The bands can be separated by gaps or ranges of forbidden energies. 58 Band Structure Band Structure The lower band is called the valence band. It represents those outer shell electrons that are bound to lattice sites in the crystal. In the case of Si or Ge, the electrons are part of a covalent bond that constitutes the interatomic force in the crystal. The upper band is called the conduction band. It represents the electrons that are free to move through the crystal. 59 Band structure for electron energies in insulators and semiconductors. 60 Radiation Detectors 15
Band Structure Electrons in the conduction band cause the electric conductivity of the material. The two bands are separated by the band gap or the forbidden band. A material is characterized as an insulator or a semiconductor based on the size of this band gap. For insulators the band gap is usually 5 ev or more. For semiconductors it is considerably less (~ 1 ev). Band Structure The number of electrons in the crystal band is just sufficient to fill all available sites in the valence band. In the absence of thermal excitation both insulators and semiconductors will have a full valence band and an empty conduction band. Under these conditions neither material shows any electric conductivity. The electron would have to cross the band gap to induce conductivity. 61 62 Band Structure In a metal the highest occupied energy band is not completely filled. Electrons can therefore easily migrate throughout the material. They only need to obtain a small amount of additional energy to be above the occupied states. Metals therefore always show a very high electrical conductivity. Charge Carriers At any temperature above absolute zero, the electrons will have a certain thermal energy. In some cases this energy can be sufficient to elevate an electron from the valence band to the conduction band. Physically, this process means that an electron that is part of a covalent bond becomes excited. It is able to leave its bonding site in the lattice and drift through the crystal. 63 64 Radiation Detectors 16
Charge Carriers The excitation process creates an electron in the otherwise empty conduction band. As a result a vacancy is left in the otherwise full valence band. This vacancy is called a hole. The combination of the two is called an electron hole pair. It is the solid state analogue of an ion pair created in a gas. Charge Carriers The electron in the conduction band will move if an electric field is applied. The hole, representing a net positive charge, will move as well. The hole will move in the opposite direction of the electron. The motion of these two charges contributes to the observed conductivity of the material. 65 66 Charge Carriers The probability per unit time of thermally creating an electron hole pair is given by: where p(t) C T 3 2 Eg exp 2 k T T = Absolute temperature E g = Bandgap energy k = Boltzmann constant C = Proportionality constant characteristic of material Charge Carriers The probability for thermal excitation depends critically on the ratio of the band gap energy to the temperature. Materials with a large band gap have a low probability for thermal excitation and will exhibit the behavior of an insulator. If the band gap is small, thermal excitation will cause the material to behave like a semiconductor. If no electric field is applied, the thermally created pairs will recombine and an equilibrium will be established. 67 68 Radiation Detectors 17
Charge Carriers As T decreases, the factor exp( E g / 2kT) becomes much smaller and thermal excitation decreases significantly. The band gap for Si is 1.115 ev. For large energy deposition (heavy charged particles), the detector can be used without cooling. For small energy deposition (X ray or electron spectroscopy), the detector needs to be cooled. Ge has a band gap of 0.665 ev. The band gap is so small that the detector must always be cooled to 77 K (liquid N 2 ) to be used. Migration of Charge Carriers If an electric field is applied to the semiconductor, both the electrons and the holes will start to migrate. The motion will be a combination of their random thermal velocity and a drift velocity parallel to the direction of the electric field. It is easy to visualize the motion of the electrons. The fact that moving holes contribute to the conductivity as well is less obvious. 69 70 Migration of Charge Carriers For a hole to move from one position to another, the remaining positive charge caused by the excited electron has to move. This is achieved if an electron moves from a different binding site to the vacancy to fill the hole. In this case the vacancy will now exist at the binding site from which the new electron moved. The hole has switched positions. Migration of Charge Carriers In gases the mobility of the electron is much higher than that of the positive ion. In contrast, the mobility of electrons and holes in semiconductors are of the same order. At higher electric field values, the drift velocity increases more slowly with the field. Eventually, it becomes independent of increases in the electric field strength. At this point a saturated drift velocity is reached. 71 72 Radiation Detectors 18
Migration of Charge Carriers Migration of Charge Carriers Semiconductor detectors are generally operated with a field strength high enough to reach a saturated drift velocity. The resulting drift velocity is on the order of 10 7 cm/s. The time required to collect charge carriers over typical dimensions of 0.1 cm or less will be under 10 ns. Semiconductor detectors belong therefore to the fastestresponding radiation detector types. Drift velocity as a function of electric field applied parallel to the {111} crystallographic direction. Absolute temperature is the parameter for the different curves. (a) Electrons in silicon; (b) holes in silicon; (c) electrons in germanium; (d) holes in germanium. 73 74 Migration of Charge Carriers Effect of Impurities In a completely pure semiconductor, all electrons in the conduction band and all holes in the valence band would be caused by thermal excitation. In this case the number of electrons in the conduction band would be exactly the same as the number of holes in the valence band. This is due to the fact that each electron thermally elevated must leave a hole behind. Such a material is called an intrinsic semi conductor. 75 76 Radiation Detectors 19
Effect of Impurities The properties of such a material can be described theoretically. However, it is impossible to create such a material in reality. The electronic properties of real materials are typically dominated by the residual small levels of impurities in the material. This is true even for Si and Ge, the semiconductors with the highest purity available. Effect of Impurities Let n represent the number of electrons present in the conduction band. Let p represent the number of holes present in the valence band. Then the equilibrium existing in an intrinsic semiconductor can be described as: ni p i The quantities n i and p i are called the intrinsic carrier densities. 77 78 Effect of Impurities The densities will be lowest for materials with large band gap energy. The densities will also decrease with decreasing temperature. Intrinsic hole or electron density at room temperature: Si: 1.5 x 10 10 cm 3 Ge: 1.5 x 10 13 cm 3 n Type Semiconductors We will use Si as an example to describe the effect of doping on semiconductor properties. The behavior of Ge and other semiconductor materials is similar. Si is tetravalent. It forms covalent bonds with the four closest neighboring silicon atoms in the normal crystal. Thermal excitation consists of breaking one of these covalent bonds and leaving behind a hole. 79 80 Radiation Detectors 20
n Type Semiconductors n Type Semiconductors However, there will be a small amount of impurities present in the material. These impurities can be residues remaining after the best available purification step. Or they could have been added intentionally to tailor the properties of the material. Let s assume that the impurity is pentavalent. The impurity will occupy a site in the crystal lattice and take the place of a Si atom. 81 (a) Representation of a donor impurity (phosphorus) occupying a substitutional site in a silicon crystal (b) Corresponding donor levels created in the silicon bandgap. 82 n Type Semiconductors Because the impurity has five valence electrons, one electron will be left after all covalent bonds with neighboring Si atoms have been formed. The additional electron is only very lightly bound to the original impurity site. It takes very little energy to dislodge it, resulting in a conduction electron. This electron does not have a corresponding hole in the valence band. The remaining donor atom becomes ionized. n Type Semiconductors Impurities of this type are called donor impurities. They readily contribute electrons to the conduction band. Because the donor atoms are not part of the regular crystal lattice, the extra electrons released can occupy a position within the forbidden gap. These loosely bound electrons will have an energy close to the top of the band gap. 83 84 Radiation Detectors 21
n Type Semiconductors The spacing between the valence band and the donor levels is small enough to ensure that a large fraction of the donor impurities are thermally excited. The concentration of impurities N D is normally always large compared with the number of intrinsic electrons in the conduction band. Therefore, the number of conduction electrons is dominated by the contribution from donor atoms. n Type Semiconductors The added concentration of electrons in the conduction band shifts the equilibrium between electrons and holes through recombination. This means the equilibrium concentration of holes is decreased, so that the product of n and p is the same as for the intrinsic material. n p n i p i n N D 85 86 Example Intrinsic carrier density for Si: 1.5 x 10 10 cm 3 Concentration of donor impurity: 1.0 x 10 17 cm 3 ni pi p n 10 1.5 10 1.5 10 1.0 10 p 17 10 2250 n Type Semiconductors The total number of charge carriers is now much greater (1 x 10 17 cm 3 vs. 3 x 10 10 cm 3 ). The electrical conductivity of a doped material is therefore always significantly larger than that of pure material. Even though the conduction electrons greatly outnumber the holes, the material itself is still charge neutral. This is due to the presence of ionized donor atoms. 87 88 Radiation Detectors 22
n Type Semiconductors These lattice sites represent net positive charges that exactly balance the excess electrons. These positive lattice sites should, however, not be confused with holes. They are fixed in the lattice and cannot migrate. The net effect of an n type material is to create a situation in which the number of conduction electrons is much greater and the number of holes much smaller than in a pure material. p Type Semiconductors The addition of a trivalent impurity to the material results in a very different situation. The impurity occupying a lattice site has one valence electron less than the neighboring atoms. This means one covalent bond with a neighbor cannot be formed. This vacancy represents a hole similar to that left behind when an electron is excited into the conduction band. It s energy characteristics are slightly different. 89 90 p Type Semiconductors p Type Semiconductors If an electron is trapped in this vacancy, it will participate in a covalent bond that is different from the others in the crystal. This is caused by the fact that one of the atoms is trivalent and not tetravalent. An electron filling the hole will still be bound to the lattice site, but it will be less attached than a valance electron at a normal lattice site. Therefore, this site will create acceptor energy levels in the forbidden gap. (a) Representation of an acceptor impurity (boron) occupying a substitutional site in a silicon crystal (b) Corresponding acceptor levels created in the silicon bandgap. 91 92 Radiation Detectors 23
p Type Semiconductors The new acceptor level will lie near the bottom of the gap, because the properties are similar to that of normal lattice sites. An impurity causing this type of energy levels in the forbidden gap is called an acceptor impurity. Because the energy difference between the valence band and the acceptor site is small, a large fraction of the acceptor sites will be filled with thermally excited electrons. These electrons come from normal sites in the lattice and will leave holes behind. p Type Semiconductors Approximately one additional hole is created for every acceptor impurity added. If the concentrations of acceptor impurities N A is large compared to the concentration of intrinsic holes, then the number of holes will be dominated by the concentration of acceptors. p N A The increased availability of holes increases the probability for recombination between electrons and holes. 93 94 p Type Semiconductors p Type Semiconductors Therefore the number of equilibrium conduction electrons will be decreased. As a result, the electrical conductivity of a p type semiconductor is dominated by holes. Holes are the majority carriers and electrons are the minority carriers. The filled acceptor sites represent fixed negative charges. They balance the positive charge of the additional holes. 95 Nomogram showing the relationship between electron and hole concentrations in a semiconductor. Lines connecting points on the two logarithmic scales always pass through the center of the diagram for any type or degree of doping. 96 Radiation Detectors 24
Ionization Energy Action of Ionizing Radiation in Semiconductors When a charged particle passes through a semi conductor, it will produce many electron hole pairs along its track. The amount of energy spent by the charged particle to create an electron hole pair is a very important quantity for detector applications. This quantity is often referred to as the ionization energy ε. It is largely independent of the energy and type of the incident radiation. 98 Ionization Energy This simplification allows to interpret the energy of the incident radiation in terms of the number of electron hole pairs produced. This only holds true if the radiation is completely stopped in the active volume of the detector. Interaction of radiation with a semiconductor always leads to the creation of the same number of electrons and holes. This is independent of the nature of the semi conductor (intrinsic, p or n type). Ionization Energy The same way equal numbers of electrons and positive ions are created in a gas, every conduction electron produced must leave behind a hole in the valence band. The concentration of doping materials are so low that they play no role in determining the photon interaction. p and n type detectors of equal thickness have identical probabilities for gamma ray interaction. The range of particles will be the same in either type as well. 99 100 Radiation Detectors 25
Ionization Energy The great advantage of semiconductor detectors lies in their small ionization energy (~ 3 ev). For comparison, about 30 ev are required to produce an ion pair in gas filled detectors. The number of charge carriers produced for a given energy is therefore ten time greater in a semiconductor. The greater amount of charge carriers reduces the statistical fluctuations in the number of carriers. The greater charge per pulse also improves the signal tonoise ratio for low energies. Semiconductors as Radiation Detectors 101 Electrical Contacts In a radiation detector the charge produced must be collected at either boundary of the semiconductor material. This is usually done by applying some type of contact to the material. An ohmic contact is a non correcting electrode through which charges of either sign can flow. Typical ohmic contacts on semiconductors are sputtered or evaporated metal pads that are patterned using photolithography. Electrical Contacts Lets assume a semiconductor with two ohmic contact fitted on opposite sides that are connected to a circuit. For each electron or hole collected at one electrode, the same species will be introduced at the other electrode. As a result, the equilibrium charge carrier concentration in the material will be maintained. Use of ohmic contacts causes leakage currents that are much too high to allow for their use in Si or Ge detectors. 103 104 Radiation Detectors 26
Electrical Contacts Instead, non injecting or blocking contacts are typically used to reduce leakage current. In this case the charge carriers initially removed by the application of an electric field are not replaced at the opposite electrode. Their overall concentration will therefore drop after application of an electric field. The leakage current can thus be reduced to a value that allows detection of the electron hole pairs created by ionizing radiation. Electrical Contacts The best type of blocking contact are the two sides of a p n semiconductor junction. Holes are the majority carrier on the p side of the junction and free electrons are rare. This makes it very difficult to inject electrons from this side. On the opposite side, electrons are the majority carrier and holes are few. It is very difficult to inject holes from this side. 105 106 Electrical Contacts In this lecture we will concentrate on detectors created by placing a p type material in direct contact with a n type material. This is called a p n junction. In other detectors the p and n type regions are separated by an intrinsic region (i region). Semiconductor Junction The detectors described in this lecture are based on the properties created near a p n junction in semiconductors. If the regions are brought in good contact, then charge carriers are able to migrate across the junction. The junction cannot be created by just bringing together two pieces of p type and n type material. There would be gaps between them that are large compared with the interatomic lattice spacing. 107 108 Radiation Detectors 27
Semiconductor Junction Semiconductor Junction Instead the junction is normally formed in a single crystal. A change in impurity content is caused from one side of the junction to the other. Lets assume the process starts with a p type crystal that has been doped with a uniform concentration of an acceptor impurity. The surface of the crystal is exposed to the vapor of an n type impurity that diffuses some distance into the crystal. 109 The assumed concentration profiles for the n p junction shown at the top are explained in the text. The effects of carrier diffusion across the junction gives rise to the illustrated profiles for space charge p(x), electric potential φ(x), and electric field ε(x). 110 Semiconductor Junction The resulting donor impurity concentration N D in the crystal decreases with distance from the surface. Near the surface, the concentration of donor atoms will be higher than that of acceptor atoms. This means this side of the crystal will become an n type material. Semiconductors Detector Configurations 111 Radiation Detectors 28
Diffused Junction Detectors Fabrication of this type of detector starts with a homogenous crystal of a p type semiconductor. One surface of the material is exposed to the vapor of an n type impurity (typically P). This converts the region close to the surface to from a p type to an n type material. As a result a p n junction is formed some distance from the surface. The depth of the n type layer ranges from 0.1 2.0 μm. Diffused Junction Detectors Because the impurity concentration in the n type region is much higher than that in the p type region, the depletion region will extend mainly into the p side of the junction. This results in a surface layer that is not depleted and does not act as an active volume. It represents a dead layer through which the radiation must pass before it can be detected. This dead layer can cause a real disadvantage for charged particle spectroscopy. 113 114 Diffused Junction Detectors Part of the particle energy will be lost before the particle reaches the active detector volume. For this reason diffused junction detectors have been replaced in most applications. They are commercially still available and offer one advantage over surface barrier detectors. They are more rugged and are less affected by accumulation of impurities, e.g. oil, on the detector surface. Surface Barrier Detectors Instead of using a p type material, a high density electron trap layer can be placed on the surface of an n type material. This results in a depletion region that behaves very much like that of a regular p n junction. The process to fabrication the electron trapping layer is mainly based on empirical observations. The procedure usually starts with etching the surface of the semiconductor. 115 116 Radiation Detectors 29
Surface Barrier Detectors Surface Barrier Detectors Afterwards a thin layer of gold is evaporated on the surface for electrical contact. The evaporation is carried out under conditions that promote oxidation of the surface layer. The resulting oxide layer between the gold and silicon plays an important role in determining the properties of the surface layer. Surface barrier detectors can also be produced by starting with a p type crystal and evaporating a layer of aluminum on the surface. Source: ORTEC Advanced Measurement Technology, Inc http://www.ortec online.com/download/introduction Charged Particle Detectors.pdf retrieved April 24, 2012 117 118 Surface Barrier Detectors Surface Barrier Detectors Construction and mounting of silicon junction detectors shown in cross sectional view. (a) Surface barrier mount with coaxial connector (M) at rear. The silicon wafer (S) is mounted in a ceramic ring (I) with electrical contact made between either side of the junction and opposite metalized surfaces of the ring. The front surface is connected to the outer case (C) and grounded, whereas the back surface is connected to the center conductor of the coaxial connector. (b) Cutaway view of a transmission mount, in which both surfaces of the silicon wafer are accessible. The coaxial connector is placed at the edge of the ceramic ring. 119 The surface barrier detectors produced in this way have a very thin dead layer. Surface barrier detectors are sensitive to light. Light photons striking the detector surface can reach the active volume. Light in the visible range has an energy of 2 4 ev. This is sufficient to create electron hole pairs. Exposure to normal room lighting would result in a very high noise level in the detector. 120 Radiation Detectors 30
Surface Barrier Detectors Fortunately the vacuum enclosure necessary for most applications provides sufficient protection from light. Due to the thin entrance window the detector is also sensitive to damage from vapor exposure. The surface of this type of detector should never be handled directly. Because surface barrier detectors are usually created using n type crystals, a positive voltage has to be applied to achieve reverse biasing. Ion Implanted Detectors Instead of exposing the surface of the detector to the vapor of a doping material, the material can be introduced through ion implantation. To produce n + or p + layers, the semiconductor is irradiated with accelerated ions, such as P or B. A fixed voltage accelerator produces mono energetic ions that have a well defined range in the material. By changing the accelerator voltage and the energy of the ions, the concentration profile of the impurity introduced can be controlled. 121 122 Ion Implanted Detectors Ion Implanted Detectors After exposure to the ion beam the detector is usually annealed at elevated temperatures. This reduces the effect of radiation damage on the detector. The temperature needed to anneal the detector (< 500 C) is significantly lower than the temperature required for thermal vapor diffusion. Ion implantation therefore results in less disturbance to the crystal structure. 123 Source: ORTEC Advanced Measurement Technology, Inc http://www.ortec online.com/download/introduction Charged Particle Detectors.pdf retrieved April 24, 2012 124 Radiation Detectors 31
Ion Implanted Detectors Ion implanted detectors tend to be more stable than surface barrier detectors. They are also less affected by ambient conditions. Detectors are available with entrance windows as thin as 34 nm. Passivated Planar Detectors The best currently available technique for the fabrication of silicon junction detectors combines the techniques of ion implantation and photo lithography. The technique is applied to a large area silicon waver. This allows for the batch production of multiple detectors simultaneously. The process starts with a silicon waver that is slightly n type due to residual impurities. 125 126 Passivated Planar Detectors The wafer is first cleaned and polished. Then the surface is passified through creation of an oxide layer at elevated temperatures. Next photolithography is used to selectively remove areas of the oxide layer where the entrance windows of the detectors will be located. A very thin layer inside the windows is converted into a p type material through boron ion implantation. Passivated Planar Detectors The rear surface of the waver is converted into an n type material through implantation of a donor (As). This creates a blocking electrical contact. Afterwards the waver is annealed at elevated temperature to reduce radiation damage. Finally aluminum is evaporated and patterned by photo lithography to serve as a thin electrical contacts on the front and rear surfaces. Then the individual detectors are separated and encapsulated. 127 128 Radiation Detectors 32
Passivated Planar Detectors Passivated Planar Detectors The detectors produced have a very low leakage current and excellent operational characteristics. The junction edges are defined by the implantation process and are contained within the bulk wafer. In surface barrier detectors the junction will extend all the way to the edge of the wafer. The oxidation layer keeps leakage current much lower than with in surface barrier detectors. Steps in the fabrication of passivated planar silicon diode detectors. 129 130 Passivated Planar Detectors Passivated Planar Detectors Leakage only occurs at the spots where the wafer is encapsulated in epoxy. Doping through ion implantation provides the detector with a very thin and uniform entrance window. This is important for good energy resolution. The evaporated aluminum surface makes the detector more rugged and it is less sensitive to damage. www.canberra.com 131 Source: Canberra Industries, Inc. http://www.canberra.com/products/505.asp retrieved April 24, 2012 132 Radiation Detectors 33
Introduction Germanium Gamma Ray Detectors The types of semiconductor detectors discussed previously are widely used for the detection of charged particle radiations. However, junction barrier detectors are not well suited for the detection of penetrating radiation. This is mainly due to the maximum depletion depth or active volume that can be achieved for such detectors. It is difficult to achieve a depletion depth of more than 2 3 mm using semiconductor grade Si or Ge. 134 Introduction Detectors for gamma ray spectroscopy require a much larger active volume. This goal can be achieved through two different approaches. One approach is to purify the semiconductor material further to reach impurity concentrations of ~ 10 10 atoms/cm 2. This would correspond to an impurity level of less than 1 part in 10 12. Introduction At this impurity level a depletion depth of 10 mm could be achieved with a bias of less than 1000 V. Production of material with this purity requires completely new refining techniques. Ultrapure crystals have been successfully created for Ge but not for Si. Detectors made from this ultrapure Ge are called High Purity Germanium Detectors (HPGe). 135 136 Radiation Detectors 34
HPGe Detector Fabrication Germanium Detector Configurations The techniques for the production of ultrapure Ge were first developed in the 1970s. The process starts with bulk Ge intended for semiconductor production. This already very pure material is further processed using zone refining. A narrow zone of the material is locally heated and melts. The wafer is then moved so that the molten zone passes from one end of the crystal to the other. 138 HPGe Detector Fabrication HPGe Detector Fabrication The impurities are typically more soluble in the molten Ge than in the solid material. They therefore tend to accumulate in the molten zone and follow it through the crystal. The molten zone is allowed to solidify at one end of the wafer. It is then separated from the remaining crystal and discarded. By repeating this process impurity levels as low as 10 9 atoms/cm 3 have been achieved. 3 Coil Zone Refiner Zone Refined Ingot 139 Source: ORTEC Advanced Measurement Technology, Inc http://www.ortec online.com/download/high Purity Germanium HPGe Detector Manufacturing.pdf retrieved April 28, 2012 140 Radiation Detectors 35
HPGe Detector Fabrication Large single crystals of germanium are grown using the Czochralski technique. A precisely cut seed crystal is dipped into the molten germanium and then withdrawn slowly. The temperature of the melt is maintained just above the freezing point during this process. The rate of crystal withdrawal and temperature of the melt are adjusted to control the growth of the crystal. HPGe Detector Fabrication Germanium Crystal Being Grown 141 Source: ORTEC Advanced Measurement Technology, Inc http://www.ortec online.com/download/high Purity Germanium HPGe Detector Manufacturing.pdf retrieved April 28, 2012 142 HPGe Detector Fabrication High purity germanium crystals suitable for detector fabrication are almost always grown in a quartz crucible under a hydrogen atmosphere. Near the completion of the growth process, the crystal is tapered gradually at the tail to minimize thermal strain. The crystal has to be grown to the exhaustion of the melt, because germanium both wets quartz and expands on freezing. The quartz crucible might be fractured if any Ge were left after completion of the crystal growth. HPGe Detector Fabrication If the impurities remaining in the material are acceptors, the electrical properties of the material will be mildly p type. This material is often referred to as π type. Alternative, if donor impurities remain, the material will be mildly n type. Such a material is called υ type. 143 144 Radiation Detectors 36
HPGe Detector Fabrication After the crystal is grown and cooled, it is mounted for slicing. The completed crystal is cut by a string saw that causes virtually no damage to the crystal. A slurry of water and silicon carbide is pulled along by a wire, resulting in a sawing action. Sections of the crystal from both top and bottom are checked to determine the impurity concentration and type (n or p). The part of the crystal which contains detector grade material is carefully selected. HPGe Detector Fabrication The rejected material is returned to the zone refining operation. The crystal is then ground perfectly cylindrical. The edge at one end is beveled to a radius to improve charge collection and timing performance. Afterwards, a hole is machined into the unbeveled end so that the central contact of the device may be made later. The detector subsequently is hand lapped all over to remove damage from the machining processes. 145 146 HPGe Detector Fabrication Planar Configuration Mounted Crystal Being Sliced Grinding the Germanium Crystal In this configuration the electrical contacts are supplied to the two flat surfaces of a Ge disk. Typically a π material is used as starting material. The n + contact is created through either ion implantation or lithium vapor diffusion of donor atoms on one surface. The resulting n + p junction is reverse biased to form the depletion region. The contact on the opposite surface has to be noninjecting for the majority carriers. Source: ORTEC Advanced Measurement Technology, Inc http://www.ortec online.com/download/high Purity Germanium HPGe Detector Manufacturing.pdf retrieved April 28, 2012 147 148 Radiation Detectors 37
Planar Configuration Coaxial Configuration The diameter of the cylindrical Ge disk used in the planar configuration is typically not more than a few centimeters. The maximum depletion depth is limited to about 1 2 cm. This means such a detector can only have an active volume of 10 30 cm 3. Gamma spectroscopy normally requires a much larger active detector volume. Therefore a different configuration is necessary. Configuration of a planar HPGe detector. The Ge semiconductor may be ν type (p + contact is rectifying), π type (n + contact is rectifying, or lithium drifted. 149 150 Coaxial Configuration Coaxial Configuration This configuration is called a coaxial geometry. The process starts with a long cylinder of Ge. The outer surface of the cylinder is covered by the first electrode. The core of the crystal is removed and a second electrical contact is applied to the resulting inner surface of the cylinder. The crystal cylinder can be extended in the axial direction and so a much larger active volume can be reached. 151 At the top are shown the three common shapes of large volume coaxial detectors. Each represents a crosssectional view through the axis of a cylindrical crystal. The outer electrode is extended over the flat front (left) surface in both closed ended cases. Cross sections perpendicular to the cylindrical axis of the crystal are sown at the bottom. The HPGE material may be either high purity p or n type. The corresponding electrode configurations are shown for each type. 152 Radiation Detectors 38
Coaxial Configuration In a closed ended coaxial configuration, only a part of the inner core is removed. In this case the outer electrode is extended over one flat surface of the crystal. This reduces leakage current from the front surface. The electrode can be made very thin on the planar front surface to serve as an entrance window for weakly penetrating radiations. Coaxial Configuration The configuration can also be modified to allow physical access to the central bore. This is called a well configuration. Small samples can be placed inside this well to be measured. In this geometry the source is almost completely surrounded by the detector. This can result in a very high detection efficiency. 153 154 Cryostat and Dewar Operational Characteristics Operation of a Ge detector at room temperature is impossible. The small band gap of the semiconductor would result in a large thermally induced leakage current. Therefore Ge detectors are cooled down to 77 K. This prevents leakage induced noise from degrading the energy resolution. To achieve this, the detector is brought into thermal contact with liquid nitrogen kept in an insulated dewar. 156 Radiation Detectors 39