Radiation Detectors of PIN type for X-Rays

F. J. Ramírez-Jiménez

Instituto Nacional de Investigaciones Nucleareskm. 36.5 Carretera México-Toluca, Ocoyoacac, 52045, MEXICO

e-mail: fjrj@nuclear.inin.mxInstituto Tecnológico de Toluca

Av Tecnológico S/N, Metepec, México

Abstract. In this laboratory session, tree experiments are proposed: the measurement of X-ray energy spectra from radioactive sources with a high resolution cooled Si-Li detector, with a room temperature PIN diode and the measurement of the response of a PIN diode to the intensity of X-rays of radiodiagnostic units.The spectra obtained with the Si-Li detector help to understand the energy distribution of X-rays and are used as a reference to compare the results obtained with the PIN diode. Measurements in medical X-ray machines are proposed. Low cost, simple electronic instruments and systems are used as tools to make measurements in X-ray units used in radiodiagnostic.

INTRODUCTION

X-rays are produced when accelerated electrons collide with a heavy material. A part of the kinetic energy of the electrons is converted in an X-ray photon. This process is the principle of operation of X-ray tubes where targets of molybdenum, tungsten, etc. are used. When an atom is excited by some mean, an electron of the inner shells can be disrupted from its original state. The atom needs to return to its base state and the electron vacancy is filled, the energy in excess of the atom is the difference between the initial and final state of the filling electron, this energy is liberated as characteristic X-ray photons. When the electron finalize in the K or L shell, the emission of photons generates K or L lines in the energy spectrum. This process is useful to determine the elemental composition of an unknown sample because the characteristic X-rays emitted are well known and unique for every chemical element, this technique is called X-ray fluorescence. In the nuclear decay process of an atom, electron capture and internal conversion processes lead to the generation of X-rays which are characteristic of the final element of the decay. Radioisotopes that follow this processes are sources of characteristic X-rays.

In this laboratory session, the experiments are proposed to measure the X-ray energy spectra from radioactive sources with a high resolution cooled Si-Li detector and a room temperature PIN diode. The measurement of the response of a PIN diode to the intensity of x-rays of radiodiagnostic units is also proposed. Low cost, simple electronic instruments and systems are used as tools to make measurements in X-ray units used in radiodiagnostic.

BASIC CONSIDERATIONS

The main characteristics of X-rays are the energy and the intensity. The energy is in the order of some keV’s and depends of the energy of the accelerated electrons in case of generation of X-rays through a collision process. In the case of X-ray fluorescence or nuclear decay, the energy depends of the chemical element.

The intensity In is defined as:

(1)

where: n is the number of photons per secondE is the energy of photons in keV A is the area in cm2

In an X-ray unit the intensity is [1]:

(2)

K is a constanti is the current through the tube V is the accelerating voltage applied to the tubeVK is the critical voltage, related with the binding energy of an electron in the atom m is a constant that has a value between 1.5 and 2.

In the case of the interaction of monoenergetic fast electrons with matter, when the electrons are slowed down and stopped in the target, a continuum in energy of electromagnetic radiation, bremsstrahlung, is generated. The energy spectrum [2] from an X-ray tube is shown in Fig. 1.The maximum energy of the generated photons is the energy of the accelerated electrons, then the applied voltage to the anode can be evaluated as the end energy of Fig. l. The characteristic X-rays of tungsten are also generated in the process and are shown as the peaks of 59.31 keV and 67.23 keV in the spectrum.

FIGURE 1. Theoretical energy spectrum of a X-ray unit for an accelerating voltage of 90 kV, the target is made of tungsten, a 0.5 mm Al filter is used.

Radioisotope X-ray sources generate characteristic X-rays with well defined energy peaks. This peaks are very useful as absolute energy reference points to make the energy calibration of a spectroscopy system. Fig. 2 shows the spectrum for an americium source. The number of counts registered in the spectrum depends on the activity of the source.

FIGURE 2. Measurement of an americium-241 x-ray source with a PIN diode in our set-up.

In the table 1, several sources used for energy calibration are listed with its energy values and its percentage per disintegration [3].

RADIOISOTOPERadiation Energy [keV]

(yield)Half-life

109Cd (cadmium) X- Rays

Gamma Rays

22.16 (86 %)24.94 (17 %)

88.03 (3.61 %)462.6 days

55Fe (Iron) X-Rays 5.89 (24.9 %)6.49 (3.4 %)

2.74 years

241Am (americium) X- Rays

Gamma Rays

11.9 (0.86 %)13.9 (13.2 %)17.8 (19.25 %)20.8 (4.85 %)26.35 (2.4 %)59.54 (35.9 %)

458 years

Table 1: Characteristics of radiactive sources used for calibration.

X-RAY DETECTORS

Silicon Lithium-Drifted planar detectors, Si-Li, are normally used for X-ray analysis due to its good energy resolution, typically 180 eV, and good efficiency in the energy range of 1 to 50 keV. The efficiency of Si-Li and PIN diodes is adequate to make measurements of X-rays in this range, Fig. 3.

Figure 3. Relative efficiency of a PIN diode with a thickness of 300 m.

The Si-Li X-ray detectors are the more sensitive and have the best resolution. They can distinguish a change of signal that corresponds to the movement of around 20 electrons. The transducers used in these experiments are the Si-Li detectors and silicon PIN diodes. The structure of a PIN diode is shown in Fig. 4.

Figure 4. Structure of a PIN diode.

The measurement of the radiation is based in the production of electron-hole pairs in the interaction with the detector material and the further collection of charges (see Fig. 5) [4].

Figure 5. Basic operation of semiconductor diodes as X-ray detectors.

The number of electron-hole pairs generated, is related to the incident energy , by:

(3)

where: is the energy needed to create an electron-hole pair; for silicon PIN diodes = 3.6 eV.Then the total charge , generated in the detector by the interaction is:

(4)

is the electron electric charge. Substituting eq. (3) in eq. (4) we have:

QeEw

(5)

An important conclusion from this equation is that the generated charge in the detector is directly proportional to the energy of the radiation. Due to the very small signal generated in the detector, the noise of the measuring system has to be considered with great care. The total noise in the measuring system is reflected in the value of resolution measured in the energy spectrum obtained in a multichannel analyzer as the Full Width at Half Maximum, FWHM, see Fig. 6.

Resolution ( eV ) = FWHM (6)

Figure 6. Definition of resolution.

The resolution is defined for x-ray detectors for the peak of 5.89 keV of a Fe-55 radiation source. The total noise has two components: the electronic noise related mainly with the preamplifier and the statistical component due to the detection process inside the detector.

(7)

(8)

where: F=0.12 is the Fano factor for Silicon, E is the energy of the incident photons in eV [3].

PREAMPLIFIERS

The interface between detector and preamplifier defines the characteristic of low noise of the measuring system, then for these applications the best performance of noise of the associated preamplifier is required. Special low noise preamplifiers have been developed to fulfill this requirement like the charge sensitive resistor and optical feedback preamplifiers. There are two possibilities to connect the PIN diodes with the preamplifier and the behavior of the detector depends on the used connection [5].

In Fig. 7.a, the diode is connected in photovoltaic mode, the radiation is measured as an integral effect, a current is generated from the interaction of radiation with the detector. The response is the average of the events inside the detector. This configuration is used in the experiment 3 to make measurement in x-ray units. The preamplifier converts the current to voltage. The output voltage Vo of the preamplifier is:

(9)

where:I is the current generated in the PIN diode by effect of the radiationR is the feedback resistor

Figure 7. Two different connections of PIN diodes with the preamplifier.

In Fig. 7.b, the diode is connected in reverse bias condition to create a depletion zone inside the intrinsic region, the radiation is measured as an individual effect, a charge is generated due to the interaction of radiation with the detector and the preamplifier converts the input charge to voltage. With this configuration, spectrometric measurements can be done in experiment 1. The output voltage Vo of the preamplifier is:

(10)

where: Qi is the charge generated in the PIN diode by effect of the radiationC is the feedback capacitor

An important contribution in the behavior of the PIN diode systems is the preamplifier; this must be a low noise one because the generated charges are very small. The preamplifiers have to be done with discrete components selected for low noise. The matching between the detector and preamplifier also defines the noise characteristics of the system, an optimal matching is desired for low noise. The preamplifier of Fig 7.b can be realized with several configurations, one is the conventional resistor feedback

charge sensitive preamplifiers and other is the configuration used in experiment 2, with capacitive feedback in which the capacitor is discharged through the input field effect transistor biased with the gate in forward mode 6.

MEASUREMENTS IN X-RAY UNITS

X-ray units are widely used in radiodiagnostic. Quality control of the performance of the X-ray units is important. Verification of the equipment periodically under a tests program of performance are the more important activities for the quality control to assure the good condition of the X-ray unit. X-ray units in the energy range from 20 keV to 35 keV are used for mammography and from 40 keV to 150 keV are used for chest radiography (conventional X-rays). Also there are special purpose units for dental X rays and fluoroscopy [7]. The more suitable instruments to make precise measurements of X-ray parameters, by definition, are ionization chambers specially designed for the energy of interest, and very sensitive electrometers to measure the small current or charge generated in the chambers [8]. PIN diodes can be used in the configuration of Fig. 7.a, to measure the main parameters of the X-ray units. Based in this configuration, the circuit of Fig. 8 can be implemented to measure the main parameters in digital displays [9]. Conventional electronic circuits like comparators, linear gates and ADCs are used.

FIGURE 8. Circuit for the measurement of Exposure time, high voltage and dose of X-ray units.

The main parameters to be measured in an X-ray unit to test his electrical behavior are: The waveform of the X-ray shot, the time in which the shot is on, the energy distribution of the generated X-rays, the current of the X-ray tube, the exposure to radiation and the high-voltage applied to the anode[10].

1.- The waveform of the X-ray shot can be measured as the response of the detector to the incident radiation with an oscilloscope at the output of circuit of Fig. 7.a, see Fig 9.

a) b)Figure 9. Response of the detectors to a shot of X-rays: a) Single phase unit, b) Three phase unit.2.- The time in which the shot is on, called exposure time in seconds can be determined from Fig. 9. The time response of the PIN detector to a shot of X rays is compared with the signal obtained in a Keithley 35080 kVp meter. Fig. 9.a shows the signals for a single-phase X-ray unit with 100 kV and 6 mA, B signal is from our test circuit. Fig. 9.b shows the signal for a three-phase unit with 75 kV, 200 ms, and 40 mA, signal 1 is from our test circuit.The exposure time is measured in Fig. 8 with a digital counter gated by the signal generated in the detector.

3.- Energy Distribution of the generated X-rays. It is recommended [10] to get the energy spectrum experimentally in the spectroscopy system of Fig. 13. The multichannel analyzer is used in the pulse height analysis mode in which we can see the effective energy of the X-ray beam and its energy dispersion can be determined with accuracy. The applied voltage to the anode can be obtained as the end energy of the graph (see Fig. 10). Also the effectivity of different filters to define a special energy pattern can be evaluated.

FIGURE 10. Experimental energy spectra of a X-ray unit for 90 kV, obtained with a Hp-Ge detector in a multichannel analyzer. The characteristic tungsten peaks of 59.31 keV and 67.23 keV can be seen, also the applied voltage to the anode can be evaluated as the end energy of the graph.

4.- The current of the X-ray tube i, is generally expressed as the product of current and time, in units of mA-s. The effect in the developed film depends on the current, the high voltage employed and the exposure time used [7]. The response of the PIN diode is related with the intensity of the X-rays as defined in equation (2), that means: more current more response.

5.- The exposure to radiation , is proportional to the charge produced by the radiation per unit of mass (C kg-1, mR) and is related approximately through a constant with the Absorbed dose , which is the absorbed energy per unit of mass due to radiation in units of ( J kg-1, Gy) [8] [3]. Dose delivered by the X-ray shot is related with its intensity as defined in equation (2). The number of electron-hole pairs produced in the detector is proportional to the dose applied. The current generated in the PIN diode has a linear relationship with the dose rate applied and produces a voltage signal in the preamplifier output, under controlled conditions it is a function of the high voltage and the mA-s of the employed technique. The total dose can be obtained in the output of the integrator of Fig. 8.

To verify the operation of the measuring circuit, dose measurements were made with the detectors at 50 cm from the X-ray focus and 15 cm apart from the table to reduce the dispersed X-rays and the results were compared with the results from a RadCal 20X6-6M ionization chamber and a 2020C electrometer [9]. A linear response with the tube current was obtained in the prototype as shown in Fig. 11. In this case the employed units for dose are mGy of Kerma in air.

0 50 100 150 200 250 300 3500

10

20

30

40

50

60

70

Ka

( mG

y )

mAs

22 kV 29 kV 35 kV

FIGURE 11. Response of the system to the mAs selected in the X-ray unit for different anode voltages.

6.- The high-voltage applied to the anode of the X-ray tube in kV can be measured invading the unit or in an indirect way in order not to invade the X-ray unit. The high voltage ( ) applied to the X-ray tube, is measured indirectly by using the principle of X-rays attenuation with different filters [12]. Here two PIN diodes with two different filters of aluminium and molybdenum are exposed to the X-rays (see Fig. 8). The voltage signal M obtained from the detector covered with the Mo filter, is divided by the voltage signal A obtained from the detector covered with the Al filter. The ratio A/M is independent of the X-ray current and proportional to the high voltage applied to the X-ray tube.

(11)where [kV] is a constant that depends on the experimental conditions. In Fig. 12 the response in the output of the analog divider is shown.

FIGURE 12. Pin diode response to the kV selected in the X–ray unit.

Some other characteristics that can be derived from the basic parameters and that are of interest to the radiologists are:- Efficiency of the unit (C kg-1-mA-s, Gy-mA-s or mR-mA-s), it is defined [10] at an anode voltage of 80 kV and a distance of one meter as:

or (12)

- Quality of the beam, or penetration power, measured as the Half Value Layer referred to the attenuation of aluminium filters.

EXPERIMENTAL SECTION

Equipment required:

- NIM standard bin- Digital Oscilloscope - Spectroscopy Amplifier- Cooled Si(Li) Detector- PIN diode detector- Low noise preamplifier for PIN diode- Current preamplifier for PIN diode- Dual low voltage power supply- NIM High voltage power supply- Multichannel Analyzer (MCA) inside a personal computer- Calibration sources( Cd 109, Am 241, Fe55)- Laser pointer- X-ray unit

EXPERIMENT 1.

Measurement of the energy spectra of X-ray sources with a Si(Li) detector

Experimental procedure

1.- Assemble the spectroscopy system as indicated in Fig. 13. Standard NIM modules are used.

FIGURE 13. Spectroscopy system for the measurement of the energy spectrum.

2.- Before applying the high voltage, verify for the correct POLARITY (-) in the high voltage power supply. Raise slowly the voltage to the recommended value.________________

3.- Adjust the controls of the amplifier to have a time constant between 10 and 12 s, a gain enough to have the 59.54 keV peak of Am 241 in channel 1700 of MCA used with 2048 channels.

4.- Measure the signals in the output of the preamplifier and amplifier. Adjust the pole-zero condition by seeing the signal in the output of the amplifier.

5.- Get a spectrum with two known energies ( one or two calibration sources). Accumulate at least 10 000 counts in the region of interest (ROI) under the peaks. Register the data in Table 2:

source energy channel

Table 2

6.- Calibrate the system by using the software of the MCA.

7.- Obtain the resolution of the system with a Fe-55 source._________________

8.- Get a spectrum for an Am241 source. Save the spectrum in ASCII format.

EXPERIMENT 2

Measurement of the energy spectra of X-ray sources with a PIN diode

Detector

There are special PIN diodes, commercially available to detect X-rays. In this experiment, we use a PIN photo diode, originally intended for optical fiber applications. The characteristics of this device are:

a) 300 m active thicknessb) 1 mm2 areac) 1 mm thick glass window.

The PIN detector has been characterized as X-ray detector for this application; the experimentally obtained electrical characteristics are shown in Fig. 14.

0 10 20 30 40 50 60 70

250

500

750

1000

1250

1500

1750

2000

Id

Cd

Reverse Voltage ( V )

Reve

rse

curr

ent (

pA

)

1

2

3

4

5

6

7

Capacitance ( pF )

FIGURE 14. Electric characteristics of the PIN diode.

The variation of the detector capacitance Cd and leakage current Id, as a function of reverse voltage are used to determine the operating voltage of the PIN diode. The selected point is where the reverse current does not increase abruptly and the full depletion is reached. In this case, the operating point is 65 V where the leakage current is less than 700 pA and the capacitance is less than 2 pF. From these characteristics, the PIN diode can be used at room temperature with good performance for X-ray spectroscopy, but requires low input capacitance and low noise preamplifiers as read-out circuits.The intrinsic efficiency of the PIN diode was experimentally obtained using the characteristic energies of the standard radioactive sources of Table 1. The results of these measurements are shown in Fig. 3. From this figure, it can be seen that the range of

operation with good efficiency is from a few keV, limited by the window, up to 60 keV when the relative efficiency is still around 2 %. To use the diode for lower energy X-ray measurements, the original glass window can be taken out or replaced by a 25 m beryllium foil.

Preamplifier

In order to get the minimum noise, the FBFA configuration without feedback resistor but well defined operating point and continuously discharging feedback capacitor was selected[6]. This configuration has proven to give a good noise performance when used with a silicon diode detector. In the FBFA, the gate of the front-end JFET is kept at a constant voltage by slightly forward biasing the gate-source junction at 0.24 V, by the effect of the flow of the reverse current of the detector (see Fig. 14). The reverse current of the detector plus the current from the gate junction of the JFET will flow through the source to ground (see Fig. 15). As the signal charge from the detector accumulates in the feedback capacitor , the FBFA will find an equilibrium, through the effect of the AC negative feedback, returning to the quiescent condition.

+65 V

Detector

+12 V

0.1 pF

Ct

Qi

Cf

0.045 pFQsignal

Io

Io

Id

Rt

R

TEST INPUT

Vi

OUTPUT

Vo

0

0 0

+65 V

Detector

+12 V

0.1 pF

Ct

Qi

Cf

0.045 pFQsignal

Io

Io

Id

Rt

R

TEST INPUT

Vi

OUTPUT

Vo

0

0 0

FIGURE 15. Biasing of the input FET in the FBFA.

The preamplifier of Fig. 15 in detail is shown in Fig 16, it has the following characteristics: Charge sensitive with a feedback capacitance of 0.045 pF and conversion gain of 22 mV/fC, the equivalent noise referred to the input is 20 electrons (rms).

FIGURE 16. Preamplifier of the FBFA type used in the experiment.

Experimental procedure

1.- Assemble the spectroscopy system as indicated in Fig. 13. In this section, standard NIM modules are used together with the special preamplifier developed for PIN diodes.

2.- Before applying the high voltage, verify for the correct POLARITY (+) in the high voltage power supply. Raise slowly the voltage to the recommended value of 50 V.__________________3.- Adjust the controls of the amplifier to have a time constant between 1.5 and 2 s, a gain

enough to have the 59.54 keV peak of Am 241 in channel 1700 of MCA used with 2048 channels.

4.- Measure the signals in the output of the preamplifier and amplifier. Adjust the pole-zero condition by seeing the signal in the output of the amplifier.

6.- Calibrate the system by using the software of the MCA.

source energy channel

Table 3

7.- Obtain the resolution of the system with a Fe-55 source._________________

8.- Obtain the optimum value of the time constant of the amplifier to get the best resolution. Plot the results in Fig 17.

Time constant ( s) Resolution obtained(eV)1.54610

Table 4

The resolution FWHM(eV) can be converted to equivalent noise charge ENC (electrons rms) by the relation:

(13)

FIGURE 17. Obtaining the optimal time constant.

9.- Get a spectrum for an Am241 source. Save the spectrum in ASCII format.

EXPERIMENT 3

Measurement of the characteristics of X-ray units with PIN diodes

PIN diodes have been proved to be good X-ray sensors in these applications and can be used to make low cost tools to verify the units. In this experiment a set of systems and circuits can measure the basic parameters of the X-ray units in a single shot. The systems can be implemented as a reference to compare the response of X-ray-units and to make acceptance tests by measuring the main parameters of X-ray units and also of new measuring instruments or tools as kVp meters, dose meters and waveform meters. The measurements are based in the use of the PIN diode in the photovoltaic mode as in Fig. 7.a. The preamplifier used is a current sensitive preamplifier, with a conversion gain of 20 mV/nA.

Experimental procedure

1.- Assemble the system as indicated in Fig. 18 and evaluate the “dark” current in the detector.

Figure 18. Arrangement for the measurement of the waveform and exposure time.

2.- Measure the waveform and exposure time of the X-ray shot with the digital oscilloscope, the waveform is saved in the memory of the oscilloscope. With the obtained waveform, the operation of the X-ray unit can be verified, if a failure in the energy conversion or stability of the unit happens, it is detected clearly.

2.- From the oscillogram obtained, evaluate the exposure time, compare with the settings in the x-ray unit.

3.- Measure the amplitude of the response of the PIN diode in the oscilloscope for different adjustments of the current in the tube and the high voltage. A graph similar to the shown in Fig. 11 should be obtained.

3.- Get the spectrum of the X-ray shot with the calibrated system used in experiment 2. Energy calibration of the system by using the well-known energy peaks of Am-241 or other sources is required before measurements of energy spectra from X-ray units. Use an appropriate filter to reduce the number of photons reaching the detector. Accumulate at least for 5 X-ray shots with the same technique (The same current and with the same high voltage) and the same geometrical conditions. As an example, the raw spectrum obtained in a PIN diode is shown in Fig. 19.

+

Figure 19. Energy spectrum obtained for a shot of X-rays from a General Electric, Senographe 700T mamograph with a technique of 30 kV and 4 mA-s; with a PIN diode.

QUESTIONS

1.- Compare the spectra obtained in experiment 1 and 2. Why the spectrum obtained in the experiment 1 is different from that obtained in the experiment 2 ?

2.- Why the spectrum obtained in the experiment 2 is different from the shown in Fig. 2 ?

3.- Why it is necessary to put a filter when the measurement of the energy spectrum of the X-ray unit is made?

4.- What kind of operational amplifier should be used in the realization of the preamplifier of Fig. 7.a?

Acknowledgments

The help of the Radiation Metrology Laboratory of ININ, the SIGLO XXI Hospital and the Santa Elena Hospital of Mexico City is acknowledged for the use of their X-ray units to design these experiments.

REFERENCES

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