Fig. 5.5.2: The detection efficiency ofthe XR-100T silicon detector [47].
5.6. SAFETY AND PROTECTIVE DEVICES
In order to provide for the safety and protection from radiation of personnel operating the spectrometer, in which the cold cathode X-ray tube was tested, a number of safety measures had to be taken. The lead lined aluminium chamber (see Figure 5.1), into which the chemical sample to be analysed was placed, had the function of absorbing any harmful X-radiation emanating from the tube that could escape to the outside environment. The chamber door was fitted with an interlock switch SW2 (see Figures 5.1 and 5.6) which only closes when the chamber itself is closed. With SW2 open the supply voltage of 30 Volts is removed from the safety interlock device (see Figure 5.6) thereby disabling the relay switch SRI, which, when closed, connects the spectrometer to the 220 Volt mains line. A further safety feature in the interlock safety device is lamp L1, which merely serves to indicate or warn that the spectrometer is in operation and that therefore X-rays are produced. With Ll dysfunctional or removed from the circuitry the base current of TI is removed resulting in 0 Volts being applied to the negative input of OPI, which serves as a comparator. This in tum results in a comparator output of 0,8 Volts which is not enough to tum on transistor T2, which requires a base voltage V B,T2 of at least V Dl + V D2
=
1.4 Volts for operation. In this way no current can flow through the coil of the relay switch SRI and so SRI remains open thus keeping the spectrometer effectively disconnected from the 220 Volt mains line.+30V
N220 VAC mains
SW1 SW2
L1
R1
R3 10k
2.2k
t--C==J---i T1
R4 8.2k
R2 BC337 3.9k R5 3.9k Sl/V
Fig. 5.6: The interlock safety device.
Uk R6 02
03 01 1N4001
T2
2N3440
to spectrometer mains supply
In order to shield the electronic circuitry of the spectrometer from the electromagnetic impulses produced by the power supply it was necessary that the power supply be placed inside an aluminium shield of 2.2 cm wall thickness. According to White [53] aluminium has the property of shielding electromagnetic pulses only in certain frequency intervals for a given aluminium shield thickness. Therefore aluminium of 2.2 cm thickness will shield frequencies given in Table 5.1 below.
Table 5.1: Electromagnetic impulse frequencies between 30 Hz and 300 kHz, at which 2.2 cm thick aluminium can be used as an electromagnetic shield [53].
Shielding Frequency (Hz) 60
110 190 305 600 1000 1580 3300 6000 10000
In Table 5.1 it can be seen that 2.2 cm thick aluminium can be used for electromagnetic shielding at an impulse frequency of 110Hz, which closely corresponds to the repetition rate of III Hz of the output voltage waveform produced by the X-ray power supply (see Section 3.l.2, Chapter 3).
5.7. SUMMARY
In this chapter the spectrometer in which the cold cathode X-ray tube was set up for testing is described. As the main concern of this study is the performance of the X-ray tube for XRF spectrometry, the spectrometer was used primarily for the characterisation of the X-ray tube.
After presenting a method for the measurement of the characteristic X-ray tube currents and voltages, a method for determining the photoefficiency of the cold cathode of the X-ray tube was illustrated in Section 5.3. It was shown that for the particular case of the X-ray tube the photoefficiency of the MgO cold cathode could be expressed as the ratio of the average photocurrent and the total current of the X-ray tube, which consists of both a photocurrent and a field current. In the measurement of the photoefficiency of the X-ray tube it was assumed that the average diode current was representative of the field emitted current in the X-ray tube. Thus it was possible to establish a value for the average photocurrent, which is the difference between the total average current and the average field current of the X-ray tube, and consequently the photoefficiency. A very important X-ray tube parameter is the intensity distribution of the primary radiation produced by an end-window X-ray tube. Since the radiation could not be measured directly at the X-ray tube window with the XR-I00T and XR-I00T-CZT detectors as this approach would have resulted in the detectors being saturated and possibly even damaged, a Geiger counter was used instead (see Section 5.4). Finally the tube was tested as an X-ray source in a spectrometer by detecting and analysing the spectra produced from samples irradiated by the tube. In Section 5.5 the functions of the XR-I00T-CZT and the XR-lOOT X-ray detector systems, in conjunction with the 'AMPTEK MCA 8000A' multichannel analyser to detect and evaluate characteristic X-ray spectra from samples, were discussed. In Section 5.5.1 the reader was introduced to the XR-l OOT -CZT detector for high-energy measurements of X-ray and gamma ray spectra, while in Section 5.5.2 a brief description of the XR-I00T silicon detector for higher energy resolution X-ray spectra measurements was given. Finally the safety features of the spectrometer were discussed, namely the safety interlock device for the prevention of operating the spectrometer with an open X-ray chamber, which would have allowed for hazardous radiation to escape to the environment, as well as the EMI shielding chamber which had the function of protecting electronic circuitry from potentially damaging electromagnetic impulses produced by the X-ray power supply (see Section 5.6).
6. Experimental Results
6.1. THE X-RAY TUBE CURRENT AND VOLTAGE CHARACTERISTICS
The measured current and corresponding voltage characteristics of the X-ray tube are shown in Figure 6.1 and Figure 6.2 respectively.
2100rv' r -300I§ 5.00~/ liD f2 RUN
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~. . . . . ..
~.. ... . :
~.. ... ... .
· . . . . . . . .
· : : . : : : : :
......... t ........
·1 ··· · · .... , ... . ... : ... . . . .. f .. .. .... ; .. ... :. . .
~.. .. .. .. ': . ... . ... .
. : .. ... .. .. ; ... ... ... ; .... ... . + .. ... ; ... .... : . .... .... .. .
. . : 1 i :
Vmax(2)=290.7mV Vavg(2)-39.48mV
Fig.6.1: The voltage drop across resistor Rl (100) which was measured to establish the current characteristics of the X-ray tube.
The waveform in Figure 6.1 was obtained by measuring the voltage drop across resistor RJ (see Section 5.2, Chapter 5), which has a resistance of 10 O. Therefore, with a maximum voltage drop of290.7 mV and an average voltage drop of 39.48 mV measured across RJ, it was found that the maximum tube current iMAX, X-RAY TUBE flowing through the tube was equal to 29.68 rnA while the average current iAVE. X·RAY TUBE was found to be equal to 3.948 rnA. In Figure 6.2 on the other hand a waveform was obtained for the voltage across the tube by measuring the voltage across a proportional resistor divider network (see Figure 5.2, Chapter 5), where 1 V displayed on the oscilloscope corresponded to 40 kV measured across the tube. Therefore, with a maximum voltage V:~u.¥. R2
=
2.125 V and an average voltage VAVE. R2=
0.327 V measured across the measuring resistor R2 (see Figure 5.2, Chapter 5), it can be seen, that the maximum voltage developed across the X-ray tube VMAX. X.RAYTUBE was 85 kV, while the actual average voltage VAVE. X-RAY TUBE was found to be equal to 13.08 kY. Therefore the average power PAVE, which is the product of the average tube current iAVE. X·RAY TUBE and the average output voltage VAVE. X-RAY TUBE(see Section 3.1.2, Chapter 3) was equal to approximately 52 Watts.
2 1.00V .0.005 5 . OO~/ III TV L 2 RUN
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: : i
Vmax(2)-2.125 V Vavg(2)-0 .327 V
Fig.6.2: The voltage drop across resistor R2 of the proportional voltage divider network, where 1 V corresponds to 40 kV discharged across the tube.
6.2. DETERMINATION OF THE PHOTOEFFICIENCY OF THE MgO COLD CATHODE The photoefficiency of the X-ray tube was determined using the measurement method described in Section 5.3 in Chapter 5. In order to establish the photoefficiency of the X-ray tube the average current flow through the X-ray tube was compared to that of a diode, which has the same structure as the X-ray tube, but does not contain the tungsten target at which the current enhancing X-radiation is produced. Using the diode it was thus possible to determine a value for the maximum and average field current, namely imax.jield and iave.jield respectively, flowing through the X-ray tube. The measured current through the X-ray tube is illustrated in Figure 6.1, where the average current iAVE. X-RAY TUBE is equal to 3:948 rnA (see Section 6.1). As in Figure 6.1 the waveform in Figure 6.4 was obtained by measuring the voltage drop across the 10
n
measuring resistor R1, which, as in the previous case of the X-ray tube (see Section 5.2, Chapter 5), was placed in series with the diode. An average voltage drop of 37.58 mV was measured across RI (see Figure 6.4) corresponding to an average diode current iAVE. DIODE=
iave./ield=
37.58 mV / 10n
=
3.758 rnA. Similarly a maximum diode current iM4.\: DIODE, or alternatively imax,jield, of 19.69 rnA was measured. The photoefficiency of the X-ray tube relative to the diode was finally determined by substituting the measured values for iAVE.X-RAY TUBE and i,./VE. DlODE into equation 5.1 (see Section 5.3, Chapter 5):h t ffi · iA1'E.x-RAYTUBE - iAVE.DJODE 100%
P 0 oe clency
= .
0=
i AVE.X-RAYTUBE
3.948 rnA - 3.758 rnA
- - - - -- - - ·100%
=
4,81%:3.948 rnA
The waveform of the voltage drop across the 10 .Q measuring resistor R\ developed during the operation of the diode is shown in the figure below:
2 100'V' .--3001§ 5.00~/ l1li :f2 RUN
... .. : ... .. : .. ...
~... ... . \ ... ... ± r ...
~.... .. .. ·l · . ... .. . ·l· .. ... . : . . ... .. . .
.
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~. :~i~ .~.~..
~..
~. hi·~· .~..
~..
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. .
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r ••••••••• : ••••••··. i · .. ••••• ! •••• · .•• ; . . •• • ••• : ..•. •. ••• : •• ••••••• : . •••• ••••
Vmax(2)-196.9mV Vavg(2)-37.58mV
Fig.6.3: The voltage drop across resistor Rl (10 Q) during the operation ofthe diode, which was used to establish the field current flowing through the X-ray tube.
6.3. MEASUREMENT OF THE PRIMARY RADIATION INTENSITY DISTRIBUTION OF