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Room-temperature mercuric iodide spectrometry for low-energy x-rays
Nuclear Instruments and Methods 193 (1982) 73-77 North-Holland Publishing Company
73
ROOM-TEMPERATURE MERCURIC IODIDE SPECTROMETRY FOR LOW-ENERGY X-RAYS* J.S. I W A N C Z Y K , * * J.H. KUSMISS, A.J. D A B R O W S K I * * , J.B. B A R T O N , G.C. H U T H University of Southern California, Medical Imaging Science Group, Marina del Rey, CA 90291, U.S.A.
T.E. E C O N O M O U and A.L. T U R K E V I C H University of Chicago, Enrico Fermi Institute, Chicago, IL 60637, U.S.A.
A discussion of the limits of energy resolution in different energy ranges is given. The energy resolution of a spectrometer is analyzed in terms of the parameters characterizing the crystal, the detector, and the amplification electronics. A high-resolution room-temperature HgI2 spectrometry system was used to measure low-energy X-ray fluorescence spectra. For the Mg K, X-ray line the measured resolution was 245 eV (Iwhm); the electronic noise linewidth of the system was 225 eV. Alpha-particles were used to excite X-ray fluorescence from low-Z elements separately or in combination. The shape of the photopeaks in the spectra is discussed.
1. Introduction
2. Charge transport in HgI2
R o o m - t e m p e r a t u r e HgI2 X-ray spectrometry has developed rapidly in the last several years [1-5]. The fast progress that has been made in achieving better energy resolution has been the result of improvements in crystal growing, in detector fabrication, and in low-noise electronics. Detector leakage currents have been reduced to values of the order of 1 p A and the contact series noise of the detector has been minimized. Studies of various types of preamplification circuits have been carried out and have resulted in significant reduction in the noise of the room-temperature system through decapsulation of the input F E T and elimination of the feedback resistor [6]. At ultralow energies the resolution is limited by the electronic noise, while at higher X-ray energies the resolution is limited by incomplete charge collection. In this paper the mechanisms which limit the energy resolution are discussed and experimental spectra measured with a roomtemperature HgI2 spectrometer are presented.
Table 1 lists charge transport parameters and their values for HgI2 at room temperature and for Ge at liquid-nitrogen temperature. From the entries in the table a comparison of the charge transport characteristics of the two materials can be made. The mobility determines the magnitude of the drift velocity of a charge carrier in a given electric field. From table 1 it can be seen that the ratio of the mobilities for electrons and holes in HgI2 is about 30, whereas for Ge the ratio is around unity. Such a disparity in the values of the mobilities for the two types of carriers in HgI2 has important consequences for charge collection. If hole collection occurs the transit time for the holes is much longer than the transit time for the electrons. For Ge the drift velocity of the carriers saturates at a value of the electric field of about 2 kV/cm. In HgI2, however, the maximum attainable drift velocity is limited by the electric field for which current injection starts to take place (rather than saturation, which occurs at above 70 kV/cm). When the transit time of cartiers starts to become comparable to the shaping time constant of the main amplifier, part of the pulse amplitude is lost. The small value of the drift velocity for holes in HgI2 thus imposes one
* This work has been supported by NASA contract NSG7535 and also by the Dept. of Energy under contract 80EV72031.001. ** On leave from Institute of Nuclear Research, 05-400 Swierk, Poland.
0029-554X/82/0000-0000/$02.75 O 1982 North-Holland
II. DETECTORS
74
J.S. l w a n c z y k et al. / Mercuric iodide spectrometry
Table 1 Charge transport parameters for Hgl2 at room temperature and for Ge at liquid-nitrogen temperature
Electron mobility ~e (cm2/V ' s) Hole mobility IZh(Cm2/V" S) Electron mobilitylifetime product (/tt'r)e ( c m 2 / V )
HgI= (300 K)
Ge (77 K)
90
3.6 ×
3
4.2 × 104
104
=
5
10 -4
> 5 X 10 -2
5 x 10 -6
> 5 x 1 0 -2
×
Hole mobilitylifetime product (/Z'r)h ( c m 2 / V )
"to the charge pulse and the efficiency will be 7(d). Conversly, 7(0) is the efficiency for pure hole collection, corresponding to the case of interaction of the incident radiation close to the positive electrode. For HgI2 7max will be nearly the same as 7 ( d ) and 7min will be exactly the same as 7(0). 7 ( d ) and 7(0) have the values 0.997 and 0.752, respectively, using values of )t e 25 cm, /~h 0.25 cm, and 0.15 cm as the detector thickness. The resulting pulse height spread is approximately 24% of the m a x i m u m pulse height. From the spectrometric point of view the influence of hole trapping on the energy resolution limits the detector thickness much more than the effect of the long transit time for the holes. The nature of the charge transport process depends on the thickness of the detector and on how much of the detector is penetrated by the incident radiation. In fig. 1 we present the mean penetration depth for X-rays as a function of energy in HgI2. The mean penetration depth is equal to the inverse of the linear attenuation coefficient; it corresponds to the thickness of detector in which 63% of the incident radiation is absorbed. Two cases of charge transport can be distinguished:
limit on the practical thickness of a detector with the m a x i m u m bias voltage applied to it. A rough calculation for HgI2 gives a m a x i m u m detector thickness of 0.15 cm for a 1/xs transit time and a drift velocity of the holes of 1.5 x 105 cm/s. The same calculation for G e gives 10 cm for a transit time of 1/x s and a carrier saturation drift velocity of 1 0 7 cm/s. The mobility-lifetime product, /.t~-, characterizes the charge collection properties for a given material. T h e drift lengths Ae =/xe~'~E and hh =/ZhrhE for electrons and holes, respectively, describe the mean distance a carrier will drift in an electric field E before it is trapped, where tz is the mobility and z is the mean trapping time. In terms of A, and Ah the charge collection effÉciency as a function of the position x of the initial ionization in the crystal is given by 7 (x) = (Ae/d)[1 - exp( - x/Ae)] + (Ah/d){1 - e x p [ - (d - x)/Ah]},
(1)
where d is the thickness of the detector. It is assumed that the length of the ionization path is negligible c o m p a r e d with the thickness of the detector. This equation is valid for the case of uniformly distributed trapping centers, a single trapping level, and no detrapping; it can be used to describe the effect of trapping on the shape of the pulse height distribution from the detector. In fact, once the efficiency is plotted as a function of the position of the initial ionization, the spread in pulse heights can be easily ascertained to be equal to the difference between 7m~ and 7~n. When the incident radiation interacts near the negative electrode, only electrons contribute
=
10q
--
I
I
I
I I I1~
~" 10-2 c~ z o ~i0-3
z .< ~10-4
10-5
E ,/ V
i
t iJJlll
i
lO ~ ENERGY (keY)
,
i Lllll
102
Fig. 1. Mean penetration depth for X-rays vs energy in HgI2.
J.S. Iwanczyk et al. / Mercuric iodid¢, spectrometry
1) Small penetration in comparison to the detector thickness: for radiation incident on the negatively biased contact the second term in eq. (1) vanishes and only electron motion contributes to the charge pulse; 2) The incident radiation interacts throughout the volume of the detector; both hole and electron collection occurs.
3. Photopeak shape and resolution Fig. 2 shows the spectrum from 2alAm taken with a room-temperature HgI2 detector. The photopeak at 59.5 keV is noticeably asymmetric and broad compared to the lower-energy X-ray peaks. The different types of charge transport behavior are reflected in the shapes and widths of the photopeaks in the spectrum. For the lowenergy peaks essentially only electron collection takes place, whereas the high-energy peak arises from both hole and electron collection from ionization events occurring throughout the volume of the detector. In this case because of the large difference between the hole and electron mobility-lifetime products the spread in pulse heights due to the spatial variation of the charge collection efficiency is the major source of broadening of the photopeak. The broadening due to the disparity in /zz products for holes and electrons affects the photopeak asymmetrically. The
Np_L a ]3.9 keV
Np-Lfl 17.7 keV
241 Am ..j 3000
H9 12 det. Bias = 850 V Temp = 295 K (det & preamp) Area = 100 rnm 2
Z < "r U 2000
'~ 59.s ~,v
PULSER
Np- L 20.8 k 'Y
Z
3,000 U
26.4 keY
Iodine X-toy e~cape
75
shape of the resulting pulse height distribution can be calculated [7-9]. Other effects which can contribute to the energy resolution include: 1) the electronic noise linewidth of the spectrometric system, which contributes the same amount at all photopeak energies in a symmetrical way; 2) the spread in the number of charge carriers produced by the incident radiation; this contribution is symmetrical and proportional to the square root of the incident energy of radiation; 3) the spread in the number of trapped charge carriers; this contributes asymmetrically to the broadening of a photopeak and is also proportional to the square root of the incident energy [101; 4) the spread in the drift lengths of carriers due to inhomogeneity of the detector material; this contribution depends on the detailed nature of the spread in A and will be proportional to the energy. The shape of the photopeak is affected by the spread in A through the dependence expressed in eq. (1) and in general will not be symmetrical. Effects (1)-(4) listed above usually do not limit the energy resolution for HgI2 at higher energies, but do limit the resolution for lower energies when only electron collection occurs. It is difficult to separate the contributions from (2), (3), and (4), since the value of the Fano factor for HgI2 is not known and neither is the form of the inhomogeneity distribution. At best only an upper limit for the Fano factor in HgI2 can be estimated from experimental results by subtracting the electronic noise linewidth from the photopeak resolution [6]. At ultralow energies the photopeak resolution should approach the electronic noise linewidth because the effects which are proportional to E m or E become negligible. For a given thickness the desired energy resolution of a detector sets a limit for the active area, since the noise linewidth depends on the capacitance. The capacitance in picofards is given by
(fro~ 59.5 kev Dea~
C = 78A/d, 0
(2)
i 100 200 C H A N N E L NUMBER
300
Fig. 2. 241Am spectrum taken with room-temperature HgI2 spectrometer.
where A is the detector area in mm 2 and d is the detector thickness in /zm. For the highest resolution the detector capacitance should be kept below 2 pF. II. D E T E C T O R S
J.S. Iwanczyk et al. I Mercuric iodide spectrometry
76
4. X R F s p e c t r o s c o p y data
The California Institute of Technology Van de Graaff accelerator provided a beam of 6.t MeV alpha-particles which were used to excite lowenergy X-ray fluorescence in various targets. Spectra were taken with a r o o m - t e m p e r a t u r e HgI2 spectrometer utilizing a pulsed-light feedback preamplifier specially designed and built for use with HgI2. The spectra are shown in figs. 3 and 4. The capability of r o o m - t e m p e r a t u r e HgI2 as an energy-dispersive detector is illustrated in fig. 3, which presents the spectrum from a standard glass rock. In fig. 4 are shown the spectra from (a) a Mg target, (b) an NaC1 target, (c) a Ti target, and (d) an Fe203 target. The system resolution measured using a pulser is also shown in fig. 3a. The p h o t o p e a k measured for the Mg K~ X-ray line at 1.25 keV is symmetrical and has a full width at half m a x i m u m of 245 eV, nearly equal to the pulser linewidth of 225 eV. It can be seen from the rest of fig. 3 that as the X-ray energy increases, the p h o t o p e a k resolution (fwhm) also increases. In additiom some asymmetry in the shape of the p h o t o p e a k starts to appear. The increasing p h o t o p e a k broadening ~: Mg Ka 1.25 keV
"6
. v
-
:,
(b]"
O
X._
Z Z < "Iu
Mg K a
O q9
.25 keV }
6F.:0K~ev
200
200 400 C H A N N E L NUMBER
and asymmetry are attributed to the statistics of charge carrier production, the statistics of trapping, and to the effects of crystalline nonuniHg 12 def. Bias = 200 V Temp = 295 K (det & preamp) Area = 4 mm2 d = 400/~m
~l~- FWHM = "~. . 2 2 5 e V I :.
Ti
.; 4.51 keV
.i :,
~
.
• (c) "
°
Fe K~ "~. 6.40 keV
300 eV
'~o .
.
.
.
.
.
j
I • ~o
_~ $
(d)
~o,~._ ,
0
, 1O0
600
Fig. 3. X-ray fluorescence spectrum for standard glass rock taken with room-temperature HgI2 detector.
FWHM = ~ 285 eV ~
"
0 u
Bias = 200 V Temp = 295 K (det. & preamp) Area = 4 mm2 d = 400/~m
Z 40O
_j j: ::
Z
STANDARD GLASS ROCK
Z Z -r 600 u
-"
cl
- ~ h W H M =2"62 keV ~. 2 4 5 e V !~! 0 :.
a
800 ._J
~ Pulser :: .:
:"
(ai "
Si Kc~ 1.74 keV
} , " .T..~...t_.....ro.....~.,~.~..r..,.. , " 200 300 400 CHANNEL NUMBER
, 500
,
k FWHM = 390 eV
:
Fe Kfl . 7.06 keV
600
700
Fig. 4. X-ray fluorescence spectra from various low-Z targets taken with room-temperature Hgl2 spectrometer.
J.S. Iwanczyk et al. / Mercuric iodide spectrometry
Hg 12 SPECTROMETER
~
600
Z
400 z O
CI
N
o 20o
si
,.i
I
I
1
2
|
I
3 4 ENERGY (keV)
l
I
5
6
Fig. 5. K, X-ray photopeak position vs energy from fluorescence spectra obtained with a room-temperature HgI2 detector.
formity. The positions of the K, X-ray photopeaks are plotted versus energy in fig. 5 to show the linearity of the room-temperature HgI2 spectrometer.
5. Conclusions
High-resolution X-ray spectrometry is possible with a room-temperature HgI2 system when only electron collection takes place (i.e., when the penetration depth of the X-rays incident on the negative electrode is small compared with the thickness of the detector). For ultralow X-ray energies the energy resolution is limited by the electronic noise linewidth of the system. At higher X-ray energies for which hole collection is still not significant, the major part of the energy resolution comes from inhomogeneities in the detector material and from the statistics of charge carrier production and of electron trapping. Once hole collection starts to become important, the energy resolution is mostly due to the spread in pulse heights due to the disparity in the values of the mobility-lifetime products for electrons and holes. At lower X-ray energies further improvements
77
in energy resolution with room-temperature HgI2 detectors can be expected with reductions in electronic noise and the growing of HgI2 crystals with better uniformity. At higher X-ray energies any improvements in energy resolution will depend on increasing the mobility-lifetime product for holes, either by producing HgI2 crystals with lower impurity content and fewer structural imperfections or perhaps by lowering the temperature of the detector. At lower temperatures the hole mobility will increase [11] but there is a danger of activating additional trapping levels. The authors are grateful to the research group at the EG & G Research Center in Santa Barbara for growing the HgI2 crystals and for fabricating some of the detectors used in this research.
References [1] W. Seibt, M. Slapa and G.C. Huth, Nucl. Instr. and Meth. 135 (1976) 573. [2] Proc. Int. Workshop on Mercuric iodide and cadmium telluride nuclear detectors, Jerusalem, Nucl. Instr. and Meth. 150 (1978). [3] A.J. Dabrowski, M. Singh, G.C. Huth and J.S. Iwanczyk, NBS Workshop on Energy dispersive X-ray spectrometry, Maryland (1979). [4] A.J. Dabrowski, G.C. Huth, M. Singh, T.E. Economou and A.L. Turkevich, Appl. Phys. Lett. 33(2) (1978) 211. [5] A.J. Dabrowski, J.S. Iwanczyk, J.B. Barton, G.C. Huth, R. Whited, C. Ortale, T.E. Economou and A.L. Turkevich, IEEE Trans. Nucl. Sci. NS-28 (1981) 536. [6] J.S. Iwanczyk, A.J. Dabrowski, G.C. Huth, A. Del Duca and W. Schnepple, IEEE Trans. Nucl. Sci. NS-28 (1981) 579. [7] R. Trammell and F.J. Walter, Nucl. Instr. and Meth. 76 (1969) 317. [8] J.S. Iwanczyk and A.J. Dabrowski, Nucl. Instr. and Meth. 134 (1976) 505. [9] T.A. McMath and M. Martini, Nucl. Instr. and Meth. 86 (1970) 245. [10] J.M. Blair and T.A. McMath, Atomic Energy of Canada, Ltd., Report AECL-3786. [11] G. Ottaviani, C. Canali and A. Alberigi Quaranta, IEEE Trans. Nucl. Sci. NS-22 (1975) 192.
II. DETECTORS
73
ROOM-TEMPERATURE MERCURIC IODIDE SPECTROMETRY FOR LOW-ENERGY X-RAYS* J.S. I W A N C Z Y K , * * J.H. KUSMISS, A.J. D A B R O W S K I * * , J.B. B A R T O N , G.C. H U T H University of Southern California, Medical Imaging Science Group, Marina del Rey, CA 90291, U.S.A.
T.E. E C O N O M O U and A.L. T U R K E V I C H University of Chicago, Enrico Fermi Institute, Chicago, IL 60637, U.S.A.
A discussion of the limits of energy resolution in different energy ranges is given. The energy resolution of a spectrometer is analyzed in terms of the parameters characterizing the crystal, the detector, and the amplification electronics. A high-resolution room-temperature HgI2 spectrometry system was used to measure low-energy X-ray fluorescence spectra. For the Mg K, X-ray line the measured resolution was 245 eV (Iwhm); the electronic noise linewidth of the system was 225 eV. Alpha-particles were used to excite X-ray fluorescence from low-Z elements separately or in combination. The shape of the photopeaks in the spectra is discussed.
1. Introduction
2. Charge transport in HgI2
R o o m - t e m p e r a t u r e HgI2 X-ray spectrometry has developed rapidly in the last several years [1-5]. The fast progress that has been made in achieving better energy resolution has been the result of improvements in crystal growing, in detector fabrication, and in low-noise electronics. Detector leakage currents have been reduced to values of the order of 1 p A and the contact series noise of the detector has been minimized. Studies of various types of preamplification circuits have been carried out and have resulted in significant reduction in the noise of the room-temperature system through decapsulation of the input F E T and elimination of the feedback resistor [6]. At ultralow energies the resolution is limited by the electronic noise, while at higher X-ray energies the resolution is limited by incomplete charge collection. In this paper the mechanisms which limit the energy resolution are discussed and experimental spectra measured with a roomtemperature HgI2 spectrometer are presented.
Table 1 lists charge transport parameters and their values for HgI2 at room temperature and for Ge at liquid-nitrogen temperature. From the entries in the table a comparison of the charge transport characteristics of the two materials can be made. The mobility determines the magnitude of the drift velocity of a charge carrier in a given electric field. From table 1 it can be seen that the ratio of the mobilities for electrons and holes in HgI2 is about 30, whereas for Ge the ratio is around unity. Such a disparity in the values of the mobilities for the two types of carriers in HgI2 has important consequences for charge collection. If hole collection occurs the transit time for the holes is much longer than the transit time for the electrons. For Ge the drift velocity of the carriers saturates at a value of the electric field of about 2 kV/cm. In HgI2, however, the maximum attainable drift velocity is limited by the electric field for which current injection starts to take place (rather than saturation, which occurs at above 70 kV/cm). When the transit time of cartiers starts to become comparable to the shaping time constant of the main amplifier, part of the pulse amplitude is lost. The small value of the drift velocity for holes in HgI2 thus imposes one
* This work has been supported by NASA contract NSG7535 and also by the Dept. of Energy under contract 80EV72031.001. ** On leave from Institute of Nuclear Research, 05-400 Swierk, Poland.
0029-554X/82/0000-0000/$02.75 O 1982 North-Holland
II. DETECTORS
74
J.S. l w a n c z y k et al. / Mercuric iodide spectrometry
Table 1 Charge transport parameters for Hgl2 at room temperature and for Ge at liquid-nitrogen temperature
Electron mobility ~e (cm2/V ' s) Hole mobility IZh(Cm2/V" S) Electron mobilitylifetime product (/tt'r)e ( c m 2 / V )
HgI= (300 K)
Ge (77 K)
90
3.6 ×
3
4.2 × 104
104
=
5
10 -4
> 5 X 10 -2
5 x 10 -6
> 5 x 1 0 -2
×
Hole mobilitylifetime product (/Z'r)h ( c m 2 / V )
"to the charge pulse and the efficiency will be 7(d). Conversly, 7(0) is the efficiency for pure hole collection, corresponding to the case of interaction of the incident radiation close to the positive electrode. For HgI2 7max will be nearly the same as 7 ( d ) and 7min will be exactly the same as 7(0). 7 ( d ) and 7(0) have the values 0.997 and 0.752, respectively, using values of )t e 25 cm, /~h 0.25 cm, and 0.15 cm as the detector thickness. The resulting pulse height spread is approximately 24% of the m a x i m u m pulse height. From the spectrometric point of view the influence of hole trapping on the energy resolution limits the detector thickness much more than the effect of the long transit time for the holes. The nature of the charge transport process depends on the thickness of the detector and on how much of the detector is penetrated by the incident radiation. In fig. 1 we present the mean penetration depth for X-rays as a function of energy in HgI2. The mean penetration depth is equal to the inverse of the linear attenuation coefficient; it corresponds to the thickness of detector in which 63% of the incident radiation is absorbed. Two cases of charge transport can be distinguished:
limit on the practical thickness of a detector with the m a x i m u m bias voltage applied to it. A rough calculation for HgI2 gives a m a x i m u m detector thickness of 0.15 cm for a 1/xs transit time and a drift velocity of the holes of 1.5 x 105 cm/s. The same calculation for G e gives 10 cm for a transit time of 1/x s and a carrier saturation drift velocity of 1 0 7 cm/s. The mobility-lifetime product, /.t~-, characterizes the charge collection properties for a given material. T h e drift lengths Ae =/xe~'~E and hh =/ZhrhE for electrons and holes, respectively, describe the mean distance a carrier will drift in an electric field E before it is trapped, where tz is the mobility and z is the mean trapping time. In terms of A, and Ah the charge collection effÉciency as a function of the position x of the initial ionization in the crystal is given by 7 (x) = (Ae/d)[1 - exp( - x/Ae)] + (Ah/d){1 - e x p [ - (d - x)/Ah]},
(1)
where d is the thickness of the detector. It is assumed that the length of the ionization path is negligible c o m p a r e d with the thickness of the detector. This equation is valid for the case of uniformly distributed trapping centers, a single trapping level, and no detrapping; it can be used to describe the effect of trapping on the shape of the pulse height distribution from the detector. In fact, once the efficiency is plotted as a function of the position of the initial ionization, the spread in pulse heights can be easily ascertained to be equal to the difference between 7m~ and 7~n. When the incident radiation interacts near the negative electrode, only electrons contribute
=
10q
--
I
I
I
I I I1~
~" 10-2 c~ z o ~i0-3
z .< ~10-4
10-5
E ,/ V
i
t iJJlll
i
lO ~ ENERGY (keY)
,
i Lllll
102
Fig. 1. Mean penetration depth for X-rays vs energy in HgI2.
J.S. Iwanczyk et al. / Mercuric iodid¢, spectrometry
1) Small penetration in comparison to the detector thickness: for radiation incident on the negatively biased contact the second term in eq. (1) vanishes and only electron motion contributes to the charge pulse; 2) The incident radiation interacts throughout the volume of the detector; both hole and electron collection occurs.
3. Photopeak shape and resolution Fig. 2 shows the spectrum from 2alAm taken with a room-temperature HgI2 detector. The photopeak at 59.5 keV is noticeably asymmetric and broad compared to the lower-energy X-ray peaks. The different types of charge transport behavior are reflected in the shapes and widths of the photopeaks in the spectrum. For the lowenergy peaks essentially only electron collection takes place, whereas the high-energy peak arises from both hole and electron collection from ionization events occurring throughout the volume of the detector. In this case because of the large difference between the hole and electron mobility-lifetime products the spread in pulse heights due to the spatial variation of the charge collection efficiency is the major source of broadening of the photopeak. The broadening due to the disparity in /zz products for holes and electrons affects the photopeak asymmetrically. The
Np_L a ]3.9 keV
Np-Lfl 17.7 keV
241 Am ..j 3000
H9 12 det. Bias = 850 V Temp = 295 K (det & preamp) Area = 100 rnm 2
Z < "r U 2000
'~ 59.s ~,v
PULSER
Np- L 20.8 k 'Y
Z
3,000 U
26.4 keY
Iodine X-toy e~cape
75
shape of the resulting pulse height distribution can be calculated [7-9]. Other effects which can contribute to the energy resolution include: 1) the electronic noise linewidth of the spectrometric system, which contributes the same amount at all photopeak energies in a symmetrical way; 2) the spread in the number of charge carriers produced by the incident radiation; this contribution is symmetrical and proportional to the square root of the incident energy of radiation; 3) the spread in the number of trapped charge carriers; this contributes asymmetrically to the broadening of a photopeak and is also proportional to the square root of the incident energy [101; 4) the spread in the drift lengths of carriers due to inhomogeneity of the detector material; this contribution depends on the detailed nature of the spread in A and will be proportional to the energy. The shape of the photopeak is affected by the spread in A through the dependence expressed in eq. (1) and in general will not be symmetrical. Effects (1)-(4) listed above usually do not limit the energy resolution for HgI2 at higher energies, but do limit the resolution for lower energies when only electron collection occurs. It is difficult to separate the contributions from (2), (3), and (4), since the value of the Fano factor for HgI2 is not known and neither is the form of the inhomogeneity distribution. At best only an upper limit for the Fano factor in HgI2 can be estimated from experimental results by subtracting the electronic noise linewidth from the photopeak resolution [6]. At ultralow energies the photopeak resolution should approach the electronic noise linewidth because the effects which are proportional to E m or E become negligible. For a given thickness the desired energy resolution of a detector sets a limit for the active area, since the noise linewidth depends on the capacitance. The capacitance in picofards is given by
(fro~ 59.5 kev Dea~
C = 78A/d, 0
(2)
i 100 200 C H A N N E L NUMBER
300
Fig. 2. 241Am spectrum taken with room-temperature HgI2 spectrometer.
where A is the detector area in mm 2 and d is the detector thickness in /zm. For the highest resolution the detector capacitance should be kept below 2 pF. II. D E T E C T O R S
J.S. Iwanczyk et al. I Mercuric iodide spectrometry
76
4. X R F s p e c t r o s c o p y data
The California Institute of Technology Van de Graaff accelerator provided a beam of 6.t MeV alpha-particles which were used to excite lowenergy X-ray fluorescence in various targets. Spectra were taken with a r o o m - t e m p e r a t u r e HgI2 spectrometer utilizing a pulsed-light feedback preamplifier specially designed and built for use with HgI2. The spectra are shown in figs. 3 and 4. The capability of r o o m - t e m p e r a t u r e HgI2 as an energy-dispersive detector is illustrated in fig. 3, which presents the spectrum from a standard glass rock. In fig. 4 are shown the spectra from (a) a Mg target, (b) an NaC1 target, (c) a Ti target, and (d) an Fe203 target. The system resolution measured using a pulser is also shown in fig. 3a. The p h o t o p e a k measured for the Mg K~ X-ray line at 1.25 keV is symmetrical and has a full width at half m a x i m u m of 245 eV, nearly equal to the pulser linewidth of 225 eV. It can be seen from the rest of fig. 3 that as the X-ray energy increases, the p h o t o p e a k resolution (fwhm) also increases. In additiom some asymmetry in the shape of the p h o t o p e a k starts to appear. The increasing p h o t o p e a k broadening ~: Mg Ka 1.25 keV
"6
. v
-
:,
(b]"
O
X._
Z Z < "Iu
Mg K a
O q9
.25 keV }
6F.:0K~ev
200
200 400 C H A N N E L NUMBER
and asymmetry are attributed to the statistics of charge carrier production, the statistics of trapping, and to the effects of crystalline nonuniHg 12 def. Bias = 200 V Temp = 295 K (det & preamp) Area = 4 mm2 d = 400/~m
~l~- FWHM = "~. . 2 2 5 e V I :.
Ti
.; 4.51 keV
.i :,
~
.
• (c) "
°
Fe K~ "~. 6.40 keV
300 eV
'~o .
.
.
.
.
.
j
I • ~o
_~ $
(d)
~o,~._ ,
0
, 1O0
600
Fig. 3. X-ray fluorescence spectrum for standard glass rock taken with room-temperature HgI2 detector.
FWHM = ~ 285 eV ~
"
0 u
Bias = 200 V Temp = 295 K (det. & preamp) Area = 4 mm2 d = 400/~m
Z 40O
_j j: ::
Z
STANDARD GLASS ROCK
Z Z -r 600 u
-"
cl
- ~ h W H M =2"62 keV ~. 2 4 5 e V !~! 0 :.
a
800 ._J
~ Pulser :: .:
:"
(ai "
Si Kc~ 1.74 keV
} , " .T..~...t_.....ro.....~.,~.~..r..,.. , " 200 300 400 CHANNEL NUMBER
, 500
,
k FWHM = 390 eV
:
Fe Kfl . 7.06 keV
600
700
Fig. 4. X-ray fluorescence spectra from various low-Z targets taken with room-temperature Hgl2 spectrometer.
J.S. Iwanczyk et al. / Mercuric iodide spectrometry
Hg 12 SPECTROMETER
~
600
Z
400 z O
CI
N
o 20o
si
,.i
I
I
1
2
|
I
3 4 ENERGY (keV)
l
I
5
6
Fig. 5. K, X-ray photopeak position vs energy from fluorescence spectra obtained with a room-temperature HgI2 detector.
formity. The positions of the K, X-ray photopeaks are plotted versus energy in fig. 5 to show the linearity of the room-temperature HgI2 spectrometer.
5. Conclusions
High-resolution X-ray spectrometry is possible with a room-temperature HgI2 system when only electron collection takes place (i.e., when the penetration depth of the X-rays incident on the negative electrode is small compared with the thickness of the detector). For ultralow X-ray energies the energy resolution is limited by the electronic noise linewidth of the system. At higher X-ray energies for which hole collection is still not significant, the major part of the energy resolution comes from inhomogeneities in the detector material and from the statistics of charge carrier production and of electron trapping. Once hole collection starts to become important, the energy resolution is mostly due to the spread in pulse heights due to the disparity in the values of the mobility-lifetime products for electrons and holes. At lower X-ray energies further improvements
77
in energy resolution with room-temperature HgI2 detectors can be expected with reductions in electronic noise and the growing of HgI2 crystals with better uniformity. At higher X-ray energies any improvements in energy resolution will depend on increasing the mobility-lifetime product for holes, either by producing HgI2 crystals with lower impurity content and fewer structural imperfections or perhaps by lowering the temperature of the detector. At lower temperatures the hole mobility will increase [11] but there is a danger of activating additional trapping levels. The authors are grateful to the research group at the EG & G Research Center in Santa Barbara for growing the HgI2 crystals and for fabricating some of the detectors used in this research.
References [1] W. Seibt, M. Slapa and G.C. Huth, Nucl. Instr. and Meth. 135 (1976) 573. [2] Proc. Int. Workshop on Mercuric iodide and cadmium telluride nuclear detectors, Jerusalem, Nucl. Instr. and Meth. 150 (1978). [3] A.J. Dabrowski, M. Singh, G.C. Huth and J.S. Iwanczyk, NBS Workshop on Energy dispersive X-ray spectrometry, Maryland (1979). [4] A.J. Dabrowski, G.C. Huth, M. Singh, T.E. Economou and A.L. Turkevich, Appl. Phys. Lett. 33(2) (1978) 211. [5] A.J. Dabrowski, J.S. Iwanczyk, J.B. Barton, G.C. Huth, R. Whited, C. Ortale, T.E. Economou and A.L. Turkevich, IEEE Trans. Nucl. Sci. NS-28 (1981) 536. [6] J.S. Iwanczyk, A.J. Dabrowski, G.C. Huth, A. Del Duca and W. Schnepple, IEEE Trans. Nucl. Sci. NS-28 (1981) 579. [7] R. Trammell and F.J. Walter, Nucl. Instr. and Meth. 76 (1969) 317. [8] J.S. Iwanczyk and A.J. Dabrowski, Nucl. Instr. and Meth. 134 (1976) 505. [9] T.A. McMath and M. Martini, Nucl. Instr. and Meth. 86 (1970) 245. [10] J.M. Blair and T.A. McMath, Atomic Energy of Canada, Ltd., Report AECL-3786. [11] G. Ottaviani, C. Canali and A. Alberigi Quaranta, IEEE Trans. Nucl. Sci. NS-22 (1975) 192.
II. DETECTORS