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Luminescence measurements of X-ray absorption spectra: An application of liquid scintillation counting in synchrotron radiation spectroscopy
530
Nuclear Instruments and Methods in Physics Research A249 (1986) 530-535 North-Holland, Amsterdam
LUMINESCENCE MEASUREMENTS OF X-RAY ABSORPTION SPECTRA: AN APPLICATION OF LIQUID SCINTILLATION COUNTING IN SYNCHROTRON RADIATION SPECTROSCOPY T.K. SHAM *, R.A . HOLROYD and R.C. MUNOZ Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA
Received 3 March 1986 Simultaneous measu . n-ray absorption spectra of tetramethyltin in organic scintillator solutions using photoconductiv ity (current yield) and n _ration (luminescence yield) techniques are reported . It is found that the luminescence spectra are very similar to those obtaineu with the photoconductivity technique. The feasibility of luminescence yield X-ray absorption measurements and its applications in the development of liquid scintillation counters are discussed. 1. Introduction Many new techniques [1] in X-ray absorption measurements have been developed recently for special situations where high sensitivity is required and the direct transmission measurement is not applicable . This development largely results from the advances in synchrotron radiation technology as well as the realization of the potential applications of synchrotron radiation. For example, dispersive X-ray absorption [2], total reflection [3], X-ray excited optical luminescence [4] and photoconductivity [5,6] are among the novel techniques [1] reported very recently. Despite the variety, the basic assumption of all these techniques is that the parameter being measured is related to the absorption cross section and can be described analytically in terms of the absorption coefficient. It has been shown that this assumption is generally valid in the EXAFS portion of the absorption spectrum, although additional considerations may be needed for each individual case to obtain the precise agreement of the EXAFS amplitude [1]. In this paper, we report the application of a liquid scintillation counting technique to record the Sn K edge X-ray absorption spectra of tetramethyltin in toluene in the presence of a small amount of scintillators. This development is related to the photoconductivity measurement [5,6] reported earlier. This new technique involves the measurement of the X-ray induced luminescence from the sample which contains organic scintillators . The luminescence yield was monitored as a function of X-ray photon energy across the Sn K edge with a photomultiplier tube (PMT). In sect . 2, the experimental arrangement and procedure are discussed . In sect. 3, ' Present address: Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T. Hong Kong . 0168-9002/86/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
the experimental correlation between luminescence and ionization (photoconductivity) yield is presented and analyzed . Finally, the feasibility of the technique and its applications are noted in the summary section. 2. Experimental arrangement The basic experimental setup is very similar to that of transmission measurements . It consists of two gas ionization chambers positioned before and behind the sample and a sample chamber. The ionization chambers are used to monitor the intensity of the incoming and the transmitted photon beam . The sample chamber was a 6 inch high, - 2-1 inch i.d . cylindrical tube with a " 4 inch X 4 inch base plate, and was mounted on an adjustable platform . The chamber was equipped with two opposing 1/2 inch i .d windows in the direction of the beam and a side port perpendicular to the beam . This port was used to house the focussing lens and the Amperex 1003 PMT. Aluminum coated Mylar was used as a window material. The inside wall of the chamber was coated with graphite to reduce background from scattered light. The top of the chamber was covered with a lid which was equipped with BNC feed throughs for easy sample-cell preparation and measurement of small currents when the conductivity cell was used. Two types of measurments have been made using this arrangement . In one configuration the liquid sample (in a plastic bag made of sealable Mylar) was positioned at the center of the chamber. The thickness and the concentration of the solution were easily adjustable. In another configuration, a glass conductivity cell was used in place of the plastic bag. The cell was a parallel plate liquid ionization chamber as described previously [5,6]. The separation of the electrodes was 3
T.K Sham et al / Luminescence measurements of X-ray absorption spectra
V t
SLIT Io
IC
Fig. 1. Schematics for the experimental arrangement of concurrent measurements of current (i.) and luminescence yield (If) of liquid scintillators. Io and I are the monitors for the incoming and transmitted photon beam . V is the applied voltage. The sample chamber is not shown for clarity.
mm and the length of the cell was 2 cm . The schematics of this arrangement is shown in fig. 1 where the sample chamber that houses the sample cell-scintillation counting assembly is omitted for clarity. The latter configuration was used to measure the current yield I,, and the luminescence yield If concurrently as a function of photon energy at different applied voltages. Several organic solutions of tetramethyltin, (CH3)4Sn, were used in the experiment . One was 2,2,4-trimethylpentane (TMP) solution which was not a scintillating liquid, and two were liquid scintillators, a 1 g/1 anthracene in toluene (An/Tol) and 2 g/l 2-(1-naphthyl)-5phenyloxazole in toluene (a-NPE/Tol) solution. X-ray absorption measurements were made at the Cornell high energy synchrotron source (CHESS) using the CZ beam line . A channel-cut Si(220) crystal was used as the monochromator. A horizontal slit was used to define the beam . A narrow slit (- 0.13 ± 0.02 mm) was used for most of the simultaneous measurements of current luminescence yields so that the current collection efficiency of the cell was close to unity when a high voltage was applied across the electrodes . For luminescence measurements alone, the slit width was not critical. The photon flux incident upon the sample was within (1-5) x 10 8 photon/s throughout the experiment [7]. Photon energy was scanned across the Sn K edge in small steps (- 2 eV/step) . The output of the Ar filled ionization chambers, the conductivity cell and the PMT were fed to Keithley amplifiers, the outputs of which were converted to counts by a voltage to frequency converter (0-10 V, 1 MHz). The pulses were recorded with a CAMAC-PDP computer data acquisition system. 3. Results and discussion 3.1 . General observations
In a preliminary experiment the relative scintillation yield of two organic liquids, - 1 g/1 anthracene in hexane and - 1 g/1 anthracene in toluene were mea-
53 1
sured at several photon energies . The samples were in plastic containers with an absorption length of - 5 mm . At 9 keV, with an incident flux of - 5 x 10 8 photons/s, the PMT voltage set at 900 V and the gain of the amplifier set at 10' V/A, the luminescence count rate for the hexane solution was - 400 counts/s while the background was 300 counts/s . The count rate for the toluene solution was at least ten times better . Subsequent experiments showed that above 7 keV the luminescence yield of the An/Tol solution increased approximately linearly with photon energy. In the absence of scintillators, such as in the case of pure tetramethyltin, pure 2,2,4-TMP and 2,2,4-TMP solutions of tetramethyltin, the luminescence was negligible at the Sn K edge (- 29 .2 keV) under our experimental conditions. The above observations can be understood on the basis of the known behavior of liquid scintillators [8] and some recent reports [9]. Upon absorption of X-rays by the solution, primary photoelectrons, Auger electrons, ions and fluorescence X-rays are produced . This is followed by secondary processes in which the energetic electrons, and a fraction of the fluorescence X-rays which are reabsorbed, further ionize the medium to produce more ions and less energetic electrons. In situations where the solution contains a small amount of scintillator molecules (A), many solvent molecules (S) are left in an excited state, S* (*denotes excited state) through ionization and recombination processes. Solute excitation can occur by excitation transfer from S* to solute : S* +A->S+A*, or as a result of charge transfer :
(1)
S+ + A -> S + A+,
(2)
A + + e- -> A*,
(3)
A* -A+hv.
(4)
followed by neutralization :
These processes result in the emission of luminescence, reaction (4). Since the solvent to solute energy transfer process plays an important role in the scintillation process, organic molecules containing 7r-bonds such as toluene with long lived excited states [10] are expected to be considerably better solvents than hexane for the preparation of liquid scintillators . Liquid scintillation counting techniques for the measurement of high energy electrons emitted during radioactive decay are well established [8]. However, this technique is rarely applied to low energy X-ray [9] and soft X-ray measurements [111 . Liquid scintillation induced by X-ray absorption is basically very similar to that induced by energetic electrons except that the electrons (photoelectron and Auger electron) and the fluorescence X-rays produced by the absorption of X-ray
TK Sham et al. / Luminescence measurements of X-ray absorption spectra
532
photons have specific energy. Thus the ionization of the solution, and the luminescence yield are two important parameters to be determined in this experiment . 3.2. Luminescence yield vs ionization yield
The tin K edge XANES spectra (XANES =X-ray absorption near edge structure) of tetramethyltin in a toluene solution containing anthracene (0.5 ml (CH 3 ) 4 Sn in 20 ml of 1 g/l An/Tol solution) are shown in fig. 2 . The upper and lower spectra were simultaneously recorded in the scintillation and the photoconductivity mode respectively . There are four sets of spectra. Each of them was recorded at a different voltage across the electrodes. The slit width was the same (- 0 .13 mm) for all these measurements and other parameters were controlled so that the voltage change should be primarily responsible for the spectral changes . The experimental parameters relevant to fig . 2 are given in table 1 . Several features are noteworthy in these results. First, the conductivity spectra exhibit a drop across the Sn K edge in contrast to the conventional absorption spectrum but in agreement with previous conductivity measurements of liquids under total absorption conditions [5,6] . Second, for the same flux the count rate and the
resolution of the conductivity spectra increase as the voltage increases . Finally, the most remarkable feature is seen in the luminescence yield spectra which surprisingly exhibit the same drop across the edge and are rather insensitive to the applied voltage . We now discuss the behavior of the conductivity XANES spectra and its implication to the luminescence spectra. Let us first consider the photocurrent i s under total absorption conditions (almost all the photons incident upon the cell are absorbed). i s çan be expressed as icl e =
YIinc ( 1 - e -"` ),
where t . is the measured ionization current, e is the electronic charge . Y is the ionization yield, Iinc (1 - e-"`) is the total photon flux absorbed by the solution, lit is the total absorption coefficient of the solution. From eq . (5), Y can be determined from el,nJ1-e-,.`)
electrons . photon
(6)
The analysis of Y has already been reported [5,61, further details will be published elsewhere [12] . Here we only want to show that an inverse XANES spectrum is expected from our sample which absorbes nearly all the X-rays. Y can be expressed in terms of the yield of the absorbing metal atom Y, and that of the surrounding
980
ô O
w 960 7 .6%
W U
w 940 U W Z
920 F 1250\
2 1000 ô 750
O W
r Z 0
90%
500 250
29150
29200
29250
PHOTON ENERGY(eV)
Fig. 2 . Sn K edge XANES spectra from simultaneous measurement of an anthracene/toluene solution of tetramethyltin (see text) . The percent drop across the edge and the applied voltage are also shown . The 0 .1 and 0 .5 kV spectra were 1 s/point accumulations while the 1 kV and 3 kV spectra were obtained with 2 and 0.38 s/point accumulations respectively . See table 1 for parameters.
TK Sham et ai. / Luminescence measurements of X-ray absorption spectra
53 3
Table 1 Parameters for voltage dependent ionization and scintillation measurements of 0 .5 ml (CH3)4 Sn in 20 ml anthracene/toluene (1 g/1) solution Voltage, Photon flux b), 1_ Relative yield `) (counts) below Ratio d) of relative yield below [10 8 photon/s] and above the edge [kV] a> and above the edge If /i . below, above ic below, above If below, above 0 .1 1 .63 (1 sec) 1230, 150 987, 916 0.77 , 9 .5 0 .5 32920, 29100 1 .54 (1 sec) 985, 920 0 .033, 0 .036 1 .0 1 .60 (2 sec) 41400, 38600 983, 918 0.029, 0.030 3 .0 2.14 (0 .38 sec) 79500, 75900 1628, 1468 0.029, 0 .028 Voltage applied across the electrodes as shown in fig. 1. Calculated from the output of Io: average time of accumulation per point is listed in parentheses. `~ For idential number of photons ( -1 .6 x 108) incident upon the solution normalized to the V = 0.1 kV case, the first minimum above the edge is used for the calculation, uncertainty is - 3% . d) Calculated with background subtracted. Background counts for If are 60, 67, 53 and 100 counts/s and dark current counts for i. are 30, 54X 102, 94X 102 , 270X 102 counts/s at voltages of 0.1, 0.5, 1.0 and 3 .0 kV respectively. a) b)
solvent, YS
Y= YMf+Ys( 1 _f),
where f is the fraction of photon intensity absorbed by the metal and (1 - f) the fraction absorbed by the surrounding solvent. We now consider the change in ionization yield below and above the absorption edge. If we label the parameters with superscripts a and b we have
ia
Ya=YAfa+YS(1 -fa)= Iinc
(1 -
e- l"')
(8)
When the total absorption condition holds, [6] then II.c(1 - e- wbr ) = I,;,ß(1 - e -"a`) and the ratio of eq . (7) to eq. (8) becomes: lb _ YMf b +Ys(1 -fb) - fe) . ia Y1iifa + Ys (1 The right hand side of eq. (9) is always greater than unity [13] . The physics behind this is that some energy is lost to channels which do not produce ionization above the edge ; that is, Ymb > Yfi . Our measurements indicate Ysn = 2.OYsn at the Sn K edge in liquid tetramethyltin. There are two reasons for this difference : one is that above the edge additional fluorescent channels of X-rays are produced, some of which can escape the solution ; the other is that lower energy electrons are produced in the photoemission above the edge and these electrons produce relatively fewer ionizations [13] . The observed voltage dependence of the current yield spectrum in fig. 2 is expected [6,13,14]. At V= 110 V, the current collection efficiency is much less than unity and a large number of free ions and electrons would recombine and escape detection. This problem is more
serious above the edge where ua > ,ub. As the voltage increases the cell becomes more efficient and more ionizations are detected . This situation is analogous to the thickness effect in transmission measurements. The remarkable similarity of all the luminescence spectra is perhaps surprising . At near constant incident flux, the optical yield spectra recorded at voltages of 0.1, 0.5 and 1 kV (table 1) are essentially insensitive to the applied voltage both in the total number of counts and the percent edge drop . This observation seems to indicate that the luminescence arises primarily from the recombination of geminate ion pairs of the solvent (recombination of ions produced by Sn KLL Auger and subsequent cascade, and fluorescence X-rays). Volume recombination of free ions, which is important at low voltage, has no significant effect on the luminescence spectra. This is expected since the free ion yield is much smaller than the geminate yield. The resemblance between the luminescence and the ion spectra at high voltages further supports the proposed mechanism that geminate recombination of solvent ions with electrons dominates the scintillation. Thus we can write (10) where y is a correlation function . A preliminary test of the validity of eq. (10) has been made. In this test, the ratio of the It and i c signals (after background substraction) was calculated for points below and above the edge . The results are also listed in table 1. It can be clearly seen that y is slightly voltage dependent when the efficiency of the cell is near unity. The increase in y at very low V is qualitatively expected [15] . Further experiments are needed to qualify the nature of y and eq . (10). It is also interesting to note that the luminescence spectra [16] in fig. 2 are in contrast to the spectra expected from X-ray fluorescence detection which would give a normal, i.e . right side up spectrum .
534 0 W
T.K. Sham et at / Luminescence measurements of X-ray absorption spectra
3300F-
w --
UN
3200 w; O U
w
Z
3100
0 J^ N
v
Z ov
36001340or 3200[II 29.0
I I 29.2 29.4 29.0 PHOTON ENERGY(
Fig. 3. Sn K edge XANES spectra of tetramethyltin in aNPO/toluene solution. While the total number of photons incident for (a) and (b) are the same, the photon flux used to record spectrum (b) was three times greater than that used to record spectrum (a). 3.3. Other results
Fig. 3 shows the XANES spectra of a solution of 0.5 ml (CH3)S Sn dissolved in 20 ml a-NPO/Tol. These spectra were concurrent measurements of ionization and luminescence yield with an applied voltage of 1 kV and varying slit widths . Again similar patterns are observed. The luminescence spectra follow closely the corresponding ion spectra. When the slit was opened to give a three-fold enhancement of the photon intensity the luminescence spectrum (fig . 3) was essentially the same as before while the count rate corresponding to the ion yield spectrum (fig . 3b) dropped slight as expected . One interesting observation is that the scintillation yield of the a-NPO/Tol is at least ten times better than that of An/Tol . This result is in qualitative accord with the resonance transfer process in which the relative overlap of the absorption spectrum of the scintillator with the fluorescence spectrum of toluene (a-NPO is considerably better than anthracene) is an important factor [8]. It should also be noted that when the concentration of (CH3)4 Sn was further diluted it gave a normal, i.e. right side up conductivity and luminescence yield XANES spectrum (see ref. [12]). 4. Summary We have reported the first simultaneous measurements of luminescence and ionization yield of liquids resulting from the absorption of X-ray photons at the
Sn K edge using a scintillation counting technique. It is found that the scintillation yield measurement gives essentially the same X-ray absorption spectra as spectra measured with the photoconductivity technique under high efficiency conditions except that it is insensitive to the applied voltage . The technique appears to be feasible and can be used in polar solvents where the conductivity technique is inapplicable. Another application is to use these solutions as liquid scintillation counting devices for X-ray detectors. For example, a selected compound may be added to a scintillation liquid (An/Tol and a-NPO/Tol) so that the energy of the X-ray absorption edge of this compound overlaps with the X-ray fluorescence spectrum of the compound to be studied. This arrangement should enhance the signal of the fluorescence X-rays . Acknowledgement X-ray absorption measurements were made at the Cornell high energy synchrotron source (CHESS) which is supported by NSF. We also thank the staff at CHESS for their assistance. Research carried out at Brookhaven National Laboratory under contrast DE-AC0276CH00016 with the US Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences . References [1] EXAFS and Near Edge Structure III, eds., K.O. Hodgson, B. Hedman and J.E . Penner-Hahn (Springer, New York, [2]
[3] [4] [5]
1984).
E. Dartyge, A. Fontaine, A. Jucha and D. Sayers, ref. [1] p. 472,
T. Matsushita, H. Oyanagi, S. Saigo and H. Kihara, ref. [1] p. 476. B.A . Bunker, S.M. Heald and J. Tranquada, ref., [1] p. 482; L. Bosio, R. Cortes and M. Froment, Ref. [1] p. 484. J. Goulon, P. Tola, J.C. Brochon, M. Lemonnier, J. Dexpert-Ghys and R. Guilard, ref. [11 p. 490. T.K. Sham and S.M . Heald, J. Am . Chem. Soc. 10 5 (1983) 5142 .
T.K. Sham and R.A . Holroyd, J. Chem. Phys. 80 (1984) 1026 and ref. [1] p. 504. [7) A 6 cm Ar filled ionization chamber was used to monitor the incoming photon flux. [8) Application of Liquid Scintillation Counting, ed., D.L. Harrocks (Academic Press, New York, 1974); Luminescence Spectroscopy, ed .,M.D . Lumb (Academic Press, New York, 1978). [9] J. Goulon, P. Tola, M. Lemonnier and J. Dexpert-Ghys, Chem . Phys. 78 (1983) 347; P. Tola, A. Retournard, J. Dexpert-Ghys, M. Lemonnier, M. Pagel and J. Goulon, Chem. Phys . 78 (1983) 339. [10] Handbook of Fluorescence Spectra of Aromatic Molecules, ed . I.B. Berlman (Academic Press, New York, 1965). [6]
T.K Sham et at. / Luminescence measurements of X-ray absorption spectra [11] B .X. Yang, J. Kirz and T.K . Sham, Nucl . Instr . and Meth. A236 (1985) 419. [12] T.K. Sham and R.A. Holroyd, to be published. [13] R.A . Holroyd and T.K . Sham, J . Phys. Chem . 89 (1985) 2909 . In this work we found the yield of one 20 keV photon was greater than the yield of two 10 keV photons . Eq . (5) can be rearranged to : fa(l-YA/Ys - fb ( 1-I,, / is ib =1 + iC
I-fa0-YM/YS)
535
assuming Ys = Ys = Ysb. The numerator of the fraction must be positive since fa > fb and YM > YM . Therefore i b/a,a > 1 . [14] R.A. Holroyd and R.L . Russell, J . Phys . Chem. 78 (1974) 2128 . [15] When more ions are counted there is less likelihood that recombination may take place . [16] Similar inversion has been reported for solid Th0Z , see ref. [4].
Nuclear Instruments and Methods in Physics Research A249 (1986) 530-535 North-Holland, Amsterdam
LUMINESCENCE MEASUREMENTS OF X-RAY ABSORPTION SPECTRA: AN APPLICATION OF LIQUID SCINTILLATION COUNTING IN SYNCHROTRON RADIATION SPECTROSCOPY T.K. SHAM *, R.A . HOLROYD and R.C. MUNOZ Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA
Received 3 March 1986 Simultaneous measu . n-ray absorption spectra of tetramethyltin in organic scintillator solutions using photoconductiv ity (current yield) and n _ration (luminescence yield) techniques are reported . It is found that the luminescence spectra are very similar to those obtaineu with the photoconductivity technique. The feasibility of luminescence yield X-ray absorption measurements and its applications in the development of liquid scintillation counters are discussed. 1. Introduction Many new techniques [1] in X-ray absorption measurements have been developed recently for special situations where high sensitivity is required and the direct transmission measurement is not applicable . This development largely results from the advances in synchrotron radiation technology as well as the realization of the potential applications of synchrotron radiation. For example, dispersive X-ray absorption [2], total reflection [3], X-ray excited optical luminescence [4] and photoconductivity [5,6] are among the novel techniques [1] reported very recently. Despite the variety, the basic assumption of all these techniques is that the parameter being measured is related to the absorption cross section and can be described analytically in terms of the absorption coefficient. It has been shown that this assumption is generally valid in the EXAFS portion of the absorption spectrum, although additional considerations may be needed for each individual case to obtain the precise agreement of the EXAFS amplitude [1]. In this paper, we report the application of a liquid scintillation counting technique to record the Sn K edge X-ray absorption spectra of tetramethyltin in toluene in the presence of a small amount of scintillators. This development is related to the photoconductivity measurement [5,6] reported earlier. This new technique involves the measurement of the X-ray induced luminescence from the sample which contains organic scintillators . The luminescence yield was monitored as a function of X-ray photon energy across the Sn K edge with a photomultiplier tube (PMT). In sect . 2, the experimental arrangement and procedure are discussed . In sect. 3, ' Present address: Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T. Hong Kong . 0168-9002/86/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
the experimental correlation between luminescence and ionization (photoconductivity) yield is presented and analyzed . Finally, the feasibility of the technique and its applications are noted in the summary section. 2. Experimental arrangement The basic experimental setup is very similar to that of transmission measurements . It consists of two gas ionization chambers positioned before and behind the sample and a sample chamber. The ionization chambers are used to monitor the intensity of the incoming and the transmitted photon beam . The sample chamber was a 6 inch high, - 2-1 inch i.d . cylindrical tube with a " 4 inch X 4 inch base plate, and was mounted on an adjustable platform . The chamber was equipped with two opposing 1/2 inch i .d windows in the direction of the beam and a side port perpendicular to the beam . This port was used to house the focussing lens and the Amperex 1003 PMT. Aluminum coated Mylar was used as a window material. The inside wall of the chamber was coated with graphite to reduce background from scattered light. The top of the chamber was covered with a lid which was equipped with BNC feed throughs for easy sample-cell preparation and measurement of small currents when the conductivity cell was used. Two types of measurments have been made using this arrangement . In one configuration the liquid sample (in a plastic bag made of sealable Mylar) was positioned at the center of the chamber. The thickness and the concentration of the solution were easily adjustable. In another configuration, a glass conductivity cell was used in place of the plastic bag. The cell was a parallel plate liquid ionization chamber as described previously [5,6]. The separation of the electrodes was 3
T.K Sham et al / Luminescence measurements of X-ray absorption spectra
V t
SLIT Io
IC
Fig. 1. Schematics for the experimental arrangement of concurrent measurements of current (i.) and luminescence yield (If) of liquid scintillators. Io and I are the monitors for the incoming and transmitted photon beam . V is the applied voltage. The sample chamber is not shown for clarity.
mm and the length of the cell was 2 cm . The schematics of this arrangement is shown in fig. 1 where the sample chamber that houses the sample cell-scintillation counting assembly is omitted for clarity. The latter configuration was used to measure the current yield I,, and the luminescence yield If concurrently as a function of photon energy at different applied voltages. Several organic solutions of tetramethyltin, (CH3)4Sn, were used in the experiment . One was 2,2,4-trimethylpentane (TMP) solution which was not a scintillating liquid, and two were liquid scintillators, a 1 g/1 anthracene in toluene (An/Tol) and 2 g/l 2-(1-naphthyl)-5phenyloxazole in toluene (a-NPE/Tol) solution. X-ray absorption measurements were made at the Cornell high energy synchrotron source (CHESS) using the CZ beam line . A channel-cut Si(220) crystal was used as the monochromator. A horizontal slit was used to define the beam . A narrow slit (- 0.13 ± 0.02 mm) was used for most of the simultaneous measurements of current luminescence yields so that the current collection efficiency of the cell was close to unity when a high voltage was applied across the electrodes . For luminescence measurements alone, the slit width was not critical. The photon flux incident upon the sample was within (1-5) x 10 8 photon/s throughout the experiment [7]. Photon energy was scanned across the Sn K edge in small steps (- 2 eV/step) . The output of the Ar filled ionization chambers, the conductivity cell and the PMT were fed to Keithley amplifiers, the outputs of which were converted to counts by a voltage to frequency converter (0-10 V, 1 MHz). The pulses were recorded with a CAMAC-PDP computer data acquisition system. 3. Results and discussion 3.1 . General observations
In a preliminary experiment the relative scintillation yield of two organic liquids, - 1 g/1 anthracene in hexane and - 1 g/1 anthracene in toluene were mea-
53 1
sured at several photon energies . The samples were in plastic containers with an absorption length of - 5 mm . At 9 keV, with an incident flux of - 5 x 10 8 photons/s, the PMT voltage set at 900 V and the gain of the amplifier set at 10' V/A, the luminescence count rate for the hexane solution was - 400 counts/s while the background was 300 counts/s . The count rate for the toluene solution was at least ten times better . Subsequent experiments showed that above 7 keV the luminescence yield of the An/Tol solution increased approximately linearly with photon energy. In the absence of scintillators, such as in the case of pure tetramethyltin, pure 2,2,4-TMP and 2,2,4-TMP solutions of tetramethyltin, the luminescence was negligible at the Sn K edge (- 29 .2 keV) under our experimental conditions. The above observations can be understood on the basis of the known behavior of liquid scintillators [8] and some recent reports [9]. Upon absorption of X-rays by the solution, primary photoelectrons, Auger electrons, ions and fluorescence X-rays are produced . This is followed by secondary processes in which the energetic electrons, and a fraction of the fluorescence X-rays which are reabsorbed, further ionize the medium to produce more ions and less energetic electrons. In situations where the solution contains a small amount of scintillator molecules (A), many solvent molecules (S) are left in an excited state, S* (*denotes excited state) through ionization and recombination processes. Solute excitation can occur by excitation transfer from S* to solute : S* +A->S+A*, or as a result of charge transfer :
(1)
S+ + A -> S + A+,
(2)
A + + e- -> A*,
(3)
A* -A+hv.
(4)
followed by neutralization :
These processes result in the emission of luminescence, reaction (4). Since the solvent to solute energy transfer process plays an important role in the scintillation process, organic molecules containing 7r-bonds such as toluene with long lived excited states [10] are expected to be considerably better solvents than hexane for the preparation of liquid scintillators . Liquid scintillation counting techniques for the measurement of high energy electrons emitted during radioactive decay are well established [8]. However, this technique is rarely applied to low energy X-ray [9] and soft X-ray measurements [111 . Liquid scintillation induced by X-ray absorption is basically very similar to that induced by energetic electrons except that the electrons (photoelectron and Auger electron) and the fluorescence X-rays produced by the absorption of X-ray
TK Sham et al. / Luminescence measurements of X-ray absorption spectra
532
photons have specific energy. Thus the ionization of the solution, and the luminescence yield are two important parameters to be determined in this experiment . 3.2. Luminescence yield vs ionization yield
The tin K edge XANES spectra (XANES =X-ray absorption near edge structure) of tetramethyltin in a toluene solution containing anthracene (0.5 ml (CH 3 ) 4 Sn in 20 ml of 1 g/l An/Tol solution) are shown in fig. 2 . The upper and lower spectra were simultaneously recorded in the scintillation and the photoconductivity mode respectively . There are four sets of spectra. Each of them was recorded at a different voltage across the electrodes. The slit width was the same (- 0 .13 mm) for all these measurements and other parameters were controlled so that the voltage change should be primarily responsible for the spectral changes . The experimental parameters relevant to fig . 2 are given in table 1 . Several features are noteworthy in these results. First, the conductivity spectra exhibit a drop across the Sn K edge in contrast to the conventional absorption spectrum but in agreement with previous conductivity measurements of liquids under total absorption conditions [5,6] . Second, for the same flux the count rate and the
resolution of the conductivity spectra increase as the voltage increases . Finally, the most remarkable feature is seen in the luminescence yield spectra which surprisingly exhibit the same drop across the edge and are rather insensitive to the applied voltage . We now discuss the behavior of the conductivity XANES spectra and its implication to the luminescence spectra. Let us first consider the photocurrent i s under total absorption conditions (almost all the photons incident upon the cell are absorbed). i s çan be expressed as icl e =
YIinc ( 1 - e -"` ),
where t . is the measured ionization current, e is the electronic charge . Y is the ionization yield, Iinc (1 - e-"`) is the total photon flux absorbed by the solution, lit is the total absorption coefficient of the solution. From eq . (5), Y can be determined from el,nJ1-e-,.`)
electrons . photon
(6)
The analysis of Y has already been reported [5,61, further details will be published elsewhere [12] . Here we only want to show that an inverse XANES spectrum is expected from our sample which absorbes nearly all the X-rays. Y can be expressed in terms of the yield of the absorbing metal atom Y, and that of the surrounding
980
ô O
w 960 7 .6%
W U
w 940 U W Z
920 F 1250\
2 1000 ô 750
O W
r Z 0
90%
500 250
29150
29200
29250
PHOTON ENERGY(eV)
Fig. 2 . Sn K edge XANES spectra from simultaneous measurement of an anthracene/toluene solution of tetramethyltin (see text) . The percent drop across the edge and the applied voltage are also shown . The 0 .1 and 0 .5 kV spectra were 1 s/point accumulations while the 1 kV and 3 kV spectra were obtained with 2 and 0.38 s/point accumulations respectively . See table 1 for parameters.
TK Sham et ai. / Luminescence measurements of X-ray absorption spectra
53 3
Table 1 Parameters for voltage dependent ionization and scintillation measurements of 0 .5 ml (CH3)4 Sn in 20 ml anthracene/toluene (1 g/1) solution Voltage, Photon flux b), 1_ Relative yield `) (counts) below Ratio d) of relative yield below [10 8 photon/s] and above the edge [kV] a> and above the edge If /i . below, above ic below, above If below, above 0 .1 1 .63 (1 sec) 1230, 150 987, 916 0.77 , 9 .5 0 .5 32920, 29100 1 .54 (1 sec) 985, 920 0 .033, 0 .036 1 .0 1 .60 (2 sec) 41400, 38600 983, 918 0.029, 0.030 3 .0 2.14 (0 .38 sec) 79500, 75900 1628, 1468 0.029, 0 .028 Voltage applied across the electrodes as shown in fig. 1. Calculated from the output of Io: average time of accumulation per point is listed in parentheses. `~ For idential number of photons ( -1 .6 x 108) incident upon the solution normalized to the V = 0.1 kV case, the first minimum above the edge is used for the calculation, uncertainty is - 3% . d) Calculated with background subtracted. Background counts for If are 60, 67, 53 and 100 counts/s and dark current counts for i. are 30, 54X 102, 94X 102 , 270X 102 counts/s at voltages of 0.1, 0.5, 1.0 and 3 .0 kV respectively. a) b)
solvent, YS
Y= YMf+Ys( 1 _f),
where f is the fraction of photon intensity absorbed by the metal and (1 - f) the fraction absorbed by the surrounding solvent. We now consider the change in ionization yield below and above the absorption edge. If we label the parameters with superscripts a and b we have
ia
Ya=YAfa+YS(1 -fa)= Iinc
(1 -
e- l"')
(8)
When the total absorption condition holds, [6] then II.c(1 - e- wbr ) = I,;,ß(1 - e -"a`) and the ratio of eq . (7) to eq. (8) becomes: lb _ YMf b +Ys(1 -fb) - fe) . ia Y1iifa + Ys (1 The right hand side of eq. (9) is always greater than unity [13] . The physics behind this is that some energy is lost to channels which do not produce ionization above the edge ; that is, Ymb > Yfi . Our measurements indicate Ysn = 2.OYsn at the Sn K edge in liquid tetramethyltin. There are two reasons for this difference : one is that above the edge additional fluorescent channels of X-rays are produced, some of which can escape the solution ; the other is that lower energy electrons are produced in the photoemission above the edge and these electrons produce relatively fewer ionizations [13] . The observed voltage dependence of the current yield spectrum in fig. 2 is expected [6,13,14]. At V= 110 V, the current collection efficiency is much less than unity and a large number of free ions and electrons would recombine and escape detection. This problem is more
serious above the edge where ua > ,ub. As the voltage increases the cell becomes more efficient and more ionizations are detected . This situation is analogous to the thickness effect in transmission measurements. The remarkable similarity of all the luminescence spectra is perhaps surprising . At near constant incident flux, the optical yield spectra recorded at voltages of 0.1, 0.5 and 1 kV (table 1) are essentially insensitive to the applied voltage both in the total number of counts and the percent edge drop . This observation seems to indicate that the luminescence arises primarily from the recombination of geminate ion pairs of the solvent (recombination of ions produced by Sn KLL Auger and subsequent cascade, and fluorescence X-rays). Volume recombination of free ions, which is important at low voltage, has no significant effect on the luminescence spectra. This is expected since the free ion yield is much smaller than the geminate yield. The resemblance between the luminescence and the ion spectra at high voltages further supports the proposed mechanism that geminate recombination of solvent ions with electrons dominates the scintillation. Thus we can write (10) where y is a correlation function . A preliminary test of the validity of eq. (10) has been made. In this test, the ratio of the It and i c signals (after background substraction) was calculated for points below and above the edge . The results are also listed in table 1. It can be clearly seen that y is slightly voltage dependent when the efficiency of the cell is near unity. The increase in y at very low V is qualitatively expected [15] . Further experiments are needed to qualify the nature of y and eq . (10). It is also interesting to note that the luminescence spectra [16] in fig. 2 are in contrast to the spectra expected from X-ray fluorescence detection which would give a normal, i.e . right side up spectrum .
534 0 W
T.K. Sham et at / Luminescence measurements of X-ray absorption spectra
3300F-
w --
UN
3200 w; O U
w
Z
3100
0 J^ N
v
Z ov
36001340or 3200[II 29.0
I I 29.2 29.4 29.0 PHOTON ENERGY(
Fig. 3. Sn K edge XANES spectra of tetramethyltin in aNPO/toluene solution. While the total number of photons incident for (a) and (b) are the same, the photon flux used to record spectrum (b) was three times greater than that used to record spectrum (a). 3.3. Other results
Fig. 3 shows the XANES spectra of a solution of 0.5 ml (CH3)S Sn dissolved in 20 ml a-NPO/Tol. These spectra were concurrent measurements of ionization and luminescence yield with an applied voltage of 1 kV and varying slit widths . Again similar patterns are observed. The luminescence spectra follow closely the corresponding ion spectra. When the slit was opened to give a three-fold enhancement of the photon intensity the luminescence spectrum (fig . 3) was essentially the same as before while the count rate corresponding to the ion yield spectrum (fig . 3b) dropped slight as expected . One interesting observation is that the scintillation yield of the a-NPO/Tol is at least ten times better than that of An/Tol . This result is in qualitative accord with the resonance transfer process in which the relative overlap of the absorption spectrum of the scintillator with the fluorescence spectrum of toluene (a-NPO is considerably better than anthracene) is an important factor [8]. It should also be noted that when the concentration of (CH3)4 Sn was further diluted it gave a normal, i.e. right side up conductivity and luminescence yield XANES spectrum (see ref. [12]). 4. Summary We have reported the first simultaneous measurements of luminescence and ionization yield of liquids resulting from the absorption of X-ray photons at the
Sn K edge using a scintillation counting technique. It is found that the scintillation yield measurement gives essentially the same X-ray absorption spectra as spectra measured with the photoconductivity technique under high efficiency conditions except that it is insensitive to the applied voltage . The technique appears to be feasible and can be used in polar solvents where the conductivity technique is inapplicable. Another application is to use these solutions as liquid scintillation counting devices for X-ray detectors. For example, a selected compound may be added to a scintillation liquid (An/Tol and a-NPO/Tol) so that the energy of the X-ray absorption edge of this compound overlaps with the X-ray fluorescence spectrum of the compound to be studied. This arrangement should enhance the signal of the fluorescence X-rays . Acknowledgement X-ray absorption measurements were made at the Cornell high energy synchrotron source (CHESS) which is supported by NSF. We also thank the staff at CHESS for their assistance. Research carried out at Brookhaven National Laboratory under contrast DE-AC0276CH00016 with the US Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences . References [1] EXAFS and Near Edge Structure III, eds., K.O. Hodgson, B. Hedman and J.E . Penner-Hahn (Springer, New York, [2]
[3] [4] [5]
1984).
E. Dartyge, A. Fontaine, A. Jucha and D. Sayers, ref. [1] p. 472,
T. Matsushita, H. Oyanagi, S. Saigo and H. Kihara, ref. [1] p. 476. B.A . Bunker, S.M. Heald and J. Tranquada, ref., [1] p. 482; L. Bosio, R. Cortes and M. Froment, Ref. [1] p. 484. J. Goulon, P. Tola, J.C. Brochon, M. Lemonnier, J. Dexpert-Ghys and R. Guilard, ref. [11 p. 490. T.K. Sham and S.M . Heald, J. Am . Chem. Soc. 10 5 (1983) 5142 .
T.K. Sham and R.A . Holroyd, J. Chem. Phys. 80 (1984) 1026 and ref. [1] p. 504. [7) A 6 cm Ar filled ionization chamber was used to monitor the incoming photon flux. [8) Application of Liquid Scintillation Counting, ed., D.L. Harrocks (Academic Press, New York, 1974); Luminescence Spectroscopy, ed .,M.D . Lumb (Academic Press, New York, 1978). [9] J. Goulon, P. Tola, M. Lemonnier and J. Dexpert-Ghys, Chem . Phys. 78 (1983) 347; P. Tola, A. Retournard, J. Dexpert-Ghys, M. Lemonnier, M. Pagel and J. Goulon, Chem. Phys . 78 (1983) 339. [10] Handbook of Fluorescence Spectra of Aromatic Molecules, ed . I.B. Berlman (Academic Press, New York, 1965). [6]
T.K Sham et at. / Luminescence measurements of X-ray absorption spectra [11] B .X. Yang, J. Kirz and T.K . Sham, Nucl . Instr . and Meth. A236 (1985) 419. [12] T.K. Sham and R.A. Holroyd, to be published. [13] R.A . Holroyd and T.K . Sham, J . Phys. Chem . 89 (1985) 2909 . In this work we found the yield of one 20 keV photon was greater than the yield of two 10 keV photons . Eq . (5) can be rearranged to : fa(l-YA/Ys - fb ( 1-I,, / is ib =1 + iC
I-fa0-YM/YS)
535
assuming Ys = Ys = Ysb. The numerator of the fraction must be positive since fa > fb and YM > YM . Therefore i b/a,a > 1 . [14] R.A. Holroyd and R.L . Russell, J . Phys . Chem. 78 (1974) 2128 . [15] When more ions are counted there is less likelihood that recombination may take place . [16] Similar inversion has been reported for solid Th0Z , see ref. [4].