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On the energy of the W Kσ1 X-rays
Volume 47B, number 4
PHYSICS LETTERS
26 November 1973
O N T H E E N E R G Y O F T H E W K,~ 1 X - R A Y S W. BEER and J. KERN
Physics Department, University of Fribourg, CH-1700Fribourg, Switzerland Received 12 October 1973 It is shown that the W Ka I X-rays'emitted after the 182Ta#-decay are shifted by 1.8 ± 0.5 eV relative to "normal" W X-rays. This effect is explained by a chemical shift. In nuclear spectroscopy the energies of "/-rays are related to X-ray standards or to the electron rest mass energy moC2. Because the annihilation peak, under normal circumstances, is broadened and shifted, nuclear spectroscopists use instead the 411.794 + 0.007 keV "),-ray from the 198Au decay, which has been calibrated against it in a single but very careful experiment [1,2]. It has been proposed by Bearden [3] to use the tungsten Ka 1 line as reference wavelength standard in X-ray work. He defines a new unit, the A*, by the following relation ~(W Kt~l) = 0,209 0100 A * .
(1)
The new unit should be equal to the normal A to within 5 ppm. If this is true, we obtain for the WKot 1 energy, using the energy-wave length conversion factor E . X = 12.398 541 (41) keV. A proposed by Taylor et al. [4], E(W Kal) = 59 320.32 + 0.35 eV(6 p p m ) .
(2)
However, Taylor et al. [4] find that the A* differs from the true .~ by as much as 20 ppm, corresponding to a conversion factor of 12.398 301 (73) keVA*. This leads to E(W K,Vl) = 59 319.18 + 0.35 eV.
(3)
Several experiments have been performed to compare the W K~t1 X-ray energy with the electron rest mass energy or the 198Au 411 keV line. The results of these experiments are summarized by Kern [2]. Except for one measurement by Knowles [5], they agree with the value (3). In the present work it is shown that the production mode influences the energy of the W X-rays and that the use of a 182Ta radioactive source, a frequency procedure, yields biased results. We have measured the W Kc~1 X-rays emitted by
radioactive 182Ta and 187W sources with the Fribourg curved crystal spectrometer [6]. For the bent crystal, a 4 mm thick quartz plate has been used, with diffraction from the [110] planes. For each measurement the reflexes on the right and on the left have been observed. More details on the experimental conditions can be found in refs. [6] and [7]. The sources were obtained by neutron irradiation o f targets of natural metallic 0.05 X 4 X 35 mm 3 Ta and 0.1 X 4 X 35 mm3W. In the first case, the X-rays are emitted following K electron conversion in 182W. In the second, the X-rays are emitted from tungsten atoms which have been excited by the external photoeffect, due to the irradiation by the "y-rays emitted after the/~-decay of 187W to 187Re. The spectrometer has been calibrated by measuring the 198Au 411 keV transition up to the n = 10 order of reflexion. The details of this calibration are given in a separate paper [7]. The result is that sin0 2~ n - ~=(6
127934-+ 25) × 10 -9 .
(4)
The quoted error is the internal standard deviation, calculated in assuming an angular scale precision of 0.15'! [7]. The measurements have been done in the time sequence 198Au, 187W, 198Au, 182Ta. No time dependent effect could be detected. The results on the X.rays are reported in table 1. The largest of the internal and external errors is quoted for the mean. The energies obtained are reported in table 2. In column 3, the experimental error AE 1 corresponds to the error quoted in table 1. It involves only the statistical uncertainties on the position of the measured reflexes and the angular calibration errors. The quadratic composition of AE 1 with the experimental error in the calibration procedure (see relation (4) above), gives the relative error AE 2. The total 345
Volume 47B, number 4
PHYSICS LETTERS
Table 1 Value of h/2d obtained in the measurement of the W Ka 1 X-rays emitted by two different sources. 82 T a ~ 87W order
108 X (M2d)
order
108 x (h/2d)
1 2 3 3 4 4 5 5
4 253 990 4 254 060 4 254 041 4 254 041 4 254 001 4 254 018 4 254 002 4 253 993
1 1 2 2 3 3 5 5
4 254 056 ± '93 4 254 172-+ 140 4 254 165 -+ 82 4 254 136 ± 140 4 254 261 ± 89 4 254 158 ± 96 4 254 177 ± 89 4 254 029 ± 91
mean
4254018 ± 8.5
mean
4254 144 ± 34
-+ 59 _+30 ± 24 ± 24 ± 24 ± 20 _+21 ± 22
Table 2 W Ka~ X-ray energies, in a scale where the 198Aureference line has the energy 411.794 ± 0.007 keV. Source
W Ka 1 energy [eVl
&E1 leVI (ppm)
AE2 [eVl
AE3 leVI
182Ta
59 319.13 59 317.37
0.12 (2) 0.48 (8)
0.27 0.54
1.0 1.2
187W Difference
1.76
0.49
(8)
error AE 3 is obtained in compounding AE 2 with the energy uncertainty o f the 198Au reference line (17 ppm). We first note that the 182Ta results agree very well with both the value (3) proposed by Taylor et al. [4] and with the results from Piller et al. [6, 2], who had used a source of the same kind. Second, it is obvious that the difference between the energies obtained in the two experiments, which is over three times the combined standard deviation, must be considered real. In Bearden's compilation [3], the X-rays are supposed to be emitted by elements in their natural composition, the atoms being excited by electron or photon irradiation. In our experiment with 187W, the source should emit the " c o r r e c t " X-rays, since the natural composition is negligibly modified by the neutron capture process, and the energy of the exciting photons should not have any influence on the X-ray energies. With the 182Ta source, the X-rays are emitted only by the 182W nuclide, and moreover, in large part, when the nucleus is in an excited state with an energy 346
26 November 1973
about 1.3 MeV above the ground state. We expect that the X-ray energy should be shifted by the following processes. a) Isotope shift. This effect has been measured, for the W isotopes, by Chesler and Boehm [8]. The energy difference between 182W and natural material is of the order of 0.1 eV. b) Isometric shift. This is known to be generally small compared to the isotopic shift. In spite of the large excitation energy, the effect should be smaller than would be produced b y the addition o f two neutrons. Thus we argue that it is probably smaller than 0.1 eV. Since the two above effects are responsible for only a small part o f the observed energy difference, yet another phenomenon has to be considered. c) Chemical shift. By a study of the energy of X-rays emitted by several substances, amoung them W, in various different chemical compounds, Sumbaev et al. [9] succeeded in measuring the shifts due to the removal o f electrons in variot~ sub-shells. They also obtained the important result that the effect was cumulative. In our case, the ionization of the 182W atoms may be a consequence of the change in the electron binding energies when the nuclear charge increases in the/~ process. It is well k n o w n ( s e e e.g. Snell [10]) that a "shake-off" of electrons can result from the sudden change of nuclear charge. The emitting atom may also be pre-ionized, as a consequence o f a previous conversion process and the subsequent Auger effect, since there are several low-energy transitions in cascade. Thes processes are of statistical nature, so that the shift will be different in sign and magnitude from one event to the other. The natural line width should then be larger than normal. We observe this widening. F o r the 187W source a line width of 38 eV is obtained which is in good agreement with the expected value [11], while with the 182Ta source a broadening to 46 eV, i.e. an increase o f (21 +- 5)% (an exact unfolding has not been performed), is observed confirming our hypothesis. The present measurements show that there is an important energy difference between the W X.rays emitted after the 182Ta/3-decay and those from natural W. In the past, several authors have used 182Ta sources for the intercomparison o f energy scales. The conclusions drawn from their results should be
Volume 47B, number 4
PHYSICS LETTERS
reevaluated. This work suggests a new means of studying the atomic state of charge after a/~ or a conversion process. Our experiments should be repeated with a Cauchois spectrometer, using also a standard X-ray tube. The authors thank Professor. O. Huber for his constant encouragement and support, Professor H.J. Leisi, Drs. J. Egger and H.K. Walter for helpful discussions.
References [ 1] G. Murray, R.L. Graham and J.S. Geiger, NucL Phys. 45 (1963) 177.
26 November 1973
[2] J. Kern, Proc. Panel Meeting Charged-particle-induced radiative capture (IAEA, Vienna, 1972) in press. [3] J.A. Bearden, Revs. Mod. Phys. 39 (1967) 78. [4] B.N. Taylor, W.H. Parker and D.N. Langenberg, Revs. Mod. Phys. 41 (1969) 375. [5] J.W. Knowles, in Proc. 2rid Conf. Nuclidic masses, Vienna 1963 (Springer 1964) p. 113. [6] O. Pilier, W. Beer and J. Kern, Nucl. Instr. 107 (1973) 61. [7] W. Beer and J. Kern (to be published). [8] R.B. Chesler and F. Boehm, Phys. Rev. 166 (1968) 1206. [9] O.I. Sumbaev et al., Soviet Phys. JEPT 26 (1968) 891 and 29 (1969) 296; O.I. Sumbaev, Soviet Phys. JEPT 30 (1970) 927. [ 10] A.H. Snell, in: Alpha, beta and gamma-ray spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1965). [11] J.H. Scofield, Phys. Rev. 179 (1969) 9.
347
PHYSICS LETTERS
26 November 1973
O N T H E E N E R G Y O F T H E W K,~ 1 X - R A Y S W. BEER and J. KERN
Physics Department, University of Fribourg, CH-1700Fribourg, Switzerland Received 12 October 1973 It is shown that the W Ka I X-rays'emitted after the 182Ta#-decay are shifted by 1.8 ± 0.5 eV relative to "normal" W X-rays. This effect is explained by a chemical shift. In nuclear spectroscopy the energies of "/-rays are related to X-ray standards or to the electron rest mass energy moC2. Because the annihilation peak, under normal circumstances, is broadened and shifted, nuclear spectroscopists use instead the 411.794 + 0.007 keV "),-ray from the 198Au decay, which has been calibrated against it in a single but very careful experiment [1,2]. It has been proposed by Bearden [3] to use the tungsten Ka 1 line as reference wavelength standard in X-ray work. He defines a new unit, the A*, by the following relation ~(W Kt~l) = 0,209 0100 A * .
(1)
The new unit should be equal to the normal A to within 5 ppm. If this is true, we obtain for the WKot 1 energy, using the energy-wave length conversion factor E . X = 12.398 541 (41) keV. A proposed by Taylor et al. [4], E(W Kal) = 59 320.32 + 0.35 eV(6 p p m ) .
(2)
However, Taylor et al. [4] find that the A* differs from the true .~ by as much as 20 ppm, corresponding to a conversion factor of 12.398 301 (73) keVA*. This leads to E(W K,Vl) = 59 319.18 + 0.35 eV.
(3)
Several experiments have been performed to compare the W K~t1 X-ray energy with the electron rest mass energy or the 198Au 411 keV line. The results of these experiments are summarized by Kern [2]. Except for one measurement by Knowles [5], they agree with the value (3). In the present work it is shown that the production mode influences the energy of the W X-rays and that the use of a 182Ta radioactive source, a frequency procedure, yields biased results. We have measured the W Kc~1 X-rays emitted by
radioactive 182Ta and 187W sources with the Fribourg curved crystal spectrometer [6]. For the bent crystal, a 4 mm thick quartz plate has been used, with diffraction from the [110] planes. For each measurement the reflexes on the right and on the left have been observed. More details on the experimental conditions can be found in refs. [6] and [7]. The sources were obtained by neutron irradiation o f targets of natural metallic 0.05 X 4 X 35 mm 3 Ta and 0.1 X 4 X 35 mm3W. In the first case, the X-rays are emitted following K electron conversion in 182W. In the second, the X-rays are emitted from tungsten atoms which have been excited by the external photoeffect, due to the irradiation by the "y-rays emitted after the/~-decay of 187W to 187Re. The spectrometer has been calibrated by measuring the 198Au 411 keV transition up to the n = 10 order of reflexion. The details of this calibration are given in a separate paper [7]. The result is that sin0 2~ n - ~=(6
127934-+ 25) × 10 -9 .
(4)
The quoted error is the internal standard deviation, calculated in assuming an angular scale precision of 0.15'! [7]. The measurements have been done in the time sequence 198Au, 187W, 198Au, 182Ta. No time dependent effect could be detected. The results on the X.rays are reported in table 1. The largest of the internal and external errors is quoted for the mean. The energies obtained are reported in table 2. In column 3, the experimental error AE 1 corresponds to the error quoted in table 1. It involves only the statistical uncertainties on the position of the measured reflexes and the angular calibration errors. The quadratic composition of AE 1 with the experimental error in the calibration procedure (see relation (4) above), gives the relative error AE 2. The total 345
Volume 47B, number 4
PHYSICS LETTERS
Table 1 Value of h/2d obtained in the measurement of the W Ka 1 X-rays emitted by two different sources. 82 T a ~ 87W order
108 X (M2d)
order
108 x (h/2d)
1 2 3 3 4 4 5 5
4 253 990 4 254 060 4 254 041 4 254 041 4 254 001 4 254 018 4 254 002 4 253 993
1 1 2 2 3 3 5 5
4 254 056 ± '93 4 254 172-+ 140 4 254 165 -+ 82 4 254 136 ± 140 4 254 261 ± 89 4 254 158 ± 96 4 254 177 ± 89 4 254 029 ± 91
mean
4254018 ± 8.5
mean
4254 144 ± 34
-+ 59 _+30 ± 24 ± 24 ± 24 ± 20 _+21 ± 22
Table 2 W Ka~ X-ray energies, in a scale where the 198Aureference line has the energy 411.794 ± 0.007 keV. Source
W Ka 1 energy [eVl
&E1 leVI (ppm)
AE2 [eVl
AE3 leVI
182Ta
59 319.13 59 317.37
0.12 (2) 0.48 (8)
0.27 0.54
1.0 1.2
187W Difference
1.76
0.49
(8)
error AE 3 is obtained in compounding AE 2 with the energy uncertainty o f the 198Au reference line (17 ppm). We first note that the 182Ta results agree very well with both the value (3) proposed by Taylor et al. [4] and with the results from Piller et al. [6, 2], who had used a source of the same kind. Second, it is obvious that the difference between the energies obtained in the two experiments, which is over three times the combined standard deviation, must be considered real. In Bearden's compilation [3], the X-rays are supposed to be emitted by elements in their natural composition, the atoms being excited by electron or photon irradiation. In our experiment with 187W, the source should emit the " c o r r e c t " X-rays, since the natural composition is negligibly modified by the neutron capture process, and the energy of the exciting photons should not have any influence on the X-ray energies. With the 182Ta source, the X-rays are emitted only by the 182W nuclide, and moreover, in large part, when the nucleus is in an excited state with an energy 346
26 November 1973
about 1.3 MeV above the ground state. We expect that the X-ray energy should be shifted by the following processes. a) Isotope shift. This effect has been measured, for the W isotopes, by Chesler and Boehm [8]. The energy difference between 182W and natural material is of the order of 0.1 eV. b) Isometric shift. This is known to be generally small compared to the isotopic shift. In spite of the large excitation energy, the effect should be smaller than would be produced b y the addition o f two neutrons. Thus we argue that it is probably smaller than 0.1 eV. Since the two above effects are responsible for only a small part o f the observed energy difference, yet another phenomenon has to be considered. c) Chemical shift. By a study of the energy of X-rays emitted by several substances, amoung them W, in various different chemical compounds, Sumbaev et al. [9] succeeded in measuring the shifts due to the removal o f electrons in variot~ sub-shells. They also obtained the important result that the effect was cumulative. In our case, the ionization of the 182W atoms may be a consequence of the change in the electron binding energies when the nuclear charge increases in the/~ process. It is well k n o w n ( s e e e.g. Snell [10]) that a "shake-off" of electrons can result from the sudden change of nuclear charge. The emitting atom may also be pre-ionized, as a consequence o f a previous conversion process and the subsequent Auger effect, since there are several low-energy transitions in cascade. Thes processes are of statistical nature, so that the shift will be different in sign and magnitude from one event to the other. The natural line width should then be larger than normal. We observe this widening. F o r the 187W source a line width of 38 eV is obtained which is in good agreement with the expected value [11], while with the 182Ta source a broadening to 46 eV, i.e. an increase o f (21 +- 5)% (an exact unfolding has not been performed), is observed confirming our hypothesis. The present measurements show that there is an important energy difference between the W X.rays emitted after the 182Ta/3-decay and those from natural W. In the past, several authors have used 182Ta sources for the intercomparison o f energy scales. The conclusions drawn from their results should be
Volume 47B, number 4
PHYSICS LETTERS
reevaluated. This work suggests a new means of studying the atomic state of charge after a/~ or a conversion process. Our experiments should be repeated with a Cauchois spectrometer, using also a standard X-ray tube. The authors thank Professor. O. Huber for his constant encouragement and support, Professor H.J. Leisi, Drs. J. Egger and H.K. Walter for helpful discussions.
References [ 1] G. Murray, R.L. Graham and J.S. Geiger, NucL Phys. 45 (1963) 177.
26 November 1973
[2] J. Kern, Proc. Panel Meeting Charged-particle-induced radiative capture (IAEA, Vienna, 1972) in press. [3] J.A. Bearden, Revs. Mod. Phys. 39 (1967) 78. [4] B.N. Taylor, W.H. Parker and D.N. Langenberg, Revs. Mod. Phys. 41 (1969) 375. [5] J.W. Knowles, in Proc. 2rid Conf. Nuclidic masses, Vienna 1963 (Springer 1964) p. 113. [6] O. Pilier, W. Beer and J. Kern, Nucl. Instr. 107 (1973) 61. [7] W. Beer and J. Kern (to be published). [8] R.B. Chesler and F. Boehm, Phys. Rev. 166 (1968) 1206. [9] O.I. Sumbaev et al., Soviet Phys. JEPT 26 (1968) 891 and 29 (1969) 296; O.I. Sumbaev, Soviet Phys. JEPT 30 (1970) 927. [ 10] A.H. Snell, in: Alpha, beta and gamma-ray spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1965). [11] J.H. Scofield, Phys. Rev. 179 (1969) 9.
347