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A radiation standard for the vacuum ultraviolet
J. Quant. Specrrosc.
Rodia~. Transfer.
Vol. 9. pp. 1407-1418.
Pergamon
Press 1969. Printed
in Great Britain
A RADIATION STANDARD FOR THE VACUUM ULTRAVIOLET* J. C. MORRISand R. L. GARRKON Aerophysics Laboratory, Space Systems Division, Avco Corporation Wilmington, Massachusetts 01887 (Received
17 September
1968)
Abstract-A constricted dc argon arc to which nitrogen or oxygen has been added has been calibrated as a radiation standard for the vacuum ultraviolet. The calibration data are given for the strongest atomic lines in the wavelength range of 18004000 A. Operation for calibrated output of continuum or blackbody radiation down to 600 A is given for a helium or neon arc using hydrogen or argon as the radiating gas. The source has been found to be stable and reproducible, thereby allowing use of the comparison method to measure the number of photons incident on the entrance slit of a spectrometer. I. IWTRODUCTION
interest in vacuum ultraviolet studies in astronomy and plasma physics has increased the demand for more reliable and direct measurements of the photon flux from stellar atmosphere or a laboratory plasma. Of the techniques available for making this measurement the comparison method is the most direct ; however, it has not received much use because of the lack of a suitable standard radiationsource. For wavelengths between 600 and about 18OOA, the dc constricted arc described in this paper shows good promise as a comparatively easy-to-use high-quality radiation standard.‘1*2) Similar arcs have proven their reliability as standard tools for studies of line-transition probabilities, recombination and Bremsstrahlung radiation cross sections, and electrical and thermal conductivities. These arcs are not difficult to use ; they are stable and can be made to radiate as high-quality blackbodies or with an output which may be predicted by using Planck’s law and a measured or calculated emissivity. In this paper, we present intensity calibration and operational data for a constricted arc generator for selected wavelengths between 1800 and 1100 A. The required operation is described for obtaining calibrated output of continuum or blackbody radiation to wavelengths as short as 600 A. INCREASED
II. PREDICTING
RADIATION
FROM
A CONSTRICTED
ARC
The radiation emitted by plasmas which are in thermal equilibrium may be determined from Planck’s law modified by the inclusion of the emissivity of the gas : I,, = @T)X’(exp
C2/1T-
l)-‘C,
(1)
l The research reported herein is part of the research program on radiation from arc-heated plasma of the Aerospace Research Laboratories, Office of Aerospace Research of the U.S. Air Force, whose support is acknowledged. (Contract #F33(615)-68-C-1081.)
1407
J. C. MORRIS and R. L. GARRISON
1408
where Zbb= radiant flux per unit area per unit increment of wavelength, s(AT) = the emissivity of the source, C1 = 3.7413 x 10-r’ watts cm’, A = wavelength of emitted radiation in cm, C2 = 1.4388 cm-OK, T = temperature in “K. To make use of an arc as a radiation standard, it is necessary to know both the emissivity and the temperature of the gas and to choose a plasma configuration which will produce the desired radiation from an isothermal environment. The latter condition is very important for a high-quality radiation standard, as is clearly evident from the appearance of Tin the exponent of Planck’s law. In the following paragraphs, each of the preceding requirements is discussed and it is shown how they are achieved with the dc-constricted arc. III.
DETAILS
OF
THE
DC-CONSTRICTED
ARC
RADIATION
SOURCE
Figure 1 shows the essential details of the arc generator and gas-flow system. The generator is made after the Gerdin type. It consists of a series of water-cooled copper washers, 0.300 in. thick with an inside diameter of 0.187 in. The sections are separated from each other by OUO in., and they are also insulated from one another, except for the residual resistance from the water-cooling system which comes from a common supply. Rubber gaskets are inserted between the sections to serve as insulators and as gas seals to isolate the arc column from the atmosphere. Because we desired to study the arc in some detail, we inserted windows into these gaskets, which allowed side-on intensity measurements to be made between any two sections or in the electrode regions. If the arc is to be used solely as a standard source, these gaskets could be replaced with “0” rings. The only critical component of the arc affecting stable operation is the cathode. The arc runs best if the cathode is allowed to reach incandescence at a single point. For stable operation, a 2 per cent thoriated tungsten tube-about 8 in. long with a& in. inside diameter and a J$ in. wall is satisfactory. Figure 1 shows the details of construction and mounting of this cathode to the end copper section. The power supply for the arc is dc and has an open-circuit voltage of about 180 V. A ballast resistor is placed in series with the arc to control the current. To start the arc, the ballast resistor is set to a value which would give a current of 10-20 amp. The arc is started by inserting a tungsten rod connecting the anode to the cathode and then drawing the arc out between the electrodes. The striker is made from a & in. tungsten rod mounted to an insulating handle. With the arc running, the ballast resistor is adjusted to give an arc current of 100 amps. The operating voltage for an eleven-section arc is about 100 V. The arc is run at atmospheric pressure to achieve a high concentration of radiating and absorbing species, and also to ensure that the plasma is in thermal equilibrium. The radiation from the arc is allowed to enter the spectrometer through a set of five apertures with openings of 0.013 in. in diameter. The apertures are made from & in. metal plates. They are spaced about $ in. apart and are placed near the end of the arc so that only radiation emitted from the axis, or very near to the axis of the arc, can pass through them and into the spectrometer. These apertures also serve as baffles for a differential pumping system, therefore permitting the arc to operate at pressures higher than the viewing spectrometer. The pump for the differential chamber closest to the arc had a speed of 20 ft3/ min at 1 torr and was capable of maintaining this chamber at 1 torr with the arc at 1 atm. Other sections of the chambers were connected to pumps with speeds of about 1.7 ft3/min at 1 torr. This procedure allowed the spectrometer to reach a pressure of 10v6 torr.
A radiation standard for the vacuum ultraviolet
-0’
c------__-
(
,c_--_-_
--
k
/_________/’
_--
4
.
J. C. MORRIS and R. L. GARRISON
1410
The relative temperature distribution along the arc axis is a constant over most of the axis, except at the electrodes where the arc terminates and it drops to the wall temperature of the generator. It is because of this end effect that a transparent noble gas is selected as the running medium. Returning to Fig. 1, one sees that premixed radiating and nobie gases are directed into and out of the center sections of the generator. Flows to the various inlets and outlets are limited to between 4 and 8 ft3/hr at l-atm pressure. The absolute flow rates are not important; however, the relative flows should be monitored to a precision of 5-10 per cent. The flows are adjusted so that the combined flow of the noble gases into the electrode regions is slightly greater than that of the mixture into the center. The exits are adjusted for equal output, taking care not to restrict the flow sufficiently to increase the pressure in the arc chamber. With this arrangement of flows to inlets and outlets, the radiating gas appears only in the isothermal center regions of the arc column and can be kept from diffusing into the cooler ends of the arc. The resulting axial density distribution of the test gas is also shown in Fig. 1. With this operating arrangement, the noble gas acts as a nonradiating, transparent heating element for the radiating gas. Calibrations of this radiation standard have been made using argon as a heating plasma with nitrogen and oxygen acting as the radiating element. The concentration of the nitrogen or oxygen is made low enough so that it does not significantly alter the temperature distribution established in the argon arc ; however, the concentration is made high enough so that the plasma is optically thick over a comparatively large region of the center of many of the atomic lines. The temperature of this standard is a function of arc current, pressure, and arc diameter. Typically, a & in. diameter, 100 amp, 1-atm-pressure argon arc has an axial temperature of 13 600°K. TABLE 1. NUMBEROF ATOMSAS FUNCTIONS OF THE NUMBER OF ARC SECTIONS AND INLET GASMIXTURES Concentration of N, in argon Number of sections
05 per cent
2.5 per cent
2 3 4
5.27 x 1Or5 7.91 x 10’5 1.05 x 10’6
2.59 x 1016 3.89 x 10’6 5.18 x 1Or6
2 3 4
Concentration of 0, in argon 6.09 x 1Or5 3.0 x 10’6 914 x 1or5 4.50 x 10’6 1.21 x 10’6 6.00 x 1Or6
IV.
TEMPERATURE
MEASUREMENTS
OF
5.0 per cent 5.05 x 1016 atoms/cm’ 7.57 x 10’6 1.01 x 10” 5.84 x 1Or6 8.75 x 10’6 1.17 x 10”
THE
UV
SOURCE
The temperature of the constricted arc was measured from the absolute intensity of two argon lines : one, the atomic line at 4300 8; the other, the ionic line at 4806 A. The relation for the intensity of radiation emitted from an optically thin line, with a bandpass whose limits are A1 and AZ,is
1411
A radiation standard for the vacuum ultraviolet
where I = intensity (watts/cm3-sr), A = transition probability (set-l), g, = statistical weight of upper level, Ui = partition function of species i, Ni = concentration of species i (number/cm3), E, = energy of upper level, T = temperature, A = wavelength of the line, h, c, k = the usual constants, and L(1) is the line shape which must be integrated over the bandpass limits. The shapes of the-lines were found to be Lorentzian with half-widths of O-1 and 0*0148fw/N3 x 10’6[A-cm3] for 4300 AI and 4806 AII, respectively.‘3’ The transition probabilities were 3.6 x 10’ set- l and 1.1 x lOa set- l, respectively. The A-values represent a choice which gives the most consistent agreement with data obtained in three investigations by POPENOE and SHUMAKER,(~) RICHTER”) and the present authors. These experimental data covered pressures ranging from 05 to 5.0 atm and used both end-on and side-on viewing techniques for arcs of different sizes and power inputs. Temperatures for the radiation standard were determined from both side-on and endon intensity measurements of the arc column. The temperature as a function of radius for the 100 amps argon arc is shown in Fig. 2. At this arc current, the presence of nitrogen did not alter the measured temperatures. The precision of the temperature measurements 14-
I
12-
6-
ARC
CONSTRICTOR
_I
DIAMETER
I 4-
2-
O-
I
1 3
I
I 2
I
I
0 ARC
I
2
3
RADIUS.mm
FIG. 2. Radial temperature distribution of a lOO-amp,4.7-mm diameter and atmospheric pressure arc.
1412
J. C. MORRISand R. L.
ck4RIUSON
was f 100°K at 13 600°K; considering errors in A values and intensity measurements, the absolute accuracy is estimated to be within +2OO”K. This error estimate gives an absolute accuracy of f20 per cent in the predictions of the blackbody radiation at 1000 A. The temperature as a function of arc current at 100 amps changes approximately 0.2 per cent for a 1 per cent change in arc current, thereby showing the insensitivity of the standard to power input. V. SPECTRAL
DISTRIBUTION
OF RADIATION
EMITTED
BY THE
SOURCE
Figure 3 shows a spectral scan from this radiation source taken with a half-meter Seya spectrometer using sodium salicylate as the detector. Nitrogen was used as the radiator for this scan. It is important to note that other gases may be used to extend the number of wavelengths available for calibration. Figure 4 shows a wavelength scan for a mixture of gases including carbon, oxygen, nitrogen, hydrogen, and argon. A line has been drawn on this figure to show the spectral lines which have reached the blackbody ceiling. Using helium or neon as the arc gas, one may employ the arc as a radiation standard to wavelength as short as 600 A. Figure 5 shows a wavelength scan which extends to 600 A, where neon is the basic gas and the center gas mixture consists of neon, argon and nitrogen. It is valuable to note that in the wavelength region of 600-800 A, where the continuum absorption coefficient is large, it is possible to have a continuous blackbody source for this gas mixture. VI. RESULTS
AND
CONCLUSIONS
There are two methods for using this source as a radiation standard. The first, which was used in Refs. 1 and 2, is to observe the blackbody radiation emitted from the center portion of optically thick lines. Using this method, one can control the spectral width covered by the blackbody radiation emitted from the line by a suitable adjustment of the argon nitrogen mixture ratio and the number of sections in the arc with nitrogen present. There is, of course, a practical limit to the maximum spectral width for which one can obtain blackbody radiation. Beyond this limit, the emissivity is a function of the bandpass. To expand the usefulness of this source for low-resolution spectrometers and for applications where more energy is needed, the authors have provided a calibration for the radiation output for a wider bandpass where the emissivity of the column is not unity. Such a calibration is obtained by using the blackbody portion of the line to calibrate the spectrometer, which is then used to measure in absolute units the radiation emitted in the larger bandpasses. Using this method, we have calibrated the arc for nine nitrogen and one oxygen multiplet. These calibration data appear in Figs. 6 and 7 for a number of mixtures and mixture lengths. The accuracy of these data is estimated to be rt 20 %, with errors resulting from a combination of the error in the temperature, the reproducibility of the power input to the arc, and the electronic recording reading errors. The second method which the authors have used and recommend for obtaining a predictable radiation ouput from the dc constricted arc is useful for wavelengths as short as 600 A. For this method, the heating gas is helium or neon. Hydrogen, argon, or xenon is mixed into the center of arc, using the same technique as has been described for nitrogen. The radiation observed for all of the gases used in the recombination continuum. This
A radiation standard for the vacuum ultraviolet
--‘. ..
I
I
1413
1414
J. C. Mom
and R. L. GARRISON
I 100
SIGNAL
20
I
SIGNAL
-
FIG.
5.
.w 100
WAVLLENCTH
6,
,000
110*
Spectral scan of a nitrogen-argon mixture heated in a neon arc for 1 to 600 A.
IW
l”“““‘l”“‘~“‘)“‘~~““l”“““‘l”“““‘l””~””l”~”~’
a00
1100
1416
J. C. MORRISand R. L.
GARRISON
0 0
d
0
(J+‘=‘PtK’~)
AUSN31NI
WfVlNIlN03
ONV W-l
AilA
A radiation
1417
standard for the vacuum ultraviolet
0
IO0-
6-
0 NITROGEN
6-
NITROGEN
x = 1319.5 A BANDPASS r.G9A 0 DATA T= 13,600. K
0
6.
4
I 6
I 6
2
4
6
4.
2-, 4
6
IO’6
A= 141 I.94A BANDPASS z66i 0 DATA T - 13.600* K 6’
6’
IO”
1 IO’6
I 2
I 4
1
I)
6
6
Id’
I
I: z 5 8
1
4-
I
I
I
0 f
1
0I
I
4-
0
0
0
0
w f
0
8
2
2-
0 0
?i >
0 0 0 IO’
0
-
_
IO’-
I)-
6_
6-
0 0
6NITROGEN
NITROGEN
)r = 1743.6
4-
A
A = wn.3 EAzAtd;SS
4-
BANDPASS 0 DATA T - l3,bCW
7.56i
T=l3,GCWK 2 4
6
I 6
I IO’6
1 2
4
6
NUMBER FIG. FIGS.
2. ’
6 lo;~
ii 7.46
A
K
I ’
’
lo’6
2
4
6
’ iDo’
OF ATDMS/cm*
7.
6 and 7. Intensity of radiation emitted in the specified bandpass, as a function of the number of atoms per cm*.
recombination continuum may be accurately predicted from the cross sections measured by SAMSON(~,‘) or from theory in the case of hydrogen. For this prediction, the number of atoms is needed. This number is determined spectroscopically from equation (2) and from the measured intensity of one of the atomic lines emitted in the visible or near-infrared region of the spectrum. The temperature in the equation is obtained from one of the spectral lines emitted by the heating gas in the same wavelength range. To obtain the radiation output from the arc, the recombination cross section is multiplied by the measured atomic number density. This procedure gives the absorption coefficient, which is then used to calculate the emissivity of the column. In using the latter method, estimates for scattered light should be made for the greatest possible accuracy. The resonance lines of the arc gas form absorbing filters (note Figs. 3,
1418
J. C. MORRISand R. L. GARRISON
4 and 5); at these wavelengths, the radiation observed gives a measure of scattered light in the instrument. In conclusion, the authors suggest that the vacuum ultraviolet standard described in this paper should not only be useful as a radiation standard but also as a continuous source of high-intensity vacuum ultraviolet radiation. It is flexible insofar as the operating pressure and power inputs are concerned. The arc generator can be operated stably and reproducibly at a pressure ranging from a few hundred microns of Hg to 5 atm. It can be run at a current ranging from a few amperes to 200 amps in the dc current mode, or up to 1000 amps in a pulsed operation with pulses of 8 msec duration giving temperatures greater than 20 000°K. The arc generator does not, of course, operate as a blackbody over all of these ranges of conditions, but it can be calibrated by using the data given in Figures 6 and 7. Acknowledgements-The authors contract monitors, of Aerospace progress of this work. We would and J. SHEAwho constructed the
wish to express their appreciation to Mr. E. SOEHNGM and Mr. P. SCHREIBER, Research Laboratories, whose interest and support did much to further the also like to thank R. U. KRFX who assisted in the experimental measurements apparatus for the experiments.
REFERENCES 1. G. B~LDT, Ionization in Gases Conference, 925-39, Munich (1961). 2. J. C. MORRIS and R. GARRISON, JQSRT6,899 (1966). 3. C. H. P~WNOE and J. B. SHUMAKER,JR., J. Res. mtn. Bur. Stand.-A. No. 6,495 (1954). 4. J. B. SHUMAKER (Personal Communication). 5. J. RICHTER, Z. Astrophys. 61,57 (1965). 6. J. SAMKIN, JOSA 54,420 (1964). 7. J. SAMSON, JOSA 54,842 (1964).
Physics and Chemistry, Vol. 69A,
Rodia~. Transfer.
Vol. 9. pp. 1407-1418.
Pergamon
Press 1969. Printed
in Great Britain
A RADIATION STANDARD FOR THE VACUUM ULTRAVIOLET* J. C. MORRISand R. L. GARRKON Aerophysics Laboratory, Space Systems Division, Avco Corporation Wilmington, Massachusetts 01887 (Received
17 September
1968)
Abstract-A constricted dc argon arc to which nitrogen or oxygen has been added has been calibrated as a radiation standard for the vacuum ultraviolet. The calibration data are given for the strongest atomic lines in the wavelength range of 18004000 A. Operation for calibrated output of continuum or blackbody radiation down to 600 A is given for a helium or neon arc using hydrogen or argon as the radiating gas. The source has been found to be stable and reproducible, thereby allowing use of the comparison method to measure the number of photons incident on the entrance slit of a spectrometer. I. IWTRODUCTION
interest in vacuum ultraviolet studies in astronomy and plasma physics has increased the demand for more reliable and direct measurements of the photon flux from stellar atmosphere or a laboratory plasma. Of the techniques available for making this measurement the comparison method is the most direct ; however, it has not received much use because of the lack of a suitable standard radiationsource. For wavelengths between 600 and about 18OOA, the dc constricted arc described in this paper shows good promise as a comparatively easy-to-use high-quality radiation standard.‘1*2) Similar arcs have proven their reliability as standard tools for studies of line-transition probabilities, recombination and Bremsstrahlung radiation cross sections, and electrical and thermal conductivities. These arcs are not difficult to use ; they are stable and can be made to radiate as high-quality blackbodies or with an output which may be predicted by using Planck’s law and a measured or calculated emissivity. In this paper, we present intensity calibration and operational data for a constricted arc generator for selected wavelengths between 1800 and 1100 A. The required operation is described for obtaining calibrated output of continuum or blackbody radiation to wavelengths as short as 600 A. INCREASED
II. PREDICTING
RADIATION
FROM
A CONSTRICTED
ARC
The radiation emitted by plasmas which are in thermal equilibrium may be determined from Planck’s law modified by the inclusion of the emissivity of the gas : I,, = @T)X’(exp
C2/1T-
l)-‘C,
(1)
l The research reported herein is part of the research program on radiation from arc-heated plasma of the Aerospace Research Laboratories, Office of Aerospace Research of the U.S. Air Force, whose support is acknowledged. (Contract #F33(615)-68-C-1081.)
1407
J. C. MORRIS and R. L. GARRISON
1408
where Zbb= radiant flux per unit area per unit increment of wavelength, s(AT) = the emissivity of the source, C1 = 3.7413 x 10-r’ watts cm’, A = wavelength of emitted radiation in cm, C2 = 1.4388 cm-OK, T = temperature in “K. To make use of an arc as a radiation standard, it is necessary to know both the emissivity and the temperature of the gas and to choose a plasma configuration which will produce the desired radiation from an isothermal environment. The latter condition is very important for a high-quality radiation standard, as is clearly evident from the appearance of Tin the exponent of Planck’s law. In the following paragraphs, each of the preceding requirements is discussed and it is shown how they are achieved with the dc-constricted arc. III.
DETAILS
OF
THE
DC-CONSTRICTED
ARC
RADIATION
SOURCE
Figure 1 shows the essential details of the arc generator and gas-flow system. The generator is made after the Gerdin type. It consists of a series of water-cooled copper washers, 0.300 in. thick with an inside diameter of 0.187 in. The sections are separated from each other by OUO in., and they are also insulated from one another, except for the residual resistance from the water-cooling system which comes from a common supply. Rubber gaskets are inserted between the sections to serve as insulators and as gas seals to isolate the arc column from the atmosphere. Because we desired to study the arc in some detail, we inserted windows into these gaskets, which allowed side-on intensity measurements to be made between any two sections or in the electrode regions. If the arc is to be used solely as a standard source, these gaskets could be replaced with “0” rings. The only critical component of the arc affecting stable operation is the cathode. The arc runs best if the cathode is allowed to reach incandescence at a single point. For stable operation, a 2 per cent thoriated tungsten tube-about 8 in. long with a& in. inside diameter and a J$ in. wall is satisfactory. Figure 1 shows the details of construction and mounting of this cathode to the end copper section. The power supply for the arc is dc and has an open-circuit voltage of about 180 V. A ballast resistor is placed in series with the arc to control the current. To start the arc, the ballast resistor is set to a value which would give a current of 10-20 amp. The arc is started by inserting a tungsten rod connecting the anode to the cathode and then drawing the arc out between the electrodes. The striker is made from a & in. tungsten rod mounted to an insulating handle. With the arc running, the ballast resistor is adjusted to give an arc current of 100 amps. The operating voltage for an eleven-section arc is about 100 V. The arc is run at atmospheric pressure to achieve a high concentration of radiating and absorbing species, and also to ensure that the plasma is in thermal equilibrium. The radiation from the arc is allowed to enter the spectrometer through a set of five apertures with openings of 0.013 in. in diameter. The apertures are made from & in. metal plates. They are spaced about $ in. apart and are placed near the end of the arc so that only radiation emitted from the axis, or very near to the axis of the arc, can pass through them and into the spectrometer. These apertures also serve as baffles for a differential pumping system, therefore permitting the arc to operate at pressures higher than the viewing spectrometer. The pump for the differential chamber closest to the arc had a speed of 20 ft3/ min at 1 torr and was capable of maintaining this chamber at 1 torr with the arc at 1 atm. Other sections of the chambers were connected to pumps with speeds of about 1.7 ft3/min at 1 torr. This procedure allowed the spectrometer to reach a pressure of 10v6 torr.
A radiation standard for the vacuum ultraviolet
-0’
c------__-
(
,c_--_-_
--
k
/_________/’
_--
4
.
J. C. MORRIS and R. L. GARRISON
1410
The relative temperature distribution along the arc axis is a constant over most of the axis, except at the electrodes where the arc terminates and it drops to the wall temperature of the generator. It is because of this end effect that a transparent noble gas is selected as the running medium. Returning to Fig. 1, one sees that premixed radiating and nobie gases are directed into and out of the center sections of the generator. Flows to the various inlets and outlets are limited to between 4 and 8 ft3/hr at l-atm pressure. The absolute flow rates are not important; however, the relative flows should be monitored to a precision of 5-10 per cent. The flows are adjusted so that the combined flow of the noble gases into the electrode regions is slightly greater than that of the mixture into the center. The exits are adjusted for equal output, taking care not to restrict the flow sufficiently to increase the pressure in the arc chamber. With this arrangement of flows to inlets and outlets, the radiating gas appears only in the isothermal center regions of the arc column and can be kept from diffusing into the cooler ends of the arc. The resulting axial density distribution of the test gas is also shown in Fig. 1. With this operating arrangement, the noble gas acts as a nonradiating, transparent heating element for the radiating gas. Calibrations of this radiation standard have been made using argon as a heating plasma with nitrogen and oxygen acting as the radiating element. The concentration of the nitrogen or oxygen is made low enough so that it does not significantly alter the temperature distribution established in the argon arc ; however, the concentration is made high enough so that the plasma is optically thick over a comparatively large region of the center of many of the atomic lines. The temperature of this standard is a function of arc current, pressure, and arc diameter. Typically, a & in. diameter, 100 amp, 1-atm-pressure argon arc has an axial temperature of 13 600°K. TABLE 1. NUMBEROF ATOMSAS FUNCTIONS OF THE NUMBER OF ARC SECTIONS AND INLET GASMIXTURES Concentration of N, in argon Number of sections
05 per cent
2.5 per cent
2 3 4
5.27 x 1Or5 7.91 x 10’5 1.05 x 10’6
2.59 x 1016 3.89 x 10’6 5.18 x 1Or6
2 3 4
Concentration of 0, in argon 6.09 x 1Or5 3.0 x 10’6 914 x 1or5 4.50 x 10’6 1.21 x 10’6 6.00 x 1Or6
IV.
TEMPERATURE
MEASUREMENTS
OF
5.0 per cent 5.05 x 1016 atoms/cm’ 7.57 x 10’6 1.01 x 10” 5.84 x 1Or6 8.75 x 10’6 1.17 x 10”
THE
UV
SOURCE
The temperature of the constricted arc was measured from the absolute intensity of two argon lines : one, the atomic line at 4300 8; the other, the ionic line at 4806 A. The relation for the intensity of radiation emitted from an optically thin line, with a bandpass whose limits are A1 and AZ,is
1411
A radiation standard for the vacuum ultraviolet
where I = intensity (watts/cm3-sr), A = transition probability (set-l), g, = statistical weight of upper level, Ui = partition function of species i, Ni = concentration of species i (number/cm3), E, = energy of upper level, T = temperature, A = wavelength of the line, h, c, k = the usual constants, and L(1) is the line shape which must be integrated over the bandpass limits. The shapes of the-lines were found to be Lorentzian with half-widths of O-1 and 0*0148fw/N3 x 10’6[A-cm3] for 4300 AI and 4806 AII, respectively.‘3’ The transition probabilities were 3.6 x 10’ set- l and 1.1 x lOa set- l, respectively. The A-values represent a choice which gives the most consistent agreement with data obtained in three investigations by POPENOE and SHUMAKER,(~) RICHTER”) and the present authors. These experimental data covered pressures ranging from 05 to 5.0 atm and used both end-on and side-on viewing techniques for arcs of different sizes and power inputs. Temperatures for the radiation standard were determined from both side-on and endon intensity measurements of the arc column. The temperature as a function of radius for the 100 amps argon arc is shown in Fig. 2. At this arc current, the presence of nitrogen did not alter the measured temperatures. The precision of the temperature measurements 14-
I
12-
6-
ARC
CONSTRICTOR
_I
DIAMETER
I 4-
2-
O-
I
1 3
I
I 2
I
I
0 ARC
I
2
3
RADIUS.mm
FIG. 2. Radial temperature distribution of a lOO-amp,4.7-mm diameter and atmospheric pressure arc.
1412
J. C. MORRISand R. L.
ck4RIUSON
was f 100°K at 13 600°K; considering errors in A values and intensity measurements, the absolute accuracy is estimated to be within +2OO”K. This error estimate gives an absolute accuracy of f20 per cent in the predictions of the blackbody radiation at 1000 A. The temperature as a function of arc current at 100 amps changes approximately 0.2 per cent for a 1 per cent change in arc current, thereby showing the insensitivity of the standard to power input. V. SPECTRAL
DISTRIBUTION
OF RADIATION
EMITTED
BY THE
SOURCE
Figure 3 shows a spectral scan from this radiation source taken with a half-meter Seya spectrometer using sodium salicylate as the detector. Nitrogen was used as the radiator for this scan. It is important to note that other gases may be used to extend the number of wavelengths available for calibration. Figure 4 shows a wavelength scan for a mixture of gases including carbon, oxygen, nitrogen, hydrogen, and argon. A line has been drawn on this figure to show the spectral lines which have reached the blackbody ceiling. Using helium or neon as the arc gas, one may employ the arc as a radiation standard to wavelength as short as 600 A. Figure 5 shows a wavelength scan which extends to 600 A, where neon is the basic gas and the center gas mixture consists of neon, argon and nitrogen. It is valuable to note that in the wavelength region of 600-800 A, where the continuum absorption coefficient is large, it is possible to have a continuous blackbody source for this gas mixture. VI. RESULTS
AND
CONCLUSIONS
There are two methods for using this source as a radiation standard. The first, which was used in Refs. 1 and 2, is to observe the blackbody radiation emitted from the center portion of optically thick lines. Using this method, one can control the spectral width covered by the blackbody radiation emitted from the line by a suitable adjustment of the argon nitrogen mixture ratio and the number of sections in the arc with nitrogen present. There is, of course, a practical limit to the maximum spectral width for which one can obtain blackbody radiation. Beyond this limit, the emissivity is a function of the bandpass. To expand the usefulness of this source for low-resolution spectrometers and for applications where more energy is needed, the authors have provided a calibration for the radiation output for a wider bandpass where the emissivity of the column is not unity. Such a calibration is obtained by using the blackbody portion of the line to calibrate the spectrometer, which is then used to measure in absolute units the radiation emitted in the larger bandpasses. Using this method, we have calibrated the arc for nine nitrogen and one oxygen multiplet. These calibration data appear in Figs. 6 and 7 for a number of mixtures and mixture lengths. The accuracy of these data is estimated to be rt 20 %, with errors resulting from a combination of the error in the temperature, the reproducibility of the power input to the arc, and the electronic recording reading errors. The second method which the authors have used and recommend for obtaining a predictable radiation ouput from the dc constricted arc is useful for wavelengths as short as 600 A. For this method, the heating gas is helium or neon. Hydrogen, argon, or xenon is mixed into the center of arc, using the same technique as has been described for nitrogen. The radiation observed for all of the gases used in the recombination continuum. This
A radiation standard for the vacuum ultraviolet
--‘. ..
I
I
1413
1414
J. C. Mom
and R. L. GARRISON
I 100
SIGNAL
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-
FIG.
5.
.w 100
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,000
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Spectral scan of a nitrogen-argon mixture heated in a neon arc for 1 to 600 A.
IW
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1416
J. C. MORRISand R. L.
GARRISON
0 0
d
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AUSN31NI
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A radiation
1417
standard for the vacuum ultraviolet
0
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0 NITROGEN
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NITROGEN
x = 1319.5 A BANDPASS r.G9A 0 DATA T= 13,600. K
0
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-
_
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)r = 1743.6
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6
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NUMBER FIG. FIGS.
2. ’
6 lo;~
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I ’
’
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2
4
6
’ iDo’
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7.
6 and 7. Intensity of radiation emitted in the specified bandpass, as a function of the number of atoms per cm*.
recombination continuum may be accurately predicted from the cross sections measured by SAMSON(~,‘) or from theory in the case of hydrogen. For this prediction, the number of atoms is needed. This number is determined spectroscopically from equation (2) and from the measured intensity of one of the atomic lines emitted in the visible or near-infrared region of the spectrum. The temperature in the equation is obtained from one of the spectral lines emitted by the heating gas in the same wavelength range. To obtain the radiation output from the arc, the recombination cross section is multiplied by the measured atomic number density. This procedure gives the absorption coefficient, which is then used to calculate the emissivity of the column. In using the latter method, estimates for scattered light should be made for the greatest possible accuracy. The resonance lines of the arc gas form absorbing filters (note Figs. 3,
1418
J. C. MORRISand R. L. GARRISON
4 and 5); at these wavelengths, the radiation observed gives a measure of scattered light in the instrument. In conclusion, the authors suggest that the vacuum ultraviolet standard described in this paper should not only be useful as a radiation standard but also as a continuous source of high-intensity vacuum ultraviolet radiation. It is flexible insofar as the operating pressure and power inputs are concerned. The arc generator can be operated stably and reproducibly at a pressure ranging from a few hundred microns of Hg to 5 atm. It can be run at a current ranging from a few amperes to 200 amps in the dc current mode, or up to 1000 amps in a pulsed operation with pulses of 8 msec duration giving temperatures greater than 20 000°K. The arc generator does not, of course, operate as a blackbody over all of these ranges of conditions, but it can be calibrated by using the data given in Figures 6 and 7. Acknowledgements-The authors contract monitors, of Aerospace progress of this work. We would and J. SHEAwho constructed the
wish to express their appreciation to Mr. E. SOEHNGM and Mr. P. SCHREIBER, Research Laboratories, whose interest and support did much to further the also like to thank R. U. KRFX who assisted in the experimental measurements apparatus for the experiments.
REFERENCES 1. G. B~LDT, Ionization in Gases Conference, 925-39, Munich (1961). 2. J. C. MORRIS and R. GARRISON, JQSRT6,899 (1966). 3. C. H. P~WNOE and J. B. SHUMAKER,JR., J. Res. mtn. Bur. Stand.-A. No. 6,495 (1954). 4. J. B. SHUMAKER (Personal Communication). 5. J. RICHTER, Z. Astrophys. 61,57 (1965). 6. J. SAMKIN, JOSA 54,420 (1964). 7. J. SAMSON, JOSA 54,842 (1964).
Physics and Chemistry, Vol. 69A,