Measurement of synchrotron radiation from the NBS SURF 11using a silicon radiometer

A. R. SchaeferNational Bureau of Standards

Washington, D. C. 20234

Abstract

A project is described in which the synchrotron radiation output from the NBS storagering known as SURF II, will be measured using a well characterized silicon based radiometer.The radiometer consists of a silicon photodiode coupled with two interference filters torestrict the spectral response to a finite and convenient spectral region for the measure-ment. Considerations required for the characterization of the radiometer will be discussed.The absolute radiant flux from the storage ring is also calculable from various machineparameters. A measurement of the number of circulating electrons will be derived fromelectron counting techniques at low levels. This will yield an important intercomparisonbetween two entirely different determinations of the synchrotron radiant flux.

There exist several different areas at NBS whose programs are at least partially involvedin radiometric measurements. Recent interesting developments in two of these areas havemade possible independent absolute radiometric determinations and have indicated the needfor an intercomparison. They are the NBS SURF II storage ring and the detector -based radiom-etry part of the Radiometric Physics Division.

First, we will digress briefly to describe the function of the detector -based radiometrygroup. For the past few years, this group has been involved with the applications of detec-tors and promising electro- optical devices to the problems of radiometry. Absolute radio-metric measurements are based on the use of an electrically calibrated D.C. thermopileradiometers; however, many practical transfer measurements have been made using siliconphotodiodes.

The convenience, sensitivity, stability, durability, and precision available in siliconphotodiodes have led to an extensive program to study the physics of this device, and thegroup now has at hand quite well understood and reliable silicon radiometers.2 Hence, itwas decided to employ a well characterized silicon radiometer in this intercomparison,because of its superior sensitivity, precision, and linearity.

At this point we shall touch briefly on the determination of radiant flux from the SURFII. SURF, Synchrotron Ultraviolet Radiation Facility, is a 250 MeV electron storage ringwith associated beam lines and experimental areas. Its primary use is in support of thenational measurement system far ultraviolet applications. Beam currents are available upto about 30 mA. The machine is not a separated function accelerator, but rather has acircular orbit. The pole magnet faces are adjusted to small tolerances (" ±0.25 mm) toensure orbital circularity. Synchrotron radiant flux from the beam passes through a beamdefining aperture down the beam line, thru a combined orbital place locator and intensitymonitor. As is well known, given the various machine parameters (orbital radius, energy,etc.) the synchrotron radiant energy flux per electron as a function of wavelength, posi-tion, and polarization can be predicted. D. Ederer at NBS has written a code which performsthese calculations for SURF II. If the electron current in the storage ring is known, thenthe synchrotron flux generated can be determined. Preliminary experiments in electroncounting by Hughey et al of the Far UV Physics Group at NBS have indicated that, at low beamcurrents, it is possible to detect individual electrons dropping out of the beam orbit.With sufficient observation of this electron beam decay, it is possible to determine thenumber of electrons present in the beam, the beam current, and the absolute radiant flux.

Our goal is to provide an independent measurement of this flux using a calibrated siliconradiometer. There are a number of considerations involved in the construction of thisradiometer. The general approach is to use a silicon photodiode, coupled with two inter-ference filters to restrict the spectral response to a finite and convenient spectral regionfor the measurement. Figure 1 indicates the approximate relative response, normalized atthe peak, of the chosen system. The radiometer consists of the detector, a three cavitynarrow band interference filter, and a three cavity broadband interference filter to providea sufficiently narrow and well shaped response profile. The radiometer will be situatedbehind a Brewster window to allow mating to the storage ring vacuum system. The absolutequantum efficiency of this device at its peak at 600 nm will be about 0.15. The inter-

84 / SPIE Vol. 196 Measurements of Optical Radiations (1979)

Measurement of synchrotron radiation from the NBS SURF II using a silicon radiometer

A. R. SchaeferNational Bureau of Standards

Washington, D. C. 20234

Abstract

A project is described in which the synchrotron radiation output from the NBS storage ring known as SURF II, will be measured using a well characterized silicon based radiometer. The radiometer consists of a silicon photodiode coupled with two interference filters to restrict the spectral response to a finite and convenient spectral region for the measure­ ment. Considerations required for the characterization of the radiometer will be discussed. The absolute radiant flux from the storage ring is also calculable from various machine parameters. A measurement of the number of circulating electrons will be derived from electron counting techniques at low levels. This will yield an important intercomparison between two entirely different determinations of the synchrotron radiant flux.

There exist several different areas at NBS whose programs are at least partially involved in radiometric measurements. Recent interesting developments in two of these areas have made possible independent absolute radiometric determinations and have indicated the need for an intercomparison. They are the NBS SURF II storage ring and the detector-based radiom- etry part of the Radiometric Physics Division.

First, we will digress briefly to describe the function of the detector-based radiometry group. For the past few years, this group has been involved with the applications of detec­ tors and promising electro-optical devices to the problems of radiometry. Absolute radio- metric measurements are based on the use of an electrically calibrated B.C. thermopile radiometer 1 ; however, many practical transfer measurements have been made using silicon photodiodes.

The convenience, sensitivity, stability, durability, and precision available in silicon photodiodes have led to an extensive program to study the physics of this device, and the group now has at hand quite well understood and reliable silicon radiometers. 2 Hence, it was decided to employ a well characterized silicon radiometer in this intercomparison, because of its superior sensitivity, precision, and linearity.

At this point we shall touch briefly on the determination of radiant flux from the SURF II. SURF, Synchrotron Ultraviolet Radiation Facility, is a 250 MeV electron storage ring with associated beam lines and experimental areas. Its primary use is in support of the national measurement system far ultraviolet applications. Beam currents are available up to about 30 mA. The machine is not a separated function accelerator, but rather has a circular orbit. The pole magnet faces are adjusted to small tolerances O±0.25 mm) to ensure orbital circularity. Synchrotron radiant flux from the beam passes through a beam defining aperture down the beam line, thru a combined orbital place locator and intensity monitor. As is well known, given the various machine parameters (orbital radius, energy, etc.) the synchrotron radiant energy flux per electron as a function of wavelength, posi­ tion, and polarization can be predicted. D. Ederer at NBS has written a code which performs these calculations for SURF II. If the electron current in the storage ring is known, then the synchrotron flux generated can be determined. Preliminary experiments in electron counting by Hughey et al of the Far UV Physics Group at NBS have indicated that, at low beam currents, it is possible to detect individual electrons dropping out of the beam orbit. With sufficient observation of this electron beam decay, it is possible to determine the number of electrons present in the beam, the beam current, and the absolute radiant flux.

Our goal is to provide an independent measurement of this flux using a calibrated silicon radiometer. There are a number of considerations involved in the construction of this radiometer. The general approach is to use a silicon photodiode, coupled with two inter­ ference filters to restrict the spectral response to a finite and convenient spectral region for the measurement. Figure 1 indicates the approximate relative response, normalized at the peak, of the chosen system. The radiometer consists of the detector, a three cavity narrow band interference filter, and a three cavity broadband interference filter to provide a sufficiently narrow and well shaped response profile. The radiometer will be situated behind a Brewster window to allow mating to the storage ring vacuum system. The absolute quantum efficiency of this device at its peak at 600 nm will be about 0.15. The inter-

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MEASUREMENT OF SYNCHROTRON RADIATION FROM THE NBS SURF II

USING A SILICON RADIOMETER

ference filters will block out of band to 10-6 of peak transmittance from x-ray to 1200 nm(beyond which the synchrotron flux is low and silicon responsivity has nearly cut off in any

case). The silicon radiometer will be calibrated against the electrical DC radiometer atmany wavelengths throughout its passband by using a tunable cw dye laser.

A feasibility experiment of this type was performed by the group in 1974, and is dis-

cussed in the literature.3 In that experiment, a silicon cell was coupled with a 10 nm halfband width interference filter centered at 600 nm and measured using an electrically cali-brated pyroelectric radiometer and a cw dye laser (Fig. 2). This filtered silicon detectorwas then used to measure the spectral irradiance from two NBS standards of spectral irradi-ance, and agreement within less than the limits of accuracy of the two techniques (% ±lo)

was obtained.

There are two considerations in choosing the bandpass of the silicon radiometer. First,this response region fits conveniently into the region of output available in the Rhodamine6G, argon pumped dye laser. Second, this is a well behaved, well understood region ofresponse of the silicon photodiode. Photodiodes of the type used in this radiometer havebeen shown by Budde`` of NRC to be linear over eight decades of incident radiant power.Also, these detectors have proven to possess very good stability and spatial uniformity,especially in this spectral range. Finally, early results in the theoretical silicon physicsprogram being pursued by Geist et al5 indicate a possibility of obtaining the quantum ef-ficiency of the silicon radiometer in this region directly from the silicon device physics,rather than from the thermal physics of the electrically calibrated radiometer.

In order to accurately calibrate the wavelength of the dye laser, the opto- galvaniceffect is used, described by King et ale. In this technique, the dye laser beam enters theinterior of a neon filled iron hollow cathode lamp. Whenever the wavelength of the beamcorresponds exactly to that of a neon transition, absorption results in disturbance of theelectron populations of the plasma, which in turn induces a change in the impedance of theplasma. By chopping the radiation and measuring the corresponding synchronous component ofthe hollow cathode voltage, many of the lines of the neon spectrum can be detected. In

Fig. 3 the positions are shown of the twenty -eight observed neon transitions in the bandpassof the radiometer. These, along with an interpolation scheme, should be sufficient toaccurately pinpoint the wavelength of the dye laser radiation. If necessary, another hollowcathode lamp would be obtained with a different fill gas to provide different spectrallines. Also, with the insertion of a thin etalon into the dye laser cavity, it should bepossible to detect some of the iron cathode transitions, yielding more wavelengths.

0

Figure 4 shows the synchrotron flux expected per electron -A- second in the bandpass of theradiometer. Multiplying this by 3000, the approximate upper limit to the number of electronsindividually detectable in the counting measurement, and integrating over the bandpass ofthe radiometer yields an expected radiometer photocurrent of 4.6x10 15A, assuming a 1x5 mmaperture on the SURF, a distance of 2 m from the orbital tangent point, and an electronenergy of 250 MeV, as seen in Fig. 5. This compares with photocurrents of about 0.1 mAgenerated at each wavelength by the dye laser characterization. Thus, a linearity scalefactor of 11 decades is necessary, which is not within the present capability of the radiom-eter. However, running the storage ring at a higher beam current of 20 mA, which correspondsto 2.2x109 electrons in the beam, produces a photocurrent of 3.4 nA from the radiometer,requiring linearity scaling of only 5 decades; well within the capability of the radiometer.A separate photomultiplier will be required to scale the six decades between this higherbeam current level and the lower levels necessary for counting. A previously developedtechnique is available for testing its linearity over this range. In addition, the linear-ity of the phototube can be measured directly with the output flux of the storage ring,making use of the exact flux step ratios generated by unit electron losses from the beam.Assorted neutral density filters can be inserted in front of the photomultiplier in order toobtain large radiant flux level selection.

There will be several optical effects to take into consideration. The Brewster windowwill be oriented to pass the predominant polarization parallel to the orbital plane of SURFII, but at this wavelength about 0.2% perpendicular polarization is expected which will notbe seen. Also, although the SURF II beam is reasonably well collimated, there is some offaxis contribution to be expected, particularly at these wavelengths. These can also becalculated. In addition, such contributions and other scattered light problems will beexperimentally measured using an annular silicon detector fabricated at NBS by Y. M. Liu.This detector has a diameter of 4.5 cm with a center hole of 1 cm diameter. By allowing themain beam to pass through the 1 cm hole in the center of the detector, the presence ofresidual scattered light can easily be measured with this device.

The current status of this project is as follows: The equipment has been obtained andset up for characterizing the silicon radiometer, including the laser, hollow cathode sys-tem, and associated hardware. The detector is now available, and the interference filtersare being manufactured. Some modification and improvements are slated for the SURF II for

SPIE Vol. 196 Measurements of Optical Radiations (1979) / 85

MEASUREMENT OF SYNCHROTRON RADIATION FROM THE NBS SURF II

USING A SILICON RADIOMETER

ference filters will block out of band to 10" 6 of peak transmittance from x-ray to 1200 nm (beyond which the synchrotron flux is low and silicon responsivity has nearly cut off in any case). The silicon radiometer will be calibrated against the electrical DC radiometer at many wavelengths throughout its passband by using a tunable cw dye laser.

A feasibility experiment of this type was performed by the group in 1974, and is dis­ cussed in the literature. 3 In that experiment, a silicon cell was coupled with a 10 nm half band width interference filter centered at 600 nm and measured using an electrically cali­ brated pyroelectric radiometer and a cw dye laser (Fig. 2). This filtered silicon detector was then used to measure the spectral irradiance from two NBS standards of spectral irradi- ance, and agreement within less than the limits of accuracy of the two techniques O±l%) was obtained.

There are two considerations in choosing the bandpass of the silicon radiometer. First, this response region fits conveniently into the region of output available in the Rhodamine 6G, argon pumped dye laser. Second, this is a well behaved, well understood region of response of the silicon photodiode. Photodiodes of the type used in this radiometer have been shown by Budde 4 of NRC to be linear over eight decades of incident radiant power. Also, these detectors have proven to possess very good stability and spatial uniformity, especially in this spectral range. Finally, early results in the theoretical silicon physics program being pursued by Geist et al 5 indicate a possibility of obtaining the quantum ef­ ficiency of the silicon radiometer in this region directly from the silicon device physics, rather than from the thermal physics of the electrically calibrated radiometer.

In order to accurately calibrate the wavelength of the dye laser, the opto-galvanic effect is used, described by King et al 6 . In this technique, the dye laser beam enters the interior of a neon filled iron hollow cathode lamp. Whenever the wavelength of the beam corresponds exactly to that of a neon transition, absorption results in disturbance of the electron populations of the plasma, which in turn induces a change in the impedance of the plasma. By chopping the radiation and measuring the corresponding synchronous component of the hollow cathode voltage, many of the lines of the neon spectrum can be detected. In Fig. 3 the positions are shown of the twenty-eight observed neon transitions in the bandpass of the radiometer. These, along with an interpolation scheme, should be sufficient to accurately pinpoint the wavelength of the dye laser radiation. If necessary, another hollow cathode lamp would be obtained with a different fill gas to provide different spectral lines. Also, with the insertion of a thin etalon into the dye laser cavity, it should be possible to detect some of the iron cathode transitions, yielding more wavelengths.

o

Figure 4 shows the synchrotron flux expected per electron-A-second in the bandpass of the radiometer. Multiplying this by 3000, the approximate upper limit to the number of electrons individually detectable in the counting measurement, and integrating over the bandpass of the radiometer yields an expected radiometer photocurrent of 4.6x10 15 A, assuming a 1x5 mm aperture on the SURF, a distance of 2 m from the orbital tangent point, and an electron energy of 250 MeV, as seen in Fig. 5. This compares with photocurrents of about 0.1 mA generated at each wavelength by the dye laser characterization. Thus, a linearity scale factor of 11 decades is necessary, which is not within the present capability of the radiom­ eter. However, running the storage ring at a higher beam current of 20 mA, which corresponds to 2.2xl0 9 electrons in the beam, produces a photocurrent of 3.4 nA from the radiometer, requiring linearity scaling of only 5 decades; well within the capability of the radiometer. A separate photomultiplier will be required to scale the six decades between this higher beam current level and the lower levels necessary for counting. A previously developed technique is available for testing its linearity 7 over this range. In addition, the linear­ ity of the phototube can be measured directly with the output flux of the storage ring, making use of the exact flux step ratios generated by unit electron losses from the beam. Assorted neutral density filters can be inserted in front of the photomultiplier in order to obtain large radiant flux level selection.

There will be several optical effects to take into consideration. The Brewster window will be oriented to pass the predominant polarization parallel to the orbital plane of SURF II, but at this wavelength about 0.2% perpendicular polarization is expected which will not be seen. Also, although the SURF II beam is reasonably well collimated, there is some off axis contribution to be expected, particularly at these wavelengths. These can also be calculated. In addition, such contributions and other scattered light problems will be experimentally measured using an annular silicon detector fabricated at NBS by Y. M. Liu. This detector has a diameter of 4.5 cm with a center hole of 1 cm diameter. By allowing the main beam to pass through the 1 cm hole in the center of the detector, the presence of residual scattered light can easily be measured with this device.

The current status of this project is as follows: The equipment has been obtained and set up for characterizing the silicon radiometer, including the laser, hollow cathode sys­ tem, and associated hardware. The detector is now available, and the interference filters are being manufactured. Some modification and improvements are slated for the SURF II for

SPIE Vol. 196 Measurements of Optical Radiations (1979) / 85

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SCHAEFER

the fall, during which time we shall assemble and characterize the radiometer, and aid inrefining the electron counting measurements as necessary. Then in spring of 1980, thesynchrotron radiation experiments will be performed, which should yield an intercomparisonof the two absolute measurement systems to within about ±1%.

References

1. J. Geist, L. B. Schmidt, and W. E. Case, Appl. Opt. 12, 2773 (1973).2. M. A. Lind, E. F. Zalewski and J. B. Fowler, Nat. Bur. Stand. Technical Note 950

(July 1977).3. J. Geist, B. Steiner, R. Schaefer, E. Zalewski, and A. Corrons, Appl. Phys. Lett.

26, 309 (1975).4. W. Budde, Appl. Opt. 18, 1555 (1979).S. J. Geist, Appl. Opt. 18, 760 (1979).6. D. S. King, P. K. Schenck, K. C. Smyth, and J. C. Travis, Appl. Opt. 16, 2617 (1977).7. A. R. Schaefer, E. F. Zalewski, M. A. Lind, and J. Geist, Proceedings of the Technical

Program, Electro- Optics Conference, (October 1977), page 459.

86 / SPIE Vol. 196 Measurements of Optical Radiations (1979)

SCHAEFER

the fall, during which time we shall assemble and characterize the radiometer, and aid in refining the electron counting measurements as necessary. Then in spring of 1980, the synchrotron radiation experiments will be performed, which should yield an intercomparison of the two absolute measurement systems to within about ±1%.

References

1. J. Geist, L. B. Schmidt, and W. E. Case, Appl. Opt. 12, 2773 (1973).2. M. A. Lind, E. F. Zalewski and J. B. Fowler, Nat. Bur. Stand. Technical Note 950

(July 1977).3. J. Geist, B. Steiner, R. Schaefer, E. Zalewski, and A. Corrons, Appl. Phys. Lett.

2^6, 309 (1975).4. W. Budde, Appl. Opt. 1_8, 1555 (1979).5. J. Geist, Appl. Opt. 1^8, 760 (1979).6. D. S. King, P. K. Schenck, K. C. Smyth, and J. C. Travis, Appl. Opt. 1^, 2617 (1977).7. A. R. Schaefer, E. F. Zalewski, M. A. Lind, and J. Geist, Proceedings of the Technical

Program, Electro-Optics Conference, (October 1977), page 459.

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MEASUREMENT OF SYNCHROTRON RADIATION FROM THE NBS SURF II

USING A SILICON RADIOMETER

580 590 600 610 620 630 640(nm)

Fig. 2: Response of filtered silicondetector as a function of wavelength,as determined with cw dye laser.

Fig. 1: Relative response, normalized atpeak, of the silicon radiometer system.

570 600

Wavelength (nm)

630

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MEASUREMENT OF SYNCHROTRON RADIATION FROM THE NBS SURF I

USING A SILICON RADIOMETER

560 570 580 590 600 610 620 630 640

A (nm)

Fig, 1: Relative response, normalized at peak ? of the silicon radiometer system.

190 -

Fig. 2: Response of filtered silicon detector as a function of wavelength, as determined with cw dye laser.

570 600 630

Wavelength (nm)

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1.0

10.1

10.2

i0 '

10°

i05

10'8 i i I

560 570 580I I I r

590 600 610 620 630 640A (nm)

Fig. 4: Synciarotron flux from SURF IIper electron -A -sec expected in thesilicon radiometer bandpass.

88 / SPIE Vol. 196 Measurements of Optical Radiations (1979)

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Fig. 3: Observed neon transitions occurringfrom the opto- galvanic effect within thebandpass of the silicon radiometer.

0.66

0.64

0.62

0.60

0.58

0.56

560 580 600

A (nm)

620 640

SCHAEFER

1.0r

10

Fig. 3: Observed neon transitions occurringfrom the opto-galvanic effect within thebandpass of the silicon radiometer.

560 570 580 590 600 610 620 630 640 A (nm)

0.66 -

Fig. 4: Synchrotron flux from SURF II per electron-A-sec expected in the silicon radiometer bandpass.

560 580 600A (nm)

620 640

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MEASUREMENT OF SYNCHROTRON RADIATION FROM THE NBS SURF IIUSING A SILICON RADIOMETER

SURF -II Energy: 250 MeV

Aperture: 1 X 5 mm

Distance fromOrbital Tangent Pt: 2 m

Beam Current: 2.7 nA Beam Current: 20 mA

Number Electronsin Ring: 3000

Number Photoelectrons Pro-duced by Radiometer: 29000/s

Number Electronsin Ring: 2.2 x 109

Number Photoelectrons Pro-duced by Radiometer: 2.1 x 1010 /s

Current from Radiometer: 4.6 fA Current from Radiometer: 3.4 nA

Fig. 5: Summary of photocurrent produced bythe silicon radiometer at normal and electroncounting level beam currents.

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MEASUREMENT OF SYNCHROTRON RADIATION FROM THE NBS SURF II

USING A SILICON RADIOMETER

SURF-II Energy: 250 MeV

Aperture: 1 x 5 mm

Distance from Orbital Tangent Pt: 2m

Beam Current: 2.7 nA Beam Current: 20 mA

Number Electrons Number Electrons « in Ring: 3000 in Ring: 2.2 x 10

Number Photoelectrons Pro- Number Photoelectrons Pro- -, n duced by Radiometer: 29000/s duced by Radiometer: 2.1 x 10 /s

Current from Radiometer: 4.6 fA Current from Radiometer: 3.4 nA

Fig. 5: Summary of photocurrent produced by the silicon radiometer at normal and electron counting level beam currents.

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