- Email: [email protected]
A proton beam facility for single event research
1256
Nuclear Instruments and Methods in Physics Research B56/57 (1991) 1256-1259 North-Holland
A proton beam facility for single event research KM. Murray KM Sciences, 6200 Meadow Wood Suite 22, Rena, NV 89502, VSA
W.J. Stapor Naval Research Laboratory,
Washington, DC 20375, USA
C. Castenada Cracker Nuclear Laboratory,
UC Davis, Davis, CA 95616, USA
We describe a charged particle (Z = 1 or 2) radiation system developed jointly by KM Sciences, The Naval Research Laboratory, and The Cracker Nuclear Laboratory. The system is used primarily to simulate the space en~ronmental protons and other charged particles with energies from 10 to 70 MeV. These particles in turn produce single events in devices being tested. The system provides a highly reproduciblebeam combined with precise dosimetric measurementand control to better than 2% for fluences from 1 x 10’ to 1 x lo’* particles/cm2.The system can also providechopped single pulses with durations from 0.1 to 10 s at intensities up to 3 x 10” particles/cm2s.
The facility described herein has been developed to provide highly reproducible fluences of particles in a very user-friendly environment. Such features are particularly useful in single event research because they allow the researchers to concentrate their efforts on the response of the device being tested with confidence in the exposure of the device. The cyclotron at the Cracker Nuclear Laboratory [l], University of California at Davis is capable of producing energetic beams of protons, deuterons, alphas, and other heavier ions. These beams are variable in energy from a few MeV to over 65 MeV. The protons are particularly suitable for simulating typical energetic particles in the Earth’s radiation belts [4] in which some spacecraft must operate. The incident particle fluence (and therefore dose given to the target) is monitored to better than 2% for most irradiation configurations. Average fluxes or dose rates are adjustable over a significant range. This enables the study of single particle and total dose effects.
of the beam lines. Beam line number 2 (BL-2) is dedicated to the irradiation system and is shown schematically in fig. 2. This beam line was developed jointly by the Naval Research Laboratory and the Cracker Nuclear Laboratory, University of California at Davis. The beam downstream of the switching magnet is focused with a magnetic quad~pole double to a spot on one of several degrader foils which can be externally selected by the rotation of a water-cooled wheel containing eleven different thicknesses of tantalum foils (0.006-0.0508 cm) and one open hole. Adjustable slits are placed just upstream of the degrader wheel to en-
2. System overview doOr
The beam from the Cracker Laboratory’s cyclotron can be switched into any one of several different special purpose beam lines. Fig. 1 is a diagram of the cyclotron vault and experimental rooms showing the orientation 0168-583X/91/$03.50
SOUTH CAVE
------/
Fig. 1. The Cracker laboratory cyclotron.
Q 1991 - Elsevier Science Publishers B.V. (North-Holland)
1251
K.M. Murray et al. / Single event research
Fig. 2. The radiation
testing beam line.
sure that the beam strikes the foil exactly on axis. The scattered beam is defined by a downstream collimator approximately 4 m away which subtends a half angle of 0.007 rad. This geometry ensures that the beam at the exit of the Faraday cup (F cup) box, where devices being tested are placed, has a highly reproducible profile which is relatively flat. Coulomb multiple scattering angles for typical scattering foils and ion energies are shown in table 1. Calculations based on a Gaussian distribution [3] indicate only a few percent drop-off across the beam intensity profile. The degrader foil together with the foils of the secondary emission monitor (SEM) and the exit window produce straggling in the beam energy. The calculated straggling is shown in the last column of table 1. The energy of the particles incident on the target is the circulating beam energy in the cyclotron corrected for the losses due to the degrader foil, the SEM, the exit window and the air path from the exit window to the test device. Downstream from the degrader wheel is an aperture not shown in fig. 2 that prevents scattered beam from striking beam plumbing and other equipment of the beam line. The collimator located in the F cup box defines a spot 7.2 cm in diameter at a point 60 cm further downstream outside the box. The chopper box is a chamber in which a system of shutters is installed to provide single pulses of beam with durations from 0.1 to 10 s. The F cup box is the
Table 1 The mean scattering Internal proton energy
a Straggling
and foil a
Foil thickness
Rms scattering
Straggling FWHM
[emI
angle
lMeV1
lMeV1 61.5 14.6
angle for energy
WI 0.0508 0.0062
0.0406 0.0236
0.600 0.300
for foils in beam added in quadrature.
* Fig. 3. The monitoring box. The electrometers
6
*
+
elements located in the Faraday cup and voltage supply are external to the vacuum.
chamber containing the essential beam monitoring elements (see fig. 3). The beam passes through the SEM and then into a Faraday cup which can be moved in and out of the beam while remaining inside the vacuum chamber. The vacuum is maintained at better than 1 x lop4 Torr to ensure stability of the SEM output. Secondary emission monitors have been used to monitor charged particle beams for many years [2]. The SEM used in this system consists of three aluminum foils 0.0025 cm thick and a fourth ground plane foil located upstream to ensure a field free region for a rotating wire scanner. The scanner is between the SEM and the final collimator but is not used for monitoring the degraded beam. Its primary purpose is in the diagnostics of the chopped focused high intensity beam. The center foil of the SEM is connected to ground through a Keithly model 617 electrometer. The two outer foils are connected together and biased at + 600 V with respect to ground and the surrounding vacuum chamber. This biasing configuration serves two purposes, first to provide a strong collecting field within the SEM to ensure collection of secondaries produced at the central foil, and second, a field outside the SEM which will return any secondaries produces outside the SEM. Secondary suppression was confirmed by comparing the ratio of F cup current to SEM current with and without a magnetic field superimposed on the SEM. With +600 V bias, no difference was observed, while with -600 V bias a 5% effect was evident. The Faraday cup consists of a graphite cylinder 13 cm in diameter and 7.5 cm deep. The back of the cup is 5 cm thick, which prevents penetration of the most energetic protons expected of the line. Two samarium cobalt magnets are mounted in the back of the cup in such a way that their field lines are essentially transverse to the beam axis inside the cup and parallel outside the cup. No deflection of the lowest energy beams has been observed with the cup retracted out of XVIII.
SOFT UPSETS
K.M. Murray et al. / Single event research
1258
the beam. The Faraday cup is connected to ground through another Keithly model 617 electrometer. The stainless steel 0.0025 cm thick exit window is just downstream from the ‘F cup. The normal target position is 10 cm further downstream from the window. Initially, the beam profile was measured at this point, using an array of thermoluminescent dosimeters. Currently the profile is determined by a scan of a radiochromic dye image of the beam. The use of radiochromic dye is discussed in more detail later in this paper. Normal operation of the system consists of first measuring a ratio of F cup current to SEM current. The F cup is then withdrawn from the beam and a target is irradiated while the SEM current is being integrated. This integral is multiplied by the previously measured ratio to give the total charge passing through the exit window. This number is then multiplied by the profile to give the fluence at any position in the beam.
3. The beam intensity profile The distributions of ion fluxes at the target were initially determined by irradiating an array of TLDs placed at this position. These TLDs were arranged in concentric circles. The observation region was divided into concentric circles for numeric integration of the observed doses and because the beam intensity was cylindrically symmetrical. Although TLDs exhibit considerable fluctuation in their individual response (about 15%) numerous beam distribution measurements by this method have shown a standard deviation of about 7%. For this reason, an average of many measurements by this method is taken as the beam intensity profile. Table 2 shows the average ratios together with their standard deviations based on 37 such measurements. The beam profile has also been determined by aluminum pellet activation. The averages of three such measurements are also given in table 2. During the past year and a half, the beam profile has been measured by obtaining a beam image on a radiochromic dye film and scanning its density distribution using a scanning densitometer. These measurements show far less variation than the TLD readings and can be read on a much finer mesh. Fig. 4 is a typical plot of
Table 2 Ratios of flux to total beam using 37 TLD arrays
Avg. ratio Std. dev. Al activation Avg. ratio
position top to bottom
-
+
mm north
to south
Fig. 4. Two orthogonal scans of a radiochromic the beam at the target location.
dye image of
dose versus position for two orthogonal scans. The resulting profile ratios agree with the TLD data but with only about one tenth the scatter. This method has now become the accepted means of determining beam profiles for this facility.
4. Beam chopper Some radiation effects work has required dose rates higher than could be obtained with the degraded beam. To satisfy these needs, a focused beam of high current (> 1 PA) is needed. Such a beam will produce dose rates up to about 1 X 10’ rad(Si)/s. At this rate a typical device is soon total dose damaged. One must therefore chop the beam into a pulse of from 0.1 to several seconds in duration in order not to destroy the device being tested. Such a beam chopping system was designed and constructed. It is installed in the vacuum chamber just downstream from the degrader wheel and is shown schematically in fig. 5. It consists of a water cooled aperture which normally intercepts the beam. pneumatic
driver
solenoids
II
! mo”Sbl~ stop
a
Ocm
1 cm
2cm
3 cm
4cm
0.0289 0.0013
0.0255 0.0016
0.0248 0.0022
0.0193 0.0028
0.0056 0.0028
0.028
0.027
0.025
chopper box
a Similar ratio for 3 aluminum
activations.
Fig. 5. The elements of the beam chopper system inside the chopper vacuum chamber.
which
are
1259
K. M. Murray et al. / Single event research
This is followed by a defining aperture and then two aluminum shutter blocks which are driven by solenoids. The sequence of operation is to first open the water cooled aperture. As that reaches its final position it trips a microswitch which simultaneously fires the first shutter and starts a timing delay which then fires the second shutter. The water cooled aperture is then returned to its normal beam intercept position. Dosimetry for this method of operation is provided by radiochromic foils placed on the device being tested. A system of phosphor screen and movable beam stop is provided between the exit window and the test device to allow careful adjustment of the beam just prior to making an irradiation. The stability of the cyclotron operation allows one to achieve the desired exposure to within about 20% by this method. The dosimetry provides the incident dose actually received to within 5% for most configurations.
5. Software The system so far described provides very accurate and highly reproducible irradiation situations for most work. The next step was to then make this operate as conveniently as possible. This was achieved by using a desktop PC to control and operate the cyclotron beam and the associated beam monitoring equipment via an IEEE 488 bus. The code developed for this purpose (called BEAMMON) is a menu based program that provides simple but flexible use of all aspects of the degraded beam system. BEAMMON obtains accurate current measurements from the F cup and SEM electrometers. A beam stop controlled by BEAMMON turns the cyclotron beam on and off according to preset conditions available in various menus of the code.
Initially, the user specifies the incident ion, energy, and target material. Protons, deuterons and alpha partitles are supported as incident ions, with silicon and gallium arsenide targets. Dose calculations are performed by BEAMMON using calculations of energy deposition based on a relativistic Bethe-Bloch equation using Ziegler’s empirical ionization potentials and a semi-empirical screening correction.
Acknowledgements The authors would like to take this opportunity to express their appreciation for the excellent cooperation of the accelerator operating staff and particularly to Mr. Joel Macurdy, Mr. Walter Kemmler, and Mr. Jerry Stoddel for their many helpful suggestions and most useful talents. We would like to express our thanks to Mr. Tom Ward and Mr. Jerry Netherton for their professionalism and tireless support during many late hours at UCD, and we would like to acknowledge the efforts of Mr. Patrick McDonald in providing most of the useful enhancements to the BEAMMON code.
References [l] 10th Int. Conf on Cyclotrons and Their Applications, East Lansing, MI, ed. F. Martin, 1984, p. 723. [2] G.W. ,_ ___\ Toutfest and H.R. Fechter, Rev. Sci. Instr. 26(3) (lY33).
131 Review of Particle Properties, Phys. Lett. B170 (1986) 45. [4] W.N. Spjeldvilc, J. Geophys. Res. 82 (1977) 2801. [5] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Matter (Pergamon, 1985). [6] J.F. Ziegler, Handbook of Stopping Cross-Sections for Energetic Ions in All Elements, vol. 5 (Pergamon, 1980).
XVIII.
SOFT UPSETS
Nuclear Instruments and Methods in Physics Research B56/57 (1991) 1256-1259 North-Holland
A proton beam facility for single event research KM. Murray KM Sciences, 6200 Meadow Wood Suite 22, Rena, NV 89502, VSA
W.J. Stapor Naval Research Laboratory,
Washington, DC 20375, USA
C. Castenada Cracker Nuclear Laboratory,
UC Davis, Davis, CA 95616, USA
We describe a charged particle (Z = 1 or 2) radiation system developed jointly by KM Sciences, The Naval Research Laboratory, and The Cracker Nuclear Laboratory. The system is used primarily to simulate the space en~ronmental protons and other charged particles with energies from 10 to 70 MeV. These particles in turn produce single events in devices being tested. The system provides a highly reproduciblebeam combined with precise dosimetric measurementand control to better than 2% for fluences from 1 x 10’ to 1 x lo’* particles/cm2.The system can also providechopped single pulses with durations from 0.1 to 10 s at intensities up to 3 x 10” particles/cm2s.
The facility described herein has been developed to provide highly reproducible fluences of particles in a very user-friendly environment. Such features are particularly useful in single event research because they allow the researchers to concentrate their efforts on the response of the device being tested with confidence in the exposure of the device. The cyclotron at the Cracker Nuclear Laboratory [l], University of California at Davis is capable of producing energetic beams of protons, deuterons, alphas, and other heavier ions. These beams are variable in energy from a few MeV to over 65 MeV. The protons are particularly suitable for simulating typical energetic particles in the Earth’s radiation belts [4] in which some spacecraft must operate. The incident particle fluence (and therefore dose given to the target) is monitored to better than 2% for most irradiation configurations. Average fluxes or dose rates are adjustable over a significant range. This enables the study of single particle and total dose effects.
of the beam lines. Beam line number 2 (BL-2) is dedicated to the irradiation system and is shown schematically in fig. 2. This beam line was developed jointly by the Naval Research Laboratory and the Cracker Nuclear Laboratory, University of California at Davis. The beam downstream of the switching magnet is focused with a magnetic quad~pole double to a spot on one of several degrader foils which can be externally selected by the rotation of a water-cooled wheel containing eleven different thicknesses of tantalum foils (0.006-0.0508 cm) and one open hole. Adjustable slits are placed just upstream of the degrader wheel to en-
2. System overview doOr
The beam from the Cracker Laboratory’s cyclotron can be switched into any one of several different special purpose beam lines. Fig. 1 is a diagram of the cyclotron vault and experimental rooms showing the orientation 0168-583X/91/$03.50
SOUTH CAVE
------/
Fig. 1. The Cracker laboratory cyclotron.
Q 1991 - Elsevier Science Publishers B.V. (North-Holland)
1251
K.M. Murray et al. / Single event research
Fig. 2. The radiation
testing beam line.
sure that the beam strikes the foil exactly on axis. The scattered beam is defined by a downstream collimator approximately 4 m away which subtends a half angle of 0.007 rad. This geometry ensures that the beam at the exit of the Faraday cup (F cup) box, where devices being tested are placed, has a highly reproducible profile which is relatively flat. Coulomb multiple scattering angles for typical scattering foils and ion energies are shown in table 1. Calculations based on a Gaussian distribution [3] indicate only a few percent drop-off across the beam intensity profile. The degrader foil together with the foils of the secondary emission monitor (SEM) and the exit window produce straggling in the beam energy. The calculated straggling is shown in the last column of table 1. The energy of the particles incident on the target is the circulating beam energy in the cyclotron corrected for the losses due to the degrader foil, the SEM, the exit window and the air path from the exit window to the test device. Downstream from the degrader wheel is an aperture not shown in fig. 2 that prevents scattered beam from striking beam plumbing and other equipment of the beam line. The collimator located in the F cup box defines a spot 7.2 cm in diameter at a point 60 cm further downstream outside the box. The chopper box is a chamber in which a system of shutters is installed to provide single pulses of beam with durations from 0.1 to 10 s. The F cup box is the
Table 1 The mean scattering Internal proton energy
a Straggling
and foil a
Foil thickness
Rms scattering
Straggling FWHM
[emI
angle
lMeV1
lMeV1 61.5 14.6
angle for energy
WI 0.0508 0.0062
0.0406 0.0236
0.600 0.300
for foils in beam added in quadrature.
* Fig. 3. The monitoring box. The electrometers
6
*
+
elements located in the Faraday cup and voltage supply are external to the vacuum.
chamber containing the essential beam monitoring elements (see fig. 3). The beam passes through the SEM and then into a Faraday cup which can be moved in and out of the beam while remaining inside the vacuum chamber. The vacuum is maintained at better than 1 x lop4 Torr to ensure stability of the SEM output. Secondary emission monitors have been used to monitor charged particle beams for many years [2]. The SEM used in this system consists of three aluminum foils 0.0025 cm thick and a fourth ground plane foil located upstream to ensure a field free region for a rotating wire scanner. The scanner is between the SEM and the final collimator but is not used for monitoring the degraded beam. Its primary purpose is in the diagnostics of the chopped focused high intensity beam. The center foil of the SEM is connected to ground through a Keithly model 617 electrometer. The two outer foils are connected together and biased at + 600 V with respect to ground and the surrounding vacuum chamber. This biasing configuration serves two purposes, first to provide a strong collecting field within the SEM to ensure collection of secondaries produced at the central foil, and second, a field outside the SEM which will return any secondaries produces outside the SEM. Secondary suppression was confirmed by comparing the ratio of F cup current to SEM current with and without a magnetic field superimposed on the SEM. With +600 V bias, no difference was observed, while with -600 V bias a 5% effect was evident. The Faraday cup consists of a graphite cylinder 13 cm in diameter and 7.5 cm deep. The back of the cup is 5 cm thick, which prevents penetration of the most energetic protons expected of the line. Two samarium cobalt magnets are mounted in the back of the cup in such a way that their field lines are essentially transverse to the beam axis inside the cup and parallel outside the cup. No deflection of the lowest energy beams has been observed with the cup retracted out of XVIII.
SOFT UPSETS
K.M. Murray et al. / Single event research
1258
the beam. The Faraday cup is connected to ground through another Keithly model 617 electrometer. The stainless steel 0.0025 cm thick exit window is just downstream from the ‘F cup. The normal target position is 10 cm further downstream from the window. Initially, the beam profile was measured at this point, using an array of thermoluminescent dosimeters. Currently the profile is determined by a scan of a radiochromic dye image of the beam. The use of radiochromic dye is discussed in more detail later in this paper. Normal operation of the system consists of first measuring a ratio of F cup current to SEM current. The F cup is then withdrawn from the beam and a target is irradiated while the SEM current is being integrated. This integral is multiplied by the previously measured ratio to give the total charge passing through the exit window. This number is then multiplied by the profile to give the fluence at any position in the beam.
3. The beam intensity profile The distributions of ion fluxes at the target were initially determined by irradiating an array of TLDs placed at this position. These TLDs were arranged in concentric circles. The observation region was divided into concentric circles for numeric integration of the observed doses and because the beam intensity was cylindrically symmetrical. Although TLDs exhibit considerable fluctuation in their individual response (about 15%) numerous beam distribution measurements by this method have shown a standard deviation of about 7%. For this reason, an average of many measurements by this method is taken as the beam intensity profile. Table 2 shows the average ratios together with their standard deviations based on 37 such measurements. The beam profile has also been determined by aluminum pellet activation. The averages of three such measurements are also given in table 2. During the past year and a half, the beam profile has been measured by obtaining a beam image on a radiochromic dye film and scanning its density distribution using a scanning densitometer. These measurements show far less variation than the TLD readings and can be read on a much finer mesh. Fig. 4 is a typical plot of
Table 2 Ratios of flux to total beam using 37 TLD arrays
Avg. ratio Std. dev. Al activation Avg. ratio
position top to bottom
-
+
mm north
to south
Fig. 4. Two orthogonal scans of a radiochromic the beam at the target location.
dye image of
dose versus position for two orthogonal scans. The resulting profile ratios agree with the TLD data but with only about one tenth the scatter. This method has now become the accepted means of determining beam profiles for this facility.
4. Beam chopper Some radiation effects work has required dose rates higher than could be obtained with the degraded beam. To satisfy these needs, a focused beam of high current (> 1 PA) is needed. Such a beam will produce dose rates up to about 1 X 10’ rad(Si)/s. At this rate a typical device is soon total dose damaged. One must therefore chop the beam into a pulse of from 0.1 to several seconds in duration in order not to destroy the device being tested. Such a beam chopping system was designed and constructed. It is installed in the vacuum chamber just downstream from the degrader wheel and is shown schematically in fig. 5. It consists of a water cooled aperture which normally intercepts the beam. pneumatic
driver
solenoids
II
! mo”Sbl~ stop
a
Ocm
1 cm
2cm
3 cm
4cm
0.0289 0.0013
0.0255 0.0016
0.0248 0.0022
0.0193 0.0028
0.0056 0.0028
0.028
0.027
0.025
chopper box
a Similar ratio for 3 aluminum
activations.
Fig. 5. The elements of the beam chopper system inside the chopper vacuum chamber.
which
are
1259
K. M. Murray et al. / Single event research
This is followed by a defining aperture and then two aluminum shutter blocks which are driven by solenoids. The sequence of operation is to first open the water cooled aperture. As that reaches its final position it trips a microswitch which simultaneously fires the first shutter and starts a timing delay which then fires the second shutter. The water cooled aperture is then returned to its normal beam intercept position. Dosimetry for this method of operation is provided by radiochromic foils placed on the device being tested. A system of phosphor screen and movable beam stop is provided between the exit window and the test device to allow careful adjustment of the beam just prior to making an irradiation. The stability of the cyclotron operation allows one to achieve the desired exposure to within about 20% by this method. The dosimetry provides the incident dose actually received to within 5% for most configurations.
5. Software The system so far described provides very accurate and highly reproducible irradiation situations for most work. The next step was to then make this operate as conveniently as possible. This was achieved by using a desktop PC to control and operate the cyclotron beam and the associated beam monitoring equipment via an IEEE 488 bus. The code developed for this purpose (called BEAMMON) is a menu based program that provides simple but flexible use of all aspects of the degraded beam system. BEAMMON obtains accurate current measurements from the F cup and SEM electrometers. A beam stop controlled by BEAMMON turns the cyclotron beam on and off according to preset conditions available in various menus of the code.
Initially, the user specifies the incident ion, energy, and target material. Protons, deuterons and alpha partitles are supported as incident ions, with silicon and gallium arsenide targets. Dose calculations are performed by BEAMMON using calculations of energy deposition based on a relativistic Bethe-Bloch equation using Ziegler’s empirical ionization potentials and a semi-empirical screening correction.
Acknowledgements The authors would like to take this opportunity to express their appreciation for the excellent cooperation of the accelerator operating staff and particularly to Mr. Joel Macurdy, Mr. Walter Kemmler, and Mr. Jerry Stoddel for their many helpful suggestions and most useful talents. We would like to express our thanks to Mr. Tom Ward and Mr. Jerry Netherton for their professionalism and tireless support during many late hours at UCD, and we would like to acknowledge the efforts of Mr. Patrick McDonald in providing most of the useful enhancements to the BEAMMON code.
References [l] 10th Int. Conf on Cyclotrons and Their Applications, East Lansing, MI, ed. F. Martin, 1984, p. 723. [2] G.W. ,_ ___\ Toutfest and H.R. Fechter, Rev. Sci. Instr. 26(3) (lY33).
131 Review of Particle Properties, Phys. Lett. B170 (1986) 45. [4] W.N. Spjeldvilc, J. Geophys. Res. 82 (1977) 2801. [5] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Matter (Pergamon, 1985). [6] J.F. Ziegler, Handbook of Stopping Cross-Sections for Energetic Ions in All Elements, vol. 5 (Pergamon, 1980).
XVIII.
SOFT UPSETS