- Email: [email protected]
A method for the energy determination of heavy-ion beams
472
Nuclear Instruments and Methods in Physic,, Rcsearch 221 (ioS4! 471 473 "qorlh-[ lollzmd. \mstc~d~lu~
Letter to the Editor
A METHOD FOR THE ENERGY DETERMINATION OF HEAVY-ION BEAMS S. B L I ] M N E R
a n d F. R A U C H
lnstitut fiir Kernphysik, Universiti~t Frankfurt, Frankfurt am Mare, West German)' Received 4 November 1983
The method described is based on the spectrometry of recoil protons produced by elastic scattering of the heavy ions in a hydrogen-containing carbon foil.
For many investigations involving heavy-ion beams, an accurate knowledge of the absolute beam energy is necessary. The c o m m o n way to determine beam energies at most accelerators is to use an analysing magnet which has been calibrated with respect to well-known resonance or threshold reactions [1]. However, the calibration factor of the magnet obtained from these light-ion reactions cannot be used with sufficient accuracy for the stronger fields necessary for bending heavy ions, because of saturation and fringing field effects [l]. One method to overcome this difficulty has been described by Trautvetter et al. [2]. They obtained a calibration of the magnet for high field strengths by reversing the roles of projectile and target in proton induced reactions and bombarding a hydrogen gas target. Another, rather direct way to determine the energies of heavy-ion beams, which is applicable for any kind of projectile and over a large energy range, is based on the detection of recoil protons produced by elastic scattering of the projectiles in a thin target containing hydrogen. F r o m a measurement of the recoil energy, E R, the projectile energy, E m, can be deduced. In the nonrelativistic case, the relation is E R / E H ~ = 4 M I M z cos20/(MI + M2) 2, where M l and M 2 are the masses of projectile and target nucleus and 0 is the scattering angle [1]. Such a method was applied by Olsen et al. [3] and later by Bimbot et al. [4] for beam energies in the range 3 to 5 M e V / n u c l e o n . A formvar foil was used as the target and the recoil protons were detected at v~ = 0 °. A gold foil was placed in front of the Si detector to stop the projectiles. The energy calibration of the detector was obtained with the help of a SOUrCes.
At our Van-de-Graaf laboratory, we have developed a similar method for deducing heavy-ion energies from the energies of recoil protons. The energy region of interest was about 0.5 to l M e V / n u c l e o n , but this 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
method can also be used at higher energies. It shows several distinct differences compared to the method of Olsen et al., so that we thought it would be worth communicating it here. In short, the target is a thin carbon foil containing hydrogen, the recoil protons are detected at an angle 0 ~ 0 ° and the energy calibration of the detector is established with the same scattering arrangement using a proton beam from the accelerator. In our experimental set-up, the ion beam is collimated to 2 mm before passing through the C foil, The Si detector is placed at ~ = 30 °, at a distance of 325 mm from the foil. The scattering angle is accurately fixed by two slit collimators at distances of 27 and 117 mm from the foil each with an opening of 0,5 mm. The resulting narrow angular aperture of +0.1 ° leads to a small kinematic spread of the recoil energy; for E R = 1.5 MeV, which is a typical value, the spread is _+3 keV, A 14SGd a source can be moved in front of the detector and may serve as a secondary energy standard. A scattering angle 0 4= 0 ° was selected for two reasons. Firstly, the measuring device can stay at a beamline without disturbing other experiments; only the beam collimator and the C foil have to be removed. Secondly, a range foil in front of the detector to stop the projectiles is not necessary; such a foil would lead to an energy spread of the recoil protons due to energy straggling and might further introduce an uncertainty in the correction for the proton energy loss in the foil. One might assume that the rapid variation of E R with 0 requires a very precise knowledge of ~ for an accurate determination of E m . However, the way o f calibrating the Si detector described below makes this unnecessary; it is sufficient that the angle is well defined and is kept constant in all measurements. Carbon foils of 5 to 10 # g / c m 2 were chosen as " h y d r o g e n targets", because previous experience in the course of hydrogen profiling experiments had shown that they contain appreciable amounts of hydrogen,
S. Bli~mmer, F Rauch / Energy determination of heavy-ion beams
N
C 0 U n
t S
C I
channel Fig. 1. Example of a spectrum obtained with a 14N beam of 8 MeV using a 7 /tg/cm 2 C foil. (H - hydrogen recoils, Ccarbon recoils, N - scattered projectiles, 1 - impurities).
usually several atomic percent (compare also the literature on stripper foils, e.g. [5]). It was found that particle beams of some 10 nA lead to sufficiently strong recoil proton peaks in 10 to 20 min, while the number of scattered projectiles and recoiling 12C ions relative to the number of 1H recoils was small enough to be tolerated in the detector. An example of a spectrum is shown in fig. 1. Such thin foils cause a relatively small energy loss of the projectiles, A Ef and a correspondingly small spread in the recoil energy, AE R. For the typical ase of 160 ions at 10 MeV and a 5 / ~ g / c m 2 foil, one has AEf = 50 keV and AEf= 8.3 keV (for 0 = 30°). Therefore, the width of the proton peaks is not much increased, given a detector resolution for protons of about 15 keV. Of course, in calculating the beam energy from the measured value E R derived from the peak center, one has to correct for the projectile energy loss, the correction being AEf/2; the corresponding correction for protons is negligibly small. An essential point of our technique is the energy calibration of the Si detector. It is obtained using protons of known energy, Ep, as projectiles and detecting the recoil protons and the protons scattered from hydrogen in the foil, both of which have energy E R = Ep cos20. The proton beam energies up to a few MeV are known, to better than 1 keV, from the calibration of
473
the accelerator with resonance and threshold reactions. The angle t* drops out in the calculation of E m, since it is the same as in the measurements with heavy-ion beams. This way of calibrating the Si detector has several advantages compared to calibration with a sources: (i) the calibration can be extended to low proton energies for which no a sources are available; (ii) a correction for the different energy losses of a particles and protons in the dead layer of the detector is not necessary and (iii) there is no trouble with the pulseheight discrepancy between protons and a particles [4,6]. Based on this calibration, which allows the determination of the energies of recoil protons from heavy ions with an uncertainty 8E R of 1 to 2 keV, the energies of heavy-ion beams can be deduced with an accuracy of 0.2% or better. In the foregoing example of 160 ions of 10 MeV, an uncertainty 8E R = 1.7 keV leads to an accuracy 8 E n l = 10 keV. One should note that because of the correction A Et/2 mentioned above, the foil thickness must be accurately known to prevent an error in E m . Uncertainties in the foil thickness may exist, because the nominal thickness given by the producer is not correct or because the thickness increases under beam impact due to carbon deposition from cracked gaseous hydrocarbon, as we have observed in prolonged bombardments of several hours. The recoil technique itself provides a nice way to determine the actual value of A Ef. An auxiliary C foil of about the same thickness as the primary foil is used as the scattering foil and one measures the shift in the recoil energy when the primary foil is placed into the heavy-ion beam in front of the scattering foil.
References [1] J.B. Marion and F.C. Young, Nuclear reaction analysis (North-Holland, Amsterdam, 1968). [2] H.P. Trautvetter, K. Elix and C. Rolfs, Nucl. Instr. and Meth. 161 (1979) 173. [3] D.K. Olsen, N.H. Merrill, S.F. Biagi, W.R. Phillips and A.R. Barnett, Nucl. Instr. and Meth. 114 (1974) 615. [4] R. Bimbot, D. Gardrs, H. Geissel and K.H. Schmidt, Z. Physik A286 (1978) 327. [5] N.R.S. Tait, D.W.L. Tolfree, P. John, I.M. Odeh, M.J.K. Thomas, M.J. Tricker, J.I.B. Wilson, J.B.A. England and D. Newton, Nucl. Instr. and Meth. 176 (1980) 433. [6] K.W. Kemper and J.D. Fox, Nucl. Instr. and Meth. 105 (1972) 333.
Nuclear Instruments and Methods in Physic,, Rcsearch 221 (ioS4! 471 473 "qorlh-[ lollzmd. \mstc~d~lu~
Letter to the Editor
A METHOD FOR THE ENERGY DETERMINATION OF HEAVY-ION BEAMS S. B L I ] M N E R
a n d F. R A U C H
lnstitut fiir Kernphysik, Universiti~t Frankfurt, Frankfurt am Mare, West German)' Received 4 November 1983
The method described is based on the spectrometry of recoil protons produced by elastic scattering of the heavy ions in a hydrogen-containing carbon foil.
For many investigations involving heavy-ion beams, an accurate knowledge of the absolute beam energy is necessary. The c o m m o n way to determine beam energies at most accelerators is to use an analysing magnet which has been calibrated with respect to well-known resonance or threshold reactions [1]. However, the calibration factor of the magnet obtained from these light-ion reactions cannot be used with sufficient accuracy for the stronger fields necessary for bending heavy ions, because of saturation and fringing field effects [l]. One method to overcome this difficulty has been described by Trautvetter et al. [2]. They obtained a calibration of the magnet for high field strengths by reversing the roles of projectile and target in proton induced reactions and bombarding a hydrogen gas target. Another, rather direct way to determine the energies of heavy-ion beams, which is applicable for any kind of projectile and over a large energy range, is based on the detection of recoil protons produced by elastic scattering of the projectiles in a thin target containing hydrogen. F r o m a measurement of the recoil energy, E R, the projectile energy, E m, can be deduced. In the nonrelativistic case, the relation is E R / E H ~ = 4 M I M z cos20/(MI + M2) 2, where M l and M 2 are the masses of projectile and target nucleus and 0 is the scattering angle [1]. Such a method was applied by Olsen et al. [3] and later by Bimbot et al. [4] for beam energies in the range 3 to 5 M e V / n u c l e o n . A formvar foil was used as the target and the recoil protons were detected at v~ = 0 °. A gold foil was placed in front of the Si detector to stop the projectiles. The energy calibration of the detector was obtained with the help of a SOUrCes.
At our Van-de-Graaf laboratory, we have developed a similar method for deducing heavy-ion energies from the energies of recoil protons. The energy region of interest was about 0.5 to l M e V / n u c l e o n , but this 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
method can also be used at higher energies. It shows several distinct differences compared to the method of Olsen et al., so that we thought it would be worth communicating it here. In short, the target is a thin carbon foil containing hydrogen, the recoil protons are detected at an angle 0 ~ 0 ° and the energy calibration of the detector is established with the same scattering arrangement using a proton beam from the accelerator. In our experimental set-up, the ion beam is collimated to 2 mm before passing through the C foil, The Si detector is placed at ~ = 30 °, at a distance of 325 mm from the foil. The scattering angle is accurately fixed by two slit collimators at distances of 27 and 117 mm from the foil each with an opening of 0,5 mm. The resulting narrow angular aperture of +0.1 ° leads to a small kinematic spread of the recoil energy; for E R = 1.5 MeV, which is a typical value, the spread is _+3 keV, A 14SGd a source can be moved in front of the detector and may serve as a secondary energy standard. A scattering angle 0 4= 0 ° was selected for two reasons. Firstly, the measuring device can stay at a beamline without disturbing other experiments; only the beam collimator and the C foil have to be removed. Secondly, a range foil in front of the detector to stop the projectiles is not necessary; such a foil would lead to an energy spread of the recoil protons due to energy straggling and might further introduce an uncertainty in the correction for the proton energy loss in the foil. One might assume that the rapid variation of E R with 0 requires a very precise knowledge of ~ for an accurate determination of E m . However, the way o f calibrating the Si detector described below makes this unnecessary; it is sufficient that the angle is well defined and is kept constant in all measurements. Carbon foils of 5 to 10 # g / c m 2 were chosen as " h y d r o g e n targets", because previous experience in the course of hydrogen profiling experiments had shown that they contain appreciable amounts of hydrogen,
S. Bli~mmer, F Rauch / Energy determination of heavy-ion beams
N
C 0 U n
t S
C I
channel Fig. 1. Example of a spectrum obtained with a 14N beam of 8 MeV using a 7 /tg/cm 2 C foil. (H - hydrogen recoils, Ccarbon recoils, N - scattered projectiles, 1 - impurities).
usually several atomic percent (compare also the literature on stripper foils, e.g. [5]). It was found that particle beams of some 10 nA lead to sufficiently strong recoil proton peaks in 10 to 20 min, while the number of scattered projectiles and recoiling 12C ions relative to the number of 1H recoils was small enough to be tolerated in the detector. An example of a spectrum is shown in fig. 1. Such thin foils cause a relatively small energy loss of the projectiles, A Ef and a correspondingly small spread in the recoil energy, AE R. For the typical ase of 160 ions at 10 MeV and a 5 / ~ g / c m 2 foil, one has AEf = 50 keV and AEf= 8.3 keV (for 0 = 30°). Therefore, the width of the proton peaks is not much increased, given a detector resolution for protons of about 15 keV. Of course, in calculating the beam energy from the measured value E R derived from the peak center, one has to correct for the projectile energy loss, the correction being AEf/2; the corresponding correction for protons is negligibly small. An essential point of our technique is the energy calibration of the Si detector. It is obtained using protons of known energy, Ep, as projectiles and detecting the recoil protons and the protons scattered from hydrogen in the foil, both of which have energy E R = Ep cos20. The proton beam energies up to a few MeV are known, to better than 1 keV, from the calibration of
473
the accelerator with resonance and threshold reactions. The angle t* drops out in the calculation of E m, since it is the same as in the measurements with heavy-ion beams. This way of calibrating the Si detector has several advantages compared to calibration with a sources: (i) the calibration can be extended to low proton energies for which no a sources are available; (ii) a correction for the different energy losses of a particles and protons in the dead layer of the detector is not necessary and (iii) there is no trouble with the pulseheight discrepancy between protons and a particles [4,6]. Based on this calibration, which allows the determination of the energies of recoil protons from heavy ions with an uncertainty 8E R of 1 to 2 keV, the energies of heavy-ion beams can be deduced with an accuracy of 0.2% or better. In the foregoing example of 160 ions of 10 MeV, an uncertainty 8E R = 1.7 keV leads to an accuracy 8 E n l = 10 keV. One should note that because of the correction A Et/2 mentioned above, the foil thickness must be accurately known to prevent an error in E m . Uncertainties in the foil thickness may exist, because the nominal thickness given by the producer is not correct or because the thickness increases under beam impact due to carbon deposition from cracked gaseous hydrocarbon, as we have observed in prolonged bombardments of several hours. The recoil technique itself provides a nice way to determine the actual value of A Ef. An auxiliary C foil of about the same thickness as the primary foil is used as the scattering foil and one measures the shift in the recoil energy when the primary foil is placed into the heavy-ion beam in front of the scattering foil.
References [1] J.B. Marion and F.C. Young, Nuclear reaction analysis (North-Holland, Amsterdam, 1968). [2] H.P. Trautvetter, K. Elix and C. Rolfs, Nucl. Instr. and Meth. 161 (1979) 173. [3] D.K. Olsen, N.H. Merrill, S.F. Biagi, W.R. Phillips and A.R. Barnett, Nucl. Instr. and Meth. 114 (1974) 615. [4] R. Bimbot, D. Gardrs, H. Geissel and K.H. Schmidt, Z. Physik A286 (1978) 327. [5] N.R.S. Tait, D.W.L. Tolfree, P. John, I.M. Odeh, M.J.K. Thomas, M.J. Tricker, J.I.B. Wilson, J.B.A. England and D. Newton, Nucl. Instr. and Meth. 176 (1980) 433. [6] K.W. Kemper and J.D. Fox, Nucl. Instr. and Meth. 105 (1972) 333.