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Beam identification system for a 2 MV mass separator
Vacuum/volume 39/numbers Printed in Great Britain
Beam identification separator W N Lennard,
0042-207X/89$3.00+.00 Pergamon Press plc
2_4/pages 413 to 415/l 989
Department
of Physics,
system The University
for a 2 MV mass
of Western Ontario,
London,
Canada N6A 3K7
We have developed a simple, inexpensive particle identification system for low energy heavy ions using an electrostatic device. The technique permits the analysis of fragments derived from molecular ions that are dissociated by a thin carbon foil located upstream from an electrostatic analyzer. The success of the system in identifying low intensity (- 0.5 nA) atomic and molecular ion beams produced by a universal ion source is described. For example, 27A135C12c was observed to be a contaminant in a 31P+ beam. Limitations of the technique are also discussed.
1. Introduction Most accelerators that produce heavy ion beams suffer from the periodic problem of unwanted contaminants that may arrive at the target together with desired species. Such problems are known to be caused by : (i) particles (molecular or atomic) whose kinematic properties mimic the acceleration and deflection kinematics of the desired particle ; (ii) unwanted particles that reach the target because of low probability charge exchange sequences that can occur upstream of the target ; (iii) particles that have completely incorrect kinematics, but that arrive at the target because of (wall) scattering. Although there exist ultra-sensitive particle identification systems for higher energy machines’, such devices cannot generally be used for lower energy accelerators. We have developed a simple, inexpensive particle identification system for low energy heavy ions using electrostatic deflection that is capable of illuminating problems (i) and (ii) above in order to identify most of the beams produced by the Chalk River 2 MV High Voltage Mass Separator equipped with a Danfysik 910 ion source. The success of the system in identifying very low intensity (- 0.5 nA) atomic and molecular ion beams is described. Limitations of the technique, particularly for hydrides, are also discussed.
charge state is q = 1, assuming no charge exchange after acceleration and before magnetic deflection. The projectiles were collimated to an area 1.5 mm x 1.5 mm by vertical and horizontal slits located downstream of the accelerator regulating slits. A schematic of the beam identification system is shown in Figure 1. The particles traversed in succession a thin self-supporting carbon foil (7 pg cm-‘), a circular electroformed aperture (A ,, 0.04 mm diameter) located well downstream of the foil to limit scattering from the edges of the aperture, a parallel plate electrostatic analyzer (ESA) positioned as close as possible to A ,, and an exit slit (S,) of adjustable width LIx(Ax1 mm) located a distance x (x = 12 mm) off the beam and ESA axis. The entire system downstream of the carbon foil was easily mounted and demounted from the accelerator beam line. The ion detector was similar to that described by Smith and Whaling’. Behind the ESA exit slit, ions were detected by a thin (1 mm) inexpensive CsI(T1) scintillator optically coupled to a Du Mont 6292 photomultiplier. The energy resolution of
CsI(TI) Carbon
2. Experimental Ion beams produced in a Danfysik 910 source, generally constructed of stainless steel, were injected into the acceleration region of a 2 MV Pelletron (National Electrostatics Corporation 2 UH). The ion beam is then analyzed by a 90” magnet with a momentum dispersion at the image (regulating) slits of Ap/p = 0.013 mm- ‘. Since the analyzing magnet of the accelerator has been calibrated, we then know the mass, M, and kinetic energy, E, of the beam from the relation
ME 2
4
=
k,.
(1)
The constant k, is a consequence of the field and geometry of the magnetic analyzer. For very small beams, the most probable
To ESA b Rate meter
_
Divider (+loq
SCA
_
PAM
-
ADC
I
I Preamp
_
Amp
Delay
-
Lmear
gate
*
ADC2
Figure 1. Schematic of the ESA beam identification system. OPS&oper-
ational power supply (programmable), PAM-pulse amplitude modulator, PMT-photomultiplier tube. The distances indicated are I, = 18 cm, I, = 15 cm, d = 0.6 cm, I, = 36 cm. 413
WN
Lennard;
Beam identification system
CsI(T1) is unimportant for this particular application. The light output response of CsI(T1) is discussed in refs 3,4 and 5. Smith and Whaling’ were able to use CsI(T1) to detect 190 keV 5hFe ions. We have used our device to detect 500 keV 84Kr ions. The aperture A, served to attenuate the beam intensity. With a typical 0.04 mm diameter aperture, beams of intensity 0.5 nA yielded total count rates (measured at 0” after the ESA using an exit slit that is not shown in Figure 1) of lo3 s ‘. Thus, spectra could be accumulated within a few minutes. The carbon foil served to dissociate molecular ion beams and to produce a charge state spectrum of the ions exiting from the back surface. The energy loss of the heavy ions in the 7 pg cm- ’ foil is small for projectile energies of a few hundred keV. The foil lifetime was very long for small beam current densities (several hours for beams - 1 nA mm-‘). The positive ion charge spectra were recorded by measuring ion intensities as a function of the ESA voltage (see Figure 1). The ESA voltage, variable from O-5 kV, was controlled by a programmable power supply driven by a triangular waveform generator. The sweeping voltage period was variable from 0.220 s. The PMT output pulses were amplified and routed through an SCA. The SCA constant amplitude pulses were modulated by the instantaneous analog voltage from the high voltage sweep to yield pulses whose amplitude was proportional to the ESA plate voltage, V. These pulses were accumulated in ADC 1. The PMT pulse height spectrum was kept for reference in ADC 2. Using Figure 1, we obtain the relation
qv1,
X=2E,d where x is the distance of the ESA exit slit from the axis (O”), E, is the particle energy after the foil,’ and I,, I,, d are as indicated. Equation (2) is then independent of projectile mass. If the slit width is Ax, then the system resolution is AX/X = AV/V, i.e. - 7%. ADC 1 must then be calibrated using a known beam at the same energy, since the channel number location of the charge state q peak, C(q), will be proportional to Eo/q. Thus the widths of the various charge state peaks, AC(q), are given by AC(q)/C(q) = Ax/x. In practice, the ESA was swept from V,,,,,, to V,,,,, where V,,, was determined by the projectile energy ; from Figure 1 and using equation (2), we found that the highest energy beam for which the 1 + ion could be detected in our geometry using V,,, = 5 kV was 1400 keV. Generally, V,,,,, - V,,,,,/35.
CHANNEL
NO
Figure 2. (a) Charge state spectrum for 900 keV ’ 'B"Ff exiting a 7 pg cm-l carbon foil. The charge state fractions are proportional to the heights of the appropriate peaks. The location of the 14N’+ peak used for calibration is shown by the arrow. (b) Calculated spectrum for BF: assuming equal fractions in all charge states. The solid lines and dashed lines locate the “F and ’ 'B charge state peaks, respectively.
shows a predicted spectrum for BF, assuming equal numbers of projectiles in all charge states, but with twice as many detected 19F ions as B” Ions. We also assume that the energy losses of “B, “F and 14N in the carbon foil are all identical. The beam identification was confirmed by observing a beam at M = 48 with l/4 of the intensity of the A4 = 49 beam (consistent with the natural abundance ratio ‘“B/l ‘B = l/4). We could not detect “B+, “F+ or BF: beams despite the large BFT beam intensity. 3.2. We have identified a beam at M = 59 as CaF+, shown in Figure 3(a). The arrow again shows the location of “‘SC*+ at the same machine energy of 1100 keV. Figure 3(b) shows a predicted spectrum assuming equal population of all charge states, as in Figure 2(b). 3.3. We observed two beams with the same magnetic rigidity. corresponding to M = 7 (q = 1), when running a beam from a Nz gas bottle. One beam was of course 14N2+ accelerated from the machine to an energy E, = 2E, (E, = terminal volts). The spectrum of the second beam showed a single species with the q = I+ peak indicating an energy Ez = EJ2. This beam was then identified as magnetically analyzed Nf+ , presumably formed by charge exchange NT + Nt+ + em occurring after acceleration but
3. Results We will illustrate the usefulness examples.
of the ESA device through
several
3.1. A large (30 nA)) beam was observed at M = 49 (for q = 1). We ran “‘N+ at E = 900 keV to calibrate the system. The spectrum for the M = 49 beam (reduced to 2 nA intensity for count rate purposes) also at 900 keV is shown in Figure 2(a). The position of the 14NL+ peak is indicated by the arrow. We observe two distinct charge state spectra ; however, from the positions of the 1+ peaks, we see that the molecule must be triatomic since, ignoring differences in energy loss in the carbon foil, the channel numbers of the q = 1+ peaks must sum to twice the channel number of the 14N2+ peak. Thus, M = 2M, + M2. Also, we czn get M,/M and M,/M from the I+ peaks. We then find M, = 19, Mz = 11 yielding the unknown beam to be BF+. Figure 2(b) 414
CHANNEL
NO.
Figure 3. (a, b) Same as Figure 2 except for CaF+ Incident at 1100 keV on a thin carbon foil. The calibrating beam is 100 keV “‘SC+.
W N Lennard:
Beam identification system
before magnetic deflection. The intensity ratio, N:+/N:, was - lo- 3 and increased when the vacuum prior to the magnet was deliberately made poorer. This contaminant is an example of problem (ii) discussed in Section 1. 3.4. Several examples of molecular contaminant interferences have been observed. For example, in trying to obtain a 45Sc+ beam, using ScCl, as the source material in a stainless steel source, we observed an equal admixture of ssMn3sC12f, which was also confirmed by the presence of 5sMn37C12f (A4 = 46 for q = 1) in the proper isotopic abundance (35C1/37C1 = 3/l) and 5sMn35Clc at M = 90. 3.5. After having run Al beams, we observed that *‘A13’C12+ was a contaminant in a desired “Pi beam. This is a memory effect of the ion source that can be minimized by cleaning the source before loading a different solid material. 3.6. In general, using our stainless steel ion source, we found that metallic chloride beams were often produced at the - 1 nA intensity ; those that have been identified using our ESA device in addition to those already mentioned are : FeCI+ (M = 89, 9 1, 93) FeCl*+ (M = 444,45$46+), CaCl+ (M = 75,77). 4. Limitations The ESA beam identifier system is not so useful for identifying beams of heavy triatomic hydrides. As an example, we observed a beam at M = 47 (q = 1) that showed two distinct charge state distributions. Using a *‘Ne calibration, one of the constituent species was observed to be 19F. Since the equality C(q) = E,/q (mass independent) is only approximate due to slight differences in energy loss in the carbon foil between the *‘Ne calibration beam and the constituent ions of the molecular beam, it was not clear whether the other constituent was *‘Al or **Si, implying that the M = 47 could be AlHF+ or SiF+. We measured the ratio of the charge state fractions, f,/f2 (f, = fraction of projectiles detected in charge state q), for the larger mass molecular constituent at two velocities. By comparing these data to actual measured values for *‘Al and *‘Si exciting C foils (see ref 6), we deduced that the beam was SiF+. At this point, we could have relied on another technique, see Section 5. For the M = 49 case discussed in Section 3.1, we observed a non-negligible q = - 1 component for the heavier of the two species, suggesting that it is C, 0 or F using the results of ref 6. This observation lends further support to our identification. Diatomic hydrides (e.g. HCI) are not difficult to identify, although we have not used the ESA device to investigate these species. Such hydrides are common products of heavy ion sources.
5. Other techniques Several other methods can be used to identify low energy accelerator beams by their energy or by the radiation they produce.
5.1. Rutherford scattering from a high-Z material is not very useful for low energy particles due to the non-linear response of surface barrier detectors to heavy particles’. Large pulse height defects and non-negligible energy losses in the Au window of the detector at low energies make this technique of limited applicability. 5.2. By inserting a carbon foil in the beam, the visible and nearultraviolet radiation from the transmitted particles can be viewed dispersively, but with low efficiency. This is the field of beam-foil spectroscopy, and is not viable for small intensity beams due to signal-to-noise limitations. Also, without extensive tables of absolute atomic transition probabilities, level lifetimes, and level population probabilities, it is difficult to obtain quantitative results for beam interferences.
5.3. X-ray spectroscopy using Si(Li) detectors can also be used in the same manner as the beam-foil technique. Even less is known in this area concerning projectile-target X-ray production probabilities, making quantitative constituent analysis of beam interferences difficult. However, the detection efficiency is not such a problem here. We point out that this technique could have solved the M = 47 problem discussed in Section 4 simply by looking for SiK or AlK X-rays produced by projectile-carbon collisions in the foil.
5.4. Time-of-flight analysis of the projectile velocity is useful only in cases where charge exchange sequences have occurred (problem (ii) of Section 1). 6. Summary We have demonstrated the usefulness of a compact, inexpensive beam identification system for low energy heavy ion accelerators by reference to many examples. The technique is rapid and capable of identifying very small (< 0.5 nA) projectile beams. It allows unambiguous identification of all beams except heavy hydrided triatomic molecules. The latter problem can be resolved via X-ray analysis or from a more detailed examination of the charge state distributions of the particles. References
I K H Purser, A E Litherland and H E Gove, Nucl Instrum Meth, 162, 637 (1979). * P L Smith and W Whaling, Phys Rev, 188, 36 (1969). ’ S Bashkin, R R Carlson, R A Douglas and J A Jacobs, Phys Rev, 109, 434 (1958). 4 R W Hill, Rev Sci Insrrum, 33, 1477 (1962). ‘A C Riviere and D R Sweetman, Rev Sci Instrum, 34, 1286 (1963). 6 W N Lennard, D Phillips and D A S Walker, Nucl Instrum Meth, 179, 413 (1981). ’ W N Lennard, H Geissel, K B Winterbon, D Phillips, T K Alexander and J S Forster, Nucl Instrum Meth Phys Res, A248,454 (1986).
Beam identification separator W N Lennard,
0042-207X/89$3.00+.00 Pergamon Press plc
2_4/pages 413 to 415/l 989
Department
of Physics,
system The University
for a 2 MV mass
of Western Ontario,
London,
Canada N6A 3K7
We have developed a simple, inexpensive particle identification system for low energy heavy ions using an electrostatic device. The technique permits the analysis of fragments derived from molecular ions that are dissociated by a thin carbon foil located upstream from an electrostatic analyzer. The success of the system in identifying low intensity (- 0.5 nA) atomic and molecular ion beams produced by a universal ion source is described. For example, 27A135C12c was observed to be a contaminant in a 31P+ beam. Limitations of the technique are also discussed.
1. Introduction Most accelerators that produce heavy ion beams suffer from the periodic problem of unwanted contaminants that may arrive at the target together with desired species. Such problems are known to be caused by : (i) particles (molecular or atomic) whose kinematic properties mimic the acceleration and deflection kinematics of the desired particle ; (ii) unwanted particles that reach the target because of low probability charge exchange sequences that can occur upstream of the target ; (iii) particles that have completely incorrect kinematics, but that arrive at the target because of (wall) scattering. Although there exist ultra-sensitive particle identification systems for higher energy machines’, such devices cannot generally be used for lower energy accelerators. We have developed a simple, inexpensive particle identification system for low energy heavy ions using electrostatic deflection that is capable of illuminating problems (i) and (ii) above in order to identify most of the beams produced by the Chalk River 2 MV High Voltage Mass Separator equipped with a Danfysik 910 ion source. The success of the system in identifying very low intensity (- 0.5 nA) atomic and molecular ion beams is described. Limitations of the technique, particularly for hydrides, are also discussed.
charge state is q = 1, assuming no charge exchange after acceleration and before magnetic deflection. The projectiles were collimated to an area 1.5 mm x 1.5 mm by vertical and horizontal slits located downstream of the accelerator regulating slits. A schematic of the beam identification system is shown in Figure 1. The particles traversed in succession a thin self-supporting carbon foil (7 pg cm-‘), a circular electroformed aperture (A ,, 0.04 mm diameter) located well downstream of the foil to limit scattering from the edges of the aperture, a parallel plate electrostatic analyzer (ESA) positioned as close as possible to A ,, and an exit slit (S,) of adjustable width LIx(Ax1 mm) located a distance x (x = 12 mm) off the beam and ESA axis. The entire system downstream of the carbon foil was easily mounted and demounted from the accelerator beam line. The ion detector was similar to that described by Smith and Whaling’. Behind the ESA exit slit, ions were detected by a thin (1 mm) inexpensive CsI(T1) scintillator optically coupled to a Du Mont 6292 photomultiplier. The energy resolution of
CsI(TI) Carbon
2. Experimental Ion beams produced in a Danfysik 910 source, generally constructed of stainless steel, were injected into the acceleration region of a 2 MV Pelletron (National Electrostatics Corporation 2 UH). The ion beam is then analyzed by a 90” magnet with a momentum dispersion at the image (regulating) slits of Ap/p = 0.013 mm- ‘. Since the analyzing magnet of the accelerator has been calibrated, we then know the mass, M, and kinetic energy, E, of the beam from the relation
ME 2
4
=
k,.
(1)
The constant k, is a consequence of the field and geometry of the magnetic analyzer. For very small beams, the most probable
To ESA b Rate meter
_
Divider (+loq
SCA
_
PAM
-
ADC
I
I Preamp
_
Amp
Delay
-
Lmear
gate
*
ADC2
Figure 1. Schematic of the ESA beam identification system. OPS&oper-
ational power supply (programmable), PAM-pulse amplitude modulator, PMT-photomultiplier tube. The distances indicated are I, = 18 cm, I, = 15 cm, d = 0.6 cm, I, = 36 cm. 413
WN
Lennard;
Beam identification system
CsI(T1) is unimportant for this particular application. The light output response of CsI(T1) is discussed in refs 3,4 and 5. Smith and Whaling’ were able to use CsI(T1) to detect 190 keV 5hFe ions. We have used our device to detect 500 keV 84Kr ions. The aperture A, served to attenuate the beam intensity. With a typical 0.04 mm diameter aperture, beams of intensity 0.5 nA yielded total count rates (measured at 0” after the ESA using an exit slit that is not shown in Figure 1) of lo3 s ‘. Thus, spectra could be accumulated within a few minutes. The carbon foil served to dissociate molecular ion beams and to produce a charge state spectrum of the ions exiting from the back surface. The energy loss of the heavy ions in the 7 pg cm- ’ foil is small for projectile energies of a few hundred keV. The foil lifetime was very long for small beam current densities (several hours for beams - 1 nA mm-‘). The positive ion charge spectra were recorded by measuring ion intensities as a function of the ESA voltage (see Figure 1). The ESA voltage, variable from O-5 kV, was controlled by a programmable power supply driven by a triangular waveform generator. The sweeping voltage period was variable from 0.220 s. The PMT output pulses were amplified and routed through an SCA. The SCA constant amplitude pulses were modulated by the instantaneous analog voltage from the high voltage sweep to yield pulses whose amplitude was proportional to the ESA plate voltage, V. These pulses were accumulated in ADC 1. The PMT pulse height spectrum was kept for reference in ADC 2. Using Figure 1, we obtain the relation
qv1,
X=2E,d where x is the distance of the ESA exit slit from the axis (O”), E, is the particle energy after the foil,’ and I,, I,, d are as indicated. Equation (2) is then independent of projectile mass. If the slit width is Ax, then the system resolution is AX/X = AV/V, i.e. - 7%. ADC 1 must then be calibrated using a known beam at the same energy, since the channel number location of the charge state q peak, C(q), will be proportional to Eo/q. Thus the widths of the various charge state peaks, AC(q), are given by AC(q)/C(q) = Ax/x. In practice, the ESA was swept from V,,,,,, to V,,,,, where V,,, was determined by the projectile energy ; from Figure 1 and using equation (2), we found that the highest energy beam for which the 1 + ion could be detected in our geometry using V,,, = 5 kV was 1400 keV. Generally, V,,,,, - V,,,,,/35.
CHANNEL
NO
Figure 2. (a) Charge state spectrum for 900 keV ’ 'B"Ff exiting a 7 pg cm-l carbon foil. The charge state fractions are proportional to the heights of the appropriate peaks. The location of the 14N’+ peak used for calibration is shown by the arrow. (b) Calculated spectrum for BF: assuming equal fractions in all charge states. The solid lines and dashed lines locate the “F and ’ 'B charge state peaks, respectively.
shows a predicted spectrum for BF, assuming equal numbers of projectiles in all charge states, but with twice as many detected 19F ions as B” Ions. We also assume that the energy losses of “B, “F and 14N in the carbon foil are all identical. The beam identification was confirmed by observing a beam at M = 48 with l/4 of the intensity of the A4 = 49 beam (consistent with the natural abundance ratio ‘“B/l ‘B = l/4). We could not detect “B+, “F+ or BF: beams despite the large BFT beam intensity. 3.2. We have identified a beam at M = 59 as CaF+, shown in Figure 3(a). The arrow again shows the location of “‘SC*+ at the same machine energy of 1100 keV. Figure 3(b) shows a predicted spectrum assuming equal population of all charge states, as in Figure 2(b). 3.3. We observed two beams with the same magnetic rigidity. corresponding to M = 7 (q = 1), when running a beam from a Nz gas bottle. One beam was of course 14N2+ accelerated from the machine to an energy E, = 2E, (E, = terminal volts). The spectrum of the second beam showed a single species with the q = I+ peak indicating an energy Ez = EJ2. This beam was then identified as magnetically analyzed Nf+ , presumably formed by charge exchange NT + Nt+ + em occurring after acceleration but
3. Results We will illustrate the usefulness examples.
of the ESA device through
several
3.1. A large (30 nA)) beam was observed at M = 49 (for q = 1). We ran “‘N+ at E = 900 keV to calibrate the system. The spectrum for the M = 49 beam (reduced to 2 nA intensity for count rate purposes) also at 900 keV is shown in Figure 2(a). The position of the 14NL+ peak is indicated by the arrow. We observe two distinct charge state spectra ; however, from the positions of the 1+ peaks, we see that the molecule must be triatomic since, ignoring differences in energy loss in the carbon foil, the channel numbers of the q = 1+ peaks must sum to twice the channel number of the 14N2+ peak. Thus, M = 2M, + M2. Also, we czn get M,/M and M,/M from the I+ peaks. We then find M, = 19, Mz = 11 yielding the unknown beam to be BF+. Figure 2(b) 414
CHANNEL
NO.
Figure 3. (a, b) Same as Figure 2 except for CaF+ Incident at 1100 keV on a thin carbon foil. The calibrating beam is 100 keV “‘SC+.
W N Lennard:
Beam identification system
before magnetic deflection. The intensity ratio, N:+/N:, was - lo- 3 and increased when the vacuum prior to the magnet was deliberately made poorer. This contaminant is an example of problem (ii) discussed in Section 1. 3.4. Several examples of molecular contaminant interferences have been observed. For example, in trying to obtain a 45Sc+ beam, using ScCl, as the source material in a stainless steel source, we observed an equal admixture of ssMn3sC12f, which was also confirmed by the presence of 5sMn37C12f (A4 = 46 for q = 1) in the proper isotopic abundance (35C1/37C1 = 3/l) and 5sMn35Clc at M = 90. 3.5. After having run Al beams, we observed that *‘A13’C12+ was a contaminant in a desired “Pi beam. This is a memory effect of the ion source that can be minimized by cleaning the source before loading a different solid material. 3.6. In general, using our stainless steel ion source, we found that metallic chloride beams were often produced at the - 1 nA intensity ; those that have been identified using our ESA device in addition to those already mentioned are : FeCI+ (M = 89, 9 1, 93) FeCl*+ (M = 444,45$46+), CaCl+ (M = 75,77). 4. Limitations The ESA beam identifier system is not so useful for identifying beams of heavy triatomic hydrides. As an example, we observed a beam at M = 47 (q = 1) that showed two distinct charge state distributions. Using a *‘Ne calibration, one of the constituent species was observed to be 19F. Since the equality C(q) = E,/q (mass independent) is only approximate due to slight differences in energy loss in the carbon foil between the *‘Ne calibration beam and the constituent ions of the molecular beam, it was not clear whether the other constituent was *‘Al or **Si, implying that the M = 47 could be AlHF+ or SiF+. We measured the ratio of the charge state fractions, f,/f2 (f, = fraction of projectiles detected in charge state q), for the larger mass molecular constituent at two velocities. By comparing these data to actual measured values for *‘Al and *‘Si exciting C foils (see ref 6), we deduced that the beam was SiF+. At this point, we could have relied on another technique, see Section 5. For the M = 49 case discussed in Section 3.1, we observed a non-negligible q = - 1 component for the heavier of the two species, suggesting that it is C, 0 or F using the results of ref 6. This observation lends further support to our identification. Diatomic hydrides (e.g. HCI) are not difficult to identify, although we have not used the ESA device to investigate these species. Such hydrides are common products of heavy ion sources.
5. Other techniques Several other methods can be used to identify low energy accelerator beams by their energy or by the radiation they produce.
5.1. Rutherford scattering from a high-Z material is not very useful for low energy particles due to the non-linear response of surface barrier detectors to heavy particles’. Large pulse height defects and non-negligible energy losses in the Au window of the detector at low energies make this technique of limited applicability. 5.2. By inserting a carbon foil in the beam, the visible and nearultraviolet radiation from the transmitted particles can be viewed dispersively, but with low efficiency. This is the field of beam-foil spectroscopy, and is not viable for small intensity beams due to signal-to-noise limitations. Also, without extensive tables of absolute atomic transition probabilities, level lifetimes, and level population probabilities, it is difficult to obtain quantitative results for beam interferences.
5.3. X-ray spectroscopy using Si(Li) detectors can also be used in the same manner as the beam-foil technique. Even less is known in this area concerning projectile-target X-ray production probabilities, making quantitative constituent analysis of beam interferences difficult. However, the detection efficiency is not such a problem here. We point out that this technique could have solved the M = 47 problem discussed in Section 4 simply by looking for SiK or AlK X-rays produced by projectile-carbon collisions in the foil.
5.4. Time-of-flight analysis of the projectile velocity is useful only in cases where charge exchange sequences have occurred (problem (ii) of Section 1). 6. Summary We have demonstrated the usefulness of a compact, inexpensive beam identification system for low energy heavy ion accelerators by reference to many examples. The technique is rapid and capable of identifying very small (< 0.5 nA) projectile beams. It allows unambiguous identification of all beams except heavy hydrided triatomic molecules. The latter problem can be resolved via X-ray analysis or from a more detailed examination of the charge state distributions of the particles. References
I K H Purser, A E Litherland and H E Gove, Nucl Instrum Meth, 162, 637 (1979). * P L Smith and W Whaling, Phys Rev, 188, 36 (1969). ’ S Bashkin, R R Carlson, R A Douglas and J A Jacobs, Phys Rev, 109, 434 (1958). 4 R W Hill, Rev Sci Insrrum, 33, 1477 (1962). ‘A C Riviere and D R Sweetman, Rev Sci Instrum, 34, 1286 (1963). 6 W N Lennard, D Phillips and D A S Walker, Nucl Instrum Meth, 179, 413 (1981). ’ W N Lennard, H Geissel, K B Winterbon, D Phillips, T K Alexander and J S Forster, Nucl Instrum Meth Phys Res, A248,454 (1986).