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High-resolution charged-particle spectroscopy with side-entry Ge(Li) detectors in the energy range of the Jülich isochronous cyclotron
N U C L E A R I N S T R U M E N T S AND METHODS
124 (I975)
527-53I;
© NORTH-HOLLAND
P U B L I S H I N G CO.
H I G H - R E S O L U T I O N CHARGED-PARTICLE S P E C T R O S C O P Y W I T H SIDE-ENTRY Ge(Li) D E T E C T O R S IN T H E ENERGY RANGE OF T H E J I ] L I C H I S O C H R O N O U S CYCLOTRON G. RIEPE, D. PROTIC and J. REICH
Institut fiir Kernphysik der Kernforschungsanlage Yiilich, D-517 Jiilich, Germany Received 5 December 1974 Planar Ge(Li) detectors of the side-entry type were produced and tested with protons of 44.1 MeV, deuterons of 62.9 MeV and alpha particles of 155.5 MeV. Overall resolution figures
obtained for particles elastically scattered on Au under 0lab = 1 5 ° were 15.2, 27.7, and 60.8keV (fwhm), respectively. Contributions affecting the resolution are discussed.
1. Introduction The Jtilich Isochronous Cyclotron provides beams of protons, deuterons, and :~-particles within the range of 22.5 to 45 MeV per nucleon1). The design of components makes this instrument especially suited for the study of nuclear reactions in medium- and heavyweight nuclei, where the small level spacing requires high energy resolution, both of beams and detection systems. The desired beam quality of AE/E= 10 -4 was achieved by means of a double monochromator system of high resolving power2). The detector type chosen according to the requirements for charged-particle spectroscopy was a planar Ge(Li) diode to be used in side-entry configuration mainly in view of the long ranges of various lighter reaction products, e.g. 90 MeV protons from a (d,p) reaction. The conventional thin-window Ge(Li) detector is limited to the use for particles having ranges below 10 mm. Hence it could be applied only for particles of the incident beam or heavier reaction products, e.g. in elastic scattering experiments. The idea of using a side-entry Ge(Li) diode for long-range particles is, of course, not a new one. But there has never been much emphasis for this type, mainly because of the often mentioned dead-layer problemsa-6). Therefore, most of the detector development concentrated on the thin-window type, and very good resolution figures 7,s) have been obtained (e.g. 19 keV fwhm for protons of 40 MeV). For side-entry types, on the other hand, the values for AE/E were never smaller than 10 -a 3,5,9-12). The aim of the present work was to consider all contributions affecting the resolution of a side-entry detector, and to check experimentally to what extent the estimated figures apply.
2. Contributions affecting resolution As a basis for discussion the assumption should be allowed, that all contributions follow a Gaussian distribution, even if this is not always the case. Thus the anticipated overall energy resolution may be obtained by the square root of the quadratic sum. Considering all significant contributions one can distinguish between (A) such as are connected with experimental conditions, and (B) those from the detector system itself.
527
(A) Beam. As far as a detecting system is concerned the quality of a particle beam can be described in adequate manner by energy resolution and angular spread. Kinematics. The amount of the kinematic contribution is given by the parameters of the scattering experiment (e.g. mass of incident particle and target nucleus, energy of incident particle, scattering angle) determining the change of energy with scattering angle dE~dO, as well as by the aperture of the detecting system and the already mentioned angular spread of the beam. Target. Since the target needs to be of finite thickness - depending on cross sections - an energy loss of the penetrating particle occurs, the straggling of which represents a further contribution. For thin targets, however, the spectrum of energy loss is rather unlike a Gaussian distribution. Background. Both target and Faraday cup represent the main sources of background radiation: ;)-rays and fast electrons coming straight away from the target. The intensities depend on various parameters and are proportional to the beam current as long as activation remains negligible. Background radiation results in pile-up effects (high-energy
528
G. RIEPE et al.
tailing) and at higher levels even in an increased reverse current of the detector. The quantitative contribution can only be determined experimentally as a function of beam current. Slit scattering. The problem of slit scattering causing low-energy tailing can be overcome by the installation of appropriate anti-scattering collimators 13). (B) Statistics of electron-hole pair creation. This contribution appears in the classical expression: AEst(fwhm)=2.35(EFs) 1/2, where E means the particle energy and e the energy needed for electron-hole pair creation. The Fano factor F, of course, is a matter of some concern, since almost every year smaller values are found14-16). Nuclear collisions. Near the end of a particle path the predominant processes of energy loss are nuclear collisions~7'~8), an energy transfer to the lattice virtually without ionization. This energyindependent effect reduces the amplitude of the detector signal and results in large fluctuations of the amplitude as a consequence of the rather high energy exchange for each collision. According to Lindhard and Nielsen 17) the contribution amounts to: ENc(fwhm)=0.7 Z1/2A 4/3 (keV), where Z and A are the atomic number and atomic weight of the particle. Incomplete charge collection. Trapping of created carriers causes a decrease of signal amplitude and usually shows up as low-energy tailing, especially after radiation damage. By increasing the bias voltage of the detector or the shaping time constant of the electronics this contribution can be minimized. Dead layer. As was pointed out already above this contribution represents a serious problem for side-entry detectors. In spite of this ideal geometric configuration for charged-particle spectroscopy, a decrease as well as a spread of the signal amplitude are often observed. An explanation can be found a9) by assuming a thin film of atoms adsorbed on the compensated zone, which act as n- or p-type surface states causing a component to the electrical field, that is normal to the surface. The resulting region of incomplete charge collection may be described by a dead layer in the conventional sense, and energyloss straggling in this layer has to be taken into account. Electronic noise. This contribution comprises several sources of noise, some of which are quantitatively well defined, as noise from detector
(leakage current), preamplifier, and additional amplifying stages. Further sources, which never can be eliminated completely, are hum and ripple from line power stray fields as well as microphonics. The quantitative influence of all these sources is determined by means of a pulser. Since the relative contribution of electronic noise is expected to decrease with increasing particle energy, the problem of gain stability will also become of increasing importance.
3. Experimental set-up Although in-beam testing of detectors certainly would eliminate or at least reduce some of the abovementioned contributions, the necessary experimental expense would have been rather high for the first approach. Therefore a provisional scattering chamber was installed, which has an outlet under 15° with respect to the beam direction. A special detector cryostat, similar to a type designed for dead-layer measurements2°), was directly connected to this outlet, thus eliminating the need of windows between target and detector. The dimensions of the Ge(ki) diode and the collimator (fig. 1) were chosen to suit the spectroscopy of 90 MeV protons. The length of the diode was 29 ram, which ensures stopping of 90 MeV protons having a range of approximately 20 m m in germanium. The collimator in front of the detector had an aperture of 1.5 x 6.0 m m 2. Because of multiple-scattering effects the proton beam passing the collimator opens up in the detectorS), so that at least 99% of the incident protons fall within an area of 9.5 x 14.5 m m 2 at the end of their range. Therefore a drifted depth of 11 mm was sufficient and the diode had a height of 13.4 m m at the entrance increasing to 16.4 m m at the end. n
i
p i
Ge(Li) D e t e c t o r Drifted Depth: 11 mm Height: 13.4/16.4 rnm Length: 2gmm
Ta-Collimator A p e r t u r e : 5 x 1.5 rnrnz Thickness: 6 mm
Fig. 1. Schematic view of detector and collimator mounting.
SPECTROSCOPY WITH SIDE-ENTRY Ge(Li) DETECTORS The distance between target and collimator was 0.6 m, which yields 25/~sr solid angle of acceptance. Throughout the experiments the spot size of the analysed beam at the target was smaller than 3 x 3 m m 2, and the angular spread (fwhm) resulting from particle beam and detector set-up was approximately 15 mrad, as the evaluation of data will show later. Because of a strong evidence for a correlation between dead-layer thickness and particle resolution appropriate procedures (warming-up cycles, surface treatment) were applied to minimize the dead layer of the detectors. After mounting diode as well as collimator inside the cryostat the thickness of the dead layer was determined with the conversion electrons from a 2°7Bi source. The shift of the 975.6 keV peak ranged between 1.5 and 3.1 keV for several diodes corresponding to a germanium equivalent thickness between 3.0 and 6.2/~m. The preamplifier used in the experiments (CI-1408) was reduced in gain to provide enough dynamic range21). To diminish pick-up from stray fields the preamplifier was connected in differential mode to the linear amplifier (TC-205). The following biased amplifier (0-444) served to cut off approximately 50% of the maximum signal amplitude. Multichannel pulseheight analysis was accomplished via an 8k A D C (Nuclear Data) with 4k digital zero suppression. A pulse generator (0-448) was employed to check the contributions of the electronics to the overall energy resolution and at the same time as the reference for a digital gain stabilizer (Nuclear Data). Much care was taken to avoid hum and the microphonics mainly caused by vibrations of turbo-molecular pumps. An essentail point in designing the set-up was to minimize background. A quadrupole doublet 1.36 m downstream of the target focused the beam into a well shielded Faraday cup, 1.2 m deep inside the concrete wall. Lead around the detector reduced the y-ray background caused by the particle beam or by neutrons.
4. Results
Certainly a side-entry Ge(Li) diode would have proven its advantage over the conventional type especially with long-range reaction products, e.g. 90 MeV protons from a (d,p) reaction. But for the beginning tests were limited to elastically scattered particles (p, d, c0 because of the relatively high cross sections involved. As target material for resolution tests gold was selected, to minimize the contribution from kinematics. With regard to energy-loss straggling a target
529
thickness of 240/~g/cm 2 was chosen and used in all tests. The inelastically scattered particles due to the excited states of 2VA1 served for energy calibration, after their energies had been corrected for kinematics and energy loss in the target. The thickness of this target was 2.5 mg/cm 2. Throughout the detector tests, from which the results are listed here, the beam current was kept within the range of 5 to 10 nA. Spectra obtained for protons, deuterons and ~particles, scattered both on gold and aluminium at 0LAB = 15°, are shown in fig. 2 in linear and logarithmic scale, respectively. The labels 1 through 5 correspond to the excited states of 27A1: 842.9, 1013.0, 2208.9, 2731.0, and 3000.6 keV. The resolution figures (fwhm) indicated for Au comprise all contributions, which will be discussed in the following, and are referred to as total in table 1. The energy spread of the analysed beam, as given among "experimental contributions" in keV, is assumed to be 10 .4 of the incident energy, since the energy resolution AE/E of the double monochromator 2) was determined from ray-tracing calculations based on measurements of the magnetic fields to be smaller than 10 -4" The contributions from energy-loss straggling in the target were taken as the fwhm values from calculated,
6O0 PROTONS 44iiMeV
V+ 1 PULSER
V-k
3co @
15.2 Me
8.3Ke Oz
10
l
(~)
10( 300
i'i
',#*..,&..~,'~a~.l
V__L
, DEUTERONS 15o! ~
MeV
oI
lO3 ~
27.7 Ke
@-, © @.
®©
,l
}
5O0 | 155.5
300] (~
PULSE~ ~o.5KeY
...... I[
............
MeV
60.8
17.3~
)I
o~
,o°;,d nod
KeVd ~ ,i
'el
,
............ CHANNELS
Fig. 2. Spectra obtained with a side-entry Ge(Li) detector. (Sec text for explanation of labels).
530
G. RIEPE et al. TABLE 1
Total resolution (fwhm) and contributions from experimental set-up and detector system. p
Particle
d
c~
Energy
(MeV)
44.1 62.9 155.5
Resolution total relative
(keV) 10-4
15.2 27.7 60.8 3.4 4.4 3.9
Experimental contributions beam target kinematics
(keV)
Remaining for detector system
(keV)
14.1 26.0 52.6
Contributions of detector system (keV) statistics nuclear collisions dead layer electronics
6.0 7.2 11.3 0.7 1.8 6.3 10.9 20.0 32.0 8.3 10.5 17.3
Sum detector system
15.0 23.8 38.6
4.4 3.0 1.8
(keV)
6.3 5.0 5.4
15.6 9.0 24.6
sometimes rather asymmetric energy-loss distributions22). According to the above considerations the peak half widths of corresponding Au and A1 spectra are expected to differ from each other only by the contributions from target and kinematics, as long as experimental conditions like beam current and background are kept the same. This means, on the other hand, that a figure for the overall angular beam spread may be obtained by a comparison of corresponding Au and A1 peaks after the contributions from the targets have been subtracted. The determination, which is based on calculated dE/dOvalues for Au and A1, gives an overall beam spread of approximately 15 mrad, independent of the kind of particles. After subtracting these three experimental contributions from the figure for the total resolution (in quadratic procedure) a value for the detector system alone remains, which now may be compared to the sum of the separate components. The contributions from electron-hole pair statistics as well as from nuclear collisions were calculated according to the above-mentioned equations for AEst and AENc. A Fano factor of 0.05 was assumeda4-16). T o derive a figure for the energy-loss straggling in
the dead layer of a diode, the germanium equivalent thickness as determined with conversion electrons was used as a basis. The fwhm values of the corresponding energy-loss distributions, calculated in the same way as those for the target, are listed. The contribution from electronics was obtained, a common procedure, by means of a pulser. The half width of this peak not only comprises electronic noise but also background effects to the same amount as particle peaks do. The sum of these contributions from the detector system alone (again a quadratic summation) appears in the last row of the table. These data now allow a check, to what degree the experimental figures may be explained by a series of estimated contributions. A comparison of the two sets of values obtained after taking into account the two kinds of contributions, shows best agreement for protons, but in the case of v.-particles reveals a discrepancy, which is far beyond the limits of error. Of course, inaccuracies were also introduced by the assumption, that all contributions follow a Gaussian distribution. The large difference for e-particles, however, can only be accounted for by an underestimated contribution. This may well be the contribution from the dead layer of the diode, because already two months had passed between the determination of dead-layer thickness and the run with e-particles, and an increase of the value in meantime cannot be excluded.
5. Conclusion
No doubt, the largest contribution affecting resolution (table 1), the dead-layer effect, attracts some attention, and the question arises, if a further reduction of this contribution seems feasable. Some difficulties, however, should be mentioned. The procedures for obtaining thin dead layers on a side-entry Ge(Li) diode lack the desired reproducibility, contrary to Baldinger and Haller 19). No well-established recipe can be quoted, only tendencies may be described. Furthermore, the determination of thickness using other kinds and energies of particles than in the actual experiment remains a matter of some uncertainty. Stability of the dead layer with time also cannot be predicted. In view of this rather alchemistic situation, based on experience with a high number of side-entry diodes, also a different type of a Ge detector was prepared and will b e tested very soon, mainly to compare dead-layer effects. This consists of a planar diode made of hyperpure Ge and equipped with an implanted entrance contact, which almost certainly means a defined dead
SPECTROSCOPY WITH SIDE-ENTRY Ge(Li) DETECTORS layer. Its depletion depth of 11 m m will be sufficient for stopping particles of the incident b e a m or heavier reaction products. Even if this type proves to be a n o p t i m u m solution for short-range particles, the side-entry diode remains the only possible type for particles having a range of more t h a n a b o u t 10 m m in Ge. This more suitable application will be studied in the n e a r future. A n o t h e r result of these detector tests, which m a y be derived from the p r o t o n data, is the experimental proof, that the energy spread of the particle b e a m does n o t exceed 10 . 4 . The authors wish to t h a n k the m e m b e r s of the detector l a b o r a t o r y A. H a m a c h e r , T. Kiinster, E. L a w i n a n d H. Spanier for their technical support, a n d U. S c h w i n n a n d J. Pfeiffer from the target l a b o r a t o r y for supplying the targets. J. Bojowald, M. Rogge a n d P. T u r e k are acknowledged for clarifying discussions a n d the m e m bers of the cyclotron group for their careful b e a m handling.
References 1) c. u. Mayer-B6ricke and J. Reich, AIP Conf. Proc., no. 9 (1972) p. 716. 2) j. Reich, C. U. Mayer-B6ricke, K. L. Brown, F. E. Johnson, AIP Conf. Proc., no. 9 (1972) p. 407. 3) F. E. Bertrand, R. W. Peelle, T.A. Love, R.J. Fox, N. W. Hill and H.A. Todd, 1EEE Trans. Nucl. Sci. NS-13, no. 3 (1966) 279.
531
4) F.S. Goulding and B. V. Jarrett, UCRL-16480 (1966). 5) C. R. Gruhn, R. Kuo, G. Maggiore, B. Breedom, L. Samuelson and J. Chander, IEEE Trans. Nucl. Sci. NS-15, no. 3 (1968) 337. a) G. Dearnaley, A.G. Hardacre and B.D. Rogers, Nucl. Instr. and Meth. 71 (1969) 86. 7) W. L. Hansen, Nucl. Instr. and Meth. 94 (1971) 377. 8) E.A. McClatchie, M.S. Zisman, A.D. Bacher, C. Glashausser, R. H. Pehl, E. Rivet and F.S. Goulding, UCRL18667 (1968) 332. 9) y. S. Horowitz and N. K. Sherman, Can. J. Phys. 45 (1967) 3265. 10) L. Marcus, G. Duhamel and H. Langevin-Joliot, Rev. Phys. Appl. 4 (1969) 279. 11) B. Sundqvist, E. Hagberg and P. Grafstrbm, Nucl. Instr. and Meth. 94 (1971) 343. 12) D. Dorcioman, R. Bertini, M. Hage-Ali, P. Siffert and L. Stab, Nucl. Instr. and Meth. 101 (1972) 91. 18) F.E. Bertrand, W. R. Burrus, N.W. Hill, T.A. Love and R. W. Peelle, ORNL-TM-3669 (1972). 14) H.R. Zulliger and D.W. Aitken, IEEE Trans. Nucl. Sci. NS-17, no. 3 (1970) 187. 15) R.H. Pehl and F. S. Goulding, Nucl. Instr. and Meth. 81 (1970) 329. 16) N. Strokan, V. AjdaNd and B. Lalovid, Nucl. Instr. and Meth. 94 (1971) 147. 17) j. Lindhard and V. Nielsen, Phys. Lett. 2, no. 5 (1962) 209. 18) E. L. Haines and A. B. Whitehead, Rev. Sci. Instr. 37, no. 2 (1966) 190. 19) E. Baldinger and E. Hailer, Proc. Symp. Semiconductor detectors for nuclear radiation (Munich 1970) p. 9. 20) D. Protid and G. Riepe, Nucl. Instr. and Meth. 101 (1972) 55. 21) j. Bojowald, private communication. .22) R. Clarkson, private communication.
124 (I975)
527-53I;
© NORTH-HOLLAND
P U B L I S H I N G CO.
H I G H - R E S O L U T I O N CHARGED-PARTICLE S P E C T R O S C O P Y W I T H SIDE-ENTRY Ge(Li) D E T E C T O R S IN T H E ENERGY RANGE OF T H E J I ] L I C H I S O C H R O N O U S CYCLOTRON G. RIEPE, D. PROTIC and J. REICH
Institut fiir Kernphysik der Kernforschungsanlage Yiilich, D-517 Jiilich, Germany Received 5 December 1974 Planar Ge(Li) detectors of the side-entry type were produced and tested with protons of 44.1 MeV, deuterons of 62.9 MeV and alpha particles of 155.5 MeV. Overall resolution figures
obtained for particles elastically scattered on Au under 0lab = 1 5 ° were 15.2, 27.7, and 60.8keV (fwhm), respectively. Contributions affecting the resolution are discussed.
1. Introduction The Jtilich Isochronous Cyclotron provides beams of protons, deuterons, and :~-particles within the range of 22.5 to 45 MeV per nucleon1). The design of components makes this instrument especially suited for the study of nuclear reactions in medium- and heavyweight nuclei, where the small level spacing requires high energy resolution, both of beams and detection systems. The desired beam quality of AE/E= 10 -4 was achieved by means of a double monochromator system of high resolving power2). The detector type chosen according to the requirements for charged-particle spectroscopy was a planar Ge(Li) diode to be used in side-entry configuration mainly in view of the long ranges of various lighter reaction products, e.g. 90 MeV protons from a (d,p) reaction. The conventional thin-window Ge(Li) detector is limited to the use for particles having ranges below 10 mm. Hence it could be applied only for particles of the incident beam or heavier reaction products, e.g. in elastic scattering experiments. The idea of using a side-entry Ge(Li) diode for long-range particles is, of course, not a new one. But there has never been much emphasis for this type, mainly because of the often mentioned dead-layer problemsa-6). Therefore, most of the detector development concentrated on the thin-window type, and very good resolution figures 7,s) have been obtained (e.g. 19 keV fwhm for protons of 40 MeV). For side-entry types, on the other hand, the values for AE/E were never smaller than 10 -a 3,5,9-12). The aim of the present work was to consider all contributions affecting the resolution of a side-entry detector, and to check experimentally to what extent the estimated figures apply.
2. Contributions affecting resolution As a basis for discussion the assumption should be allowed, that all contributions follow a Gaussian distribution, even if this is not always the case. Thus the anticipated overall energy resolution may be obtained by the square root of the quadratic sum. Considering all significant contributions one can distinguish between (A) such as are connected with experimental conditions, and (B) those from the detector system itself.
527
(A) Beam. As far as a detecting system is concerned the quality of a particle beam can be described in adequate manner by energy resolution and angular spread. Kinematics. The amount of the kinematic contribution is given by the parameters of the scattering experiment (e.g. mass of incident particle and target nucleus, energy of incident particle, scattering angle) determining the change of energy with scattering angle dE~dO, as well as by the aperture of the detecting system and the already mentioned angular spread of the beam. Target. Since the target needs to be of finite thickness - depending on cross sections - an energy loss of the penetrating particle occurs, the straggling of which represents a further contribution. For thin targets, however, the spectrum of energy loss is rather unlike a Gaussian distribution. Background. Both target and Faraday cup represent the main sources of background radiation: ;)-rays and fast electrons coming straight away from the target. The intensities depend on various parameters and are proportional to the beam current as long as activation remains negligible. Background radiation results in pile-up effects (high-energy
528
G. RIEPE et al.
tailing) and at higher levels even in an increased reverse current of the detector. The quantitative contribution can only be determined experimentally as a function of beam current. Slit scattering. The problem of slit scattering causing low-energy tailing can be overcome by the installation of appropriate anti-scattering collimators 13). (B) Statistics of electron-hole pair creation. This contribution appears in the classical expression: AEst(fwhm)=2.35(EFs) 1/2, where E means the particle energy and e the energy needed for electron-hole pair creation. The Fano factor F, of course, is a matter of some concern, since almost every year smaller values are found14-16). Nuclear collisions. Near the end of a particle path the predominant processes of energy loss are nuclear collisions~7'~8), an energy transfer to the lattice virtually without ionization. This energyindependent effect reduces the amplitude of the detector signal and results in large fluctuations of the amplitude as a consequence of the rather high energy exchange for each collision. According to Lindhard and Nielsen 17) the contribution amounts to: ENc(fwhm)=0.7 Z1/2A 4/3 (keV), where Z and A are the atomic number and atomic weight of the particle. Incomplete charge collection. Trapping of created carriers causes a decrease of signal amplitude and usually shows up as low-energy tailing, especially after radiation damage. By increasing the bias voltage of the detector or the shaping time constant of the electronics this contribution can be minimized. Dead layer. As was pointed out already above this contribution represents a serious problem for side-entry detectors. In spite of this ideal geometric configuration for charged-particle spectroscopy, a decrease as well as a spread of the signal amplitude are often observed. An explanation can be found a9) by assuming a thin film of atoms adsorbed on the compensated zone, which act as n- or p-type surface states causing a component to the electrical field, that is normal to the surface. The resulting region of incomplete charge collection may be described by a dead layer in the conventional sense, and energyloss straggling in this layer has to be taken into account. Electronic noise. This contribution comprises several sources of noise, some of which are quantitatively well defined, as noise from detector
(leakage current), preamplifier, and additional amplifying stages. Further sources, which never can be eliminated completely, are hum and ripple from line power stray fields as well as microphonics. The quantitative influence of all these sources is determined by means of a pulser. Since the relative contribution of electronic noise is expected to decrease with increasing particle energy, the problem of gain stability will also become of increasing importance.
3. Experimental set-up Although in-beam testing of detectors certainly would eliminate or at least reduce some of the abovementioned contributions, the necessary experimental expense would have been rather high for the first approach. Therefore a provisional scattering chamber was installed, which has an outlet under 15° with respect to the beam direction. A special detector cryostat, similar to a type designed for dead-layer measurements2°), was directly connected to this outlet, thus eliminating the need of windows between target and detector. The dimensions of the Ge(ki) diode and the collimator (fig. 1) were chosen to suit the spectroscopy of 90 MeV protons. The length of the diode was 29 ram, which ensures stopping of 90 MeV protons having a range of approximately 20 m m in germanium. The collimator in front of the detector had an aperture of 1.5 x 6.0 m m 2. Because of multiple-scattering effects the proton beam passing the collimator opens up in the detectorS), so that at least 99% of the incident protons fall within an area of 9.5 x 14.5 m m 2 at the end of their range. Therefore a drifted depth of 11 mm was sufficient and the diode had a height of 13.4 m m at the entrance increasing to 16.4 m m at the end. n
i
p i
Ge(Li) D e t e c t o r Drifted Depth: 11 mm Height: 13.4/16.4 rnm Length: 2gmm
Ta-Collimator A p e r t u r e : 5 x 1.5 rnrnz Thickness: 6 mm
Fig. 1. Schematic view of detector and collimator mounting.
SPECTROSCOPY WITH SIDE-ENTRY Ge(Li) DETECTORS The distance between target and collimator was 0.6 m, which yields 25/~sr solid angle of acceptance. Throughout the experiments the spot size of the analysed beam at the target was smaller than 3 x 3 m m 2, and the angular spread (fwhm) resulting from particle beam and detector set-up was approximately 15 mrad, as the evaluation of data will show later. Because of a strong evidence for a correlation between dead-layer thickness and particle resolution appropriate procedures (warming-up cycles, surface treatment) were applied to minimize the dead layer of the detectors. After mounting diode as well as collimator inside the cryostat the thickness of the dead layer was determined with the conversion electrons from a 2°7Bi source. The shift of the 975.6 keV peak ranged between 1.5 and 3.1 keV for several diodes corresponding to a germanium equivalent thickness between 3.0 and 6.2/~m. The preamplifier used in the experiments (CI-1408) was reduced in gain to provide enough dynamic range21). To diminish pick-up from stray fields the preamplifier was connected in differential mode to the linear amplifier (TC-205). The following biased amplifier (0-444) served to cut off approximately 50% of the maximum signal amplitude. Multichannel pulseheight analysis was accomplished via an 8k A D C (Nuclear Data) with 4k digital zero suppression. A pulse generator (0-448) was employed to check the contributions of the electronics to the overall energy resolution and at the same time as the reference for a digital gain stabilizer (Nuclear Data). Much care was taken to avoid hum and the microphonics mainly caused by vibrations of turbo-molecular pumps. An essentail point in designing the set-up was to minimize background. A quadrupole doublet 1.36 m downstream of the target focused the beam into a well shielded Faraday cup, 1.2 m deep inside the concrete wall. Lead around the detector reduced the y-ray background caused by the particle beam or by neutrons.
4. Results
Certainly a side-entry Ge(Li) diode would have proven its advantage over the conventional type especially with long-range reaction products, e.g. 90 MeV protons from a (d,p) reaction. But for the beginning tests were limited to elastically scattered particles (p, d, c0 because of the relatively high cross sections involved. As target material for resolution tests gold was selected, to minimize the contribution from kinematics. With regard to energy-loss straggling a target
529
thickness of 240/~g/cm 2 was chosen and used in all tests. The inelastically scattered particles due to the excited states of 2VA1 served for energy calibration, after their energies had been corrected for kinematics and energy loss in the target. The thickness of this target was 2.5 mg/cm 2. Throughout the detector tests, from which the results are listed here, the beam current was kept within the range of 5 to 10 nA. Spectra obtained for protons, deuterons and ~particles, scattered both on gold and aluminium at 0LAB = 15°, are shown in fig. 2 in linear and logarithmic scale, respectively. The labels 1 through 5 correspond to the excited states of 27A1: 842.9, 1013.0, 2208.9, 2731.0, and 3000.6 keV. The resolution figures (fwhm) indicated for Au comprise all contributions, which will be discussed in the following, and are referred to as total in table 1. The energy spread of the analysed beam, as given among "experimental contributions" in keV, is assumed to be 10 .4 of the incident energy, since the energy resolution AE/E of the double monochromator 2) was determined from ray-tracing calculations based on measurements of the magnetic fields to be smaller than 10 -4" The contributions from energy-loss straggling in the target were taken as the fwhm values from calculated,
6O0 PROTONS 44iiMeV
V+ 1 PULSER
V-k
3co @
15.2 Me
8.3Ke Oz
10
l
(~)
10( 300
i'i
',#*..,&..~,'~a~.l
V__L
, DEUTERONS 15o! ~
MeV
oI
lO3 ~
27.7 Ke
@-, © @.
®©
,l
}
5O0 | 155.5
300] (~
PULSE~ ~o.5KeY
...... I[
............
MeV
60.8
17.3~
)I
o~
,o°;,d nod
KeVd ~ ,i
'el
,
............ CHANNELS
Fig. 2. Spectra obtained with a side-entry Ge(Li) detector. (Sec text for explanation of labels).
530
G. RIEPE et al. TABLE 1
Total resolution (fwhm) and contributions from experimental set-up and detector system. p
Particle
d
c~
Energy
(MeV)
44.1 62.9 155.5
Resolution total relative
(keV) 10-4
15.2 27.7 60.8 3.4 4.4 3.9
Experimental contributions beam target kinematics
(keV)
Remaining for detector system
(keV)
14.1 26.0 52.6
Contributions of detector system (keV) statistics nuclear collisions dead layer electronics
6.0 7.2 11.3 0.7 1.8 6.3 10.9 20.0 32.0 8.3 10.5 17.3
Sum detector system
15.0 23.8 38.6
4.4 3.0 1.8
(keV)
6.3 5.0 5.4
15.6 9.0 24.6
sometimes rather asymmetric energy-loss distributions22). According to the above considerations the peak half widths of corresponding Au and A1 spectra are expected to differ from each other only by the contributions from target and kinematics, as long as experimental conditions like beam current and background are kept the same. This means, on the other hand, that a figure for the overall angular beam spread may be obtained by a comparison of corresponding Au and A1 peaks after the contributions from the targets have been subtracted. The determination, which is based on calculated dE/dOvalues for Au and A1, gives an overall beam spread of approximately 15 mrad, independent of the kind of particles. After subtracting these three experimental contributions from the figure for the total resolution (in quadratic procedure) a value for the detector system alone remains, which now may be compared to the sum of the separate components. The contributions from electron-hole pair statistics as well as from nuclear collisions were calculated according to the above-mentioned equations for AEst and AENc. A Fano factor of 0.05 was assumeda4-16). T o derive a figure for the energy-loss straggling in
the dead layer of a diode, the germanium equivalent thickness as determined with conversion electrons was used as a basis. The fwhm values of the corresponding energy-loss distributions, calculated in the same way as those for the target, are listed. The contribution from electronics was obtained, a common procedure, by means of a pulser. The half width of this peak not only comprises electronic noise but also background effects to the same amount as particle peaks do. The sum of these contributions from the detector system alone (again a quadratic summation) appears in the last row of the table. These data now allow a check, to what degree the experimental figures may be explained by a series of estimated contributions. A comparison of the two sets of values obtained after taking into account the two kinds of contributions, shows best agreement for protons, but in the case of v.-particles reveals a discrepancy, which is far beyond the limits of error. Of course, inaccuracies were also introduced by the assumption, that all contributions follow a Gaussian distribution. The large difference for e-particles, however, can only be accounted for by an underestimated contribution. This may well be the contribution from the dead layer of the diode, because already two months had passed between the determination of dead-layer thickness and the run with e-particles, and an increase of the value in meantime cannot be excluded.
5. Conclusion
No doubt, the largest contribution affecting resolution (table 1), the dead-layer effect, attracts some attention, and the question arises, if a further reduction of this contribution seems feasable. Some difficulties, however, should be mentioned. The procedures for obtaining thin dead layers on a side-entry Ge(Li) diode lack the desired reproducibility, contrary to Baldinger and Haller 19). No well-established recipe can be quoted, only tendencies may be described. Furthermore, the determination of thickness using other kinds and energies of particles than in the actual experiment remains a matter of some uncertainty. Stability of the dead layer with time also cannot be predicted. In view of this rather alchemistic situation, based on experience with a high number of side-entry diodes, also a different type of a Ge detector was prepared and will b e tested very soon, mainly to compare dead-layer effects. This consists of a planar diode made of hyperpure Ge and equipped with an implanted entrance contact, which almost certainly means a defined dead
SPECTROSCOPY WITH SIDE-ENTRY Ge(Li) DETECTORS layer. Its depletion depth of 11 m m will be sufficient for stopping particles of the incident b e a m or heavier reaction products. Even if this type proves to be a n o p t i m u m solution for short-range particles, the side-entry diode remains the only possible type for particles having a range of more t h a n a b o u t 10 m m in Ge. This more suitable application will be studied in the n e a r future. A n o t h e r result of these detector tests, which m a y be derived from the p r o t o n data, is the experimental proof, that the energy spread of the particle b e a m does n o t exceed 10 . 4 . The authors wish to t h a n k the m e m b e r s of the detector l a b o r a t o r y A. H a m a c h e r , T. Kiinster, E. L a w i n a n d H. Spanier for their technical support, a n d U. S c h w i n n a n d J. Pfeiffer from the target l a b o r a t o r y for supplying the targets. J. Bojowald, M. Rogge a n d P. T u r e k are acknowledged for clarifying discussions a n d the m e m bers of the cyclotron group for their careful b e a m handling.
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