- Email: [email protected]
Preparation of targets by sputter deposition
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
Preparation of targets by sputter deposition Birgit Kindler*, Willi Hartmann, Josef Klemm, Bettina Lommel, Jutta Steiner Gesellschaft fur . Schwerionenforschung (GSI), Targetlabor, Planckstrasse 1, Darmstadt 64291, Germany
Abstract For target preparation at GSI, a focused ion-beam sputter source, as described by Sletten and Knudsen, has been available for a long time. A survey of the sputtering yields of several isotopic materials was given by Folger et al. This method is applied mainly for rare refractive materials where only small target areas and medium uniformities are needed. We present the setup and discuss the method’s pros and cons. For some selected isotopic materials we describe the preparation and the achieved thickness. While for the target preparation of rare materials the focused ion-beam sputter deposition is the ideal choice for larger dimensions, for significantly thicker layers or non-sticking materials, we complemented our lab equipment with a magnetron sputtering machine. We show the principle of this method and report first results of target preparation with the apparatus. r 2003 Elsevier B.V. All rights reserved. PACS: 81.15.Cd; 81.15.Jj Keywords: Focused ion-beam sputtering; Magnetron sputtering; Heavy-ion target; Sputtering
1. Introduction and motivation The deposition of refractory materials, especially when they are scarce, demands special techniques. Thermal evaporation is not appropriate for a high-melting material since it is difficult or even impossible to find a suitable crucible material that will not contaminate the target in some way at high temperature. On the other hand, electron-beam evaporation is also not an alternative in the case of scarce isotopes because of the larger distances one has to use to protect the substrate from overheating and reflected electrons. Sputtering, in contrast to the methods mentioned above, is a ballistic process which allows *Corresponding author. Tel.: +49-6159-71-25-23, fax: +496159-71-21-66. E-mail address: [email protected] (B. Kindler).
depositing materials independently of their vapor pressures: low-melting metals, refractory materials, elements, compounds, conductors and insulators. Additionally, it is possible to sputter in the presence of a reactive gas or a gas mixture so that for example, oxides can be created during the deposition. In general, sputtered layers have a higher adhesive strength on a substrate than evaporated layers. All these properties make sputtering deposition a powerful tool for the preparation of heavy-ion targets. In the following, we characterize the focused ion-beam method, as described by Sletten and Knudsen [1], which is in most cases a good answer to the problem of depositing refractory materials with a high yield. Subsequently, we introduce the complementary magnetron sputtering technique which has become available only recently in our lab.
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.153
ARTICLE IN PRESS B. Kindler et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
223
2. Focused ion-beam sputtering In Fig. 1, a drawing of our focused ion-beam apparatus is shown. It is a Sletten-type from Danfysik, as already described by Folger et al. [2]. From a duoplasmatron ion-source with an Ar partial pressure of 2–3 10 2 mbar, an ion-current of about 1 mA is generated. By means of an Einzellens-arrangement, the Ar ions are extracted into high vacuum and focused to a diameter smaller than 2 mm. With 10 kV, they are accelerated onto the cathode where the target material in the form of a metal bead or a pressed tablet is situated inside the beam stop made of high-purity carbon. The substrates are arranged concentrically around the sputter target as depicted in Fig. 2. The gap between the sputter target and the substrates can be varied between 8 and 50 mm with about 15 mm being a standard distance. In this geometry, six of our standard frames with a 10 mm hole can be deposited in one run with a satisfactory uniformity over the diameter of the beam spot. The form of the cathode influences the directivity of the sputter yield. Since we always work with a vertical incidence beam with the substrates arranged sideways, we prefer a spherical sputter cathode instead of one with a flat surface where more material is sputtered in the vertical direction.
Fig. 2. One possible target arrangement around the sputter source.
The whole assembly can be moved in the X and Y directions for an optimum alignment of the beam relative to the sputter target. Also the substrate holder can be rotated, thus providing a better uniformity of the deposited layers.
3. Pros and cons of the method
Fig. 1. Sketch of the Sletten-type focused ion-beam apparatus.
Since sputtering is a purely ballistic process, material can be deposited independently of its vapor pressure or melting temperature. Sputtered layers adhere in general well to the substrate. The non-sticking quality of the substances like Cd, that in thermal evaporation processes are often problematic, are uncritical in sputtering processes. It is also a ‘‘cold’’ process which does not strain the backing material thermally. This allows for a close deposition geometry which makes the process extremely efficient. With the small beam focus, a deposition with a minimum of starting material is possible. Because of the small beam focus and the extremely poor sputtering yield of carbon, there is only slight danger of contamination of the target with carbon crucible material. The fact that the deposition takes place in a high vacuum
ARTICLE IN PRESS 224
B. Kindler et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
environment helps obtaining clean and well adhering target layers. Most of the advantages of the method also involve certain disadvantages. The ballistic impact has consequences on the choice of the release agent for backing-free targets. Especially betaine–sucrose layers do not always dissolve properly after sputter deposition. In fact we use the method primarily for backed targets or on Cu substrates which can be etched after deposition. The close geometry of the setup leads to a reasonable uniformity only for small target areas. The small beam focus results in low deposition rates which in turn lead to extremely long deposition times for thick or large layers. In view of the two latter arguments, the deposition of large-area targets with this technique is not recommended. Also with increasing deposition time, the danger of target contamination increases.
4. Preparation and sputter yields for some selected isotopes In the following, we will give a few examples of targets made with focused ion-beam sputtering in recent years. We used a standard target-to-substrate distance of about 15 mm for all the targets described here. If only a small amount of starting material is available, we can reduce this distance to about 8 mm therefore, accepting a considerable inhomogeneity of the target layer thickness. Particularly for isotopes of Ru and Hf, we experienced that for thicker layers on thin backings, a deposition in several steps is absolutely necessary. Otherwise the deposited layer flakes off or the backing foil is damaged. For 96Ru, we made targets of 2.15 mg/cm2 on a backing of 1 mg/cm2 92Mo in seven runs, depositing about 300 mg/cm2 each time with 3–5 days rest between the successive runs. We obtained a yield of 3% per cm2 target area. For 178Hf on B35 mg/cm2 C, we used a pressed tablet of the oxide as the sputter target. For targets with a thickness of 1 mg/cm2, we performed nine runs, depositing in each step 100–150 mg/cm2 with 2–3 days rest in between. In this process, we got a yield of 4% per cm2 target area.
The isotopes 186W with a thickness of 280 mg/ cm2 on 10 mg/cm2 Cu and 232Th with a thickness of 610 mg/cm2 on 2.3 mg/cm2 Be could each be deposited in only one step with a yield of 3.6% and 6% per cm2, respectively.
5. Magnetron sputtering A sputter coater from BOC Edwards has been available for 2 years in the GSI target laboratory. There are two 1-in. and two 3-in. planar magnetron sputtering sources available of which three can be mounted simultaneously in the machine. In Fig. 3 shows the inside of the chamber with a 1in. source mounted in front and a 3-in. source located in the rear. In Fig. 4, the principle of such a planar magnetron is depicted. The magnetic field created from permanent magnets below the sputter target together with the vertical electric field, confines the low-pressure plasma to a torus above the sputter cathode. Magnetrons can be operated either in the DC mode with a maximum power of 500 W or in the RF mode with a maximum power of 300 W.
Fig. 3. View inside the Edwards 500 DC and RF sputter coater with a 1-in. source mounted in the front and a 3-in. source in the rear.
ARTICLE IN PRESS B. Kindler et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
Fig. 4. Principle of a planar magnetron source [3]: The magnetic fields drawn above in combination with a vertical E-field confine the low-pressure plasma to a torus above the sputter target that serves as cathode. N, north; S, south.
As a rule of thumb, the minimum distance between the sputter target and the substrate should be of the same order as the diameter of the sputter source used, namely 1-in. or 3-in., to avoid damage of the substrate by the plasma. This rather large gap implies only a medium efficiency in terms of material consumption. A maximum reasonable distance is about 10 cm for a 1-in. source and about 15 cm for a 3-in. source. We next describe some examples of targets successfully prepared with this method, with which we gained our first experience. 5.1. Cadmium For nuclear chemistry experiments, we had a request for the deposition of Cd on 6 mm Tibacking in a 2 cm2 banana-shape. Cd is a difficult candidate to evaporate since it deposits only on the coldest surfaces which means in general the watercooled parts of the chamber and the pump baffle. With magnetron sputtering, we produced targets with 250 and 650 mg/cm2 without problems. The sputtering distance was about 60 mm. 5.2. Tantalum For the heavy-element program at SHIP, we were asked to produce two mountings, each with eight targets of 181Ta with a target area of about 18.5 cm2 and a thickness of 400 mg/cm2 on a C backing of 40 mg/cm2.
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Since Ta is a refractory material, thermal evaporation which is the standard method for the SHIP targets, was not applicable in this case. Ta can only be evaporated with an electron gun but for the banana-shaped SHIP targets, a large distance between the crucible and the target was necessary to get a tolerable uniformity of the layer. But anyhow, the stress on these large backings was so high that even 45 mg/cm2 of carbon did not survive. Since 181Ta has a natural abundance of 99.988%, it was possible to use a standard plate of Ta as sputtering source and make a first test for the production of SHIP targets with magnetron sputtering. The setup used in this case is shown in Fig. 3 with the Ta target mounted in the 1-in. source in the front and the evaporation wheel mounted on the rotation axis at the top of the chamber. Eight targets were deposited in one run with the sputtering done in two steps for a total of about 45 min and a target-tosubstrate distance of 55 mm. The targets showed a good uniformity and a satisfactory durability during irradiation with a heavy ion beam. In other cases, magnetron sputtering would possibly not be so easy, since for experiments with low reaction cross-sections, it is necessary to use enriched material. To get this in the form of a metal sheet at least 1-in. in diameter with a sufficient thickness would be difficult and expensive. Also the material consumption is rather high. Nevertheless this new method opens up a perspective for heavy-ion targets of materials considered until now as impossible. 5.3. Cobalt and nickel We had a first try at depositing magnetic materials with this method. On 50 mm Ta backings, 100 mg/cm2 Co or 200 mg/cm2 Ni were to be deposited with a target of 45 mm in diameter. The requirements for the layers were high adhesive strength, a variation of less than 710% from the desired thickness and less than 75% in thickness across the diameter. Therefore, magnetron sputtering was the most promising method to do the job.
ARTICLE IN PRESS B. Kindler et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
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1
250 245 240 235 230 225 220 215 210 205 200 2
3
4
5
1
2
3
4
thickness in µg/cm²
6. Summary average 237 µ/cm² deviation +/- 9.94 µ/cm² +/- 4.19 %
5
cm
Fig. 5. Distribution of the weight per unit area of a sputtered Ni-layer (left) and the corresponding tolerances (right).
We found that it was not practicable to use standard sputter targets which have a thickness of 3–5 mm. The magnetic susceptibility of Co and Ni interfered with the magnetic field of the permanent Sm–Co magnets situated below the sputtering target (see Fig. 4) so that it was impossible to ignite a stable plasma. We finally succeeded by stacking several thin Co or Ni foils with a thickness not more than 250–300 mm each and replaced the upper foil after some time. The sputtering distance was 65 mm in this case. The bar diagram in Fig. 5 shows the thickness variation for one of the test targets which had a somewhat higher thickness than the required ones. With a deviation of 74.19%, we were within the tolerance we sought.
We have shown that focused ion-beam sputtering is a powerful method for high-melting-point substances when only small amounts of starting material are available. For small target areas, a satisfactory uniformity with low material consumption can be had. For larger target areas, where focused ion-beam sputtering is not recommended, magnetron sputtering can be an alternative for refractory or non-sticking materials and for all cases where a good adhesion of the target layer to the substrate is required. For expensive material, magnetron sputtering is not a good option since the material consumption is rather high.
Acknowledgements The authors thank Gabi Otto and Achim Zschau for the photographs.
References [1] G. Sletten, P. Knudsen, Nucl. Instr. and Meth. 102 (1972) 459. [2] H. Folger, J. Klemm, M. Muller, . IEEE Trans. Nucl. Sci. NS-30 (2) (1983) 1568. [3] http://www.lesker.com
Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
Preparation of targets by sputter deposition Birgit Kindler*, Willi Hartmann, Josef Klemm, Bettina Lommel, Jutta Steiner Gesellschaft fur . Schwerionenforschung (GSI), Targetlabor, Planckstrasse 1, Darmstadt 64291, Germany
Abstract For target preparation at GSI, a focused ion-beam sputter source, as described by Sletten and Knudsen, has been available for a long time. A survey of the sputtering yields of several isotopic materials was given by Folger et al. This method is applied mainly for rare refractive materials where only small target areas and medium uniformities are needed. We present the setup and discuss the method’s pros and cons. For some selected isotopic materials we describe the preparation and the achieved thickness. While for the target preparation of rare materials the focused ion-beam sputter deposition is the ideal choice for larger dimensions, for significantly thicker layers or non-sticking materials, we complemented our lab equipment with a magnetron sputtering machine. We show the principle of this method and report first results of target preparation with the apparatus. r 2003 Elsevier B.V. All rights reserved. PACS: 81.15.Cd; 81.15.Jj Keywords: Focused ion-beam sputtering; Magnetron sputtering; Heavy-ion target; Sputtering
1. Introduction and motivation The deposition of refractory materials, especially when they are scarce, demands special techniques. Thermal evaporation is not appropriate for a high-melting material since it is difficult or even impossible to find a suitable crucible material that will not contaminate the target in some way at high temperature. On the other hand, electron-beam evaporation is also not an alternative in the case of scarce isotopes because of the larger distances one has to use to protect the substrate from overheating and reflected electrons. Sputtering, in contrast to the methods mentioned above, is a ballistic process which allows *Corresponding author. Tel.: +49-6159-71-25-23, fax: +496159-71-21-66. E-mail address: [email protected] (B. Kindler).
depositing materials independently of their vapor pressures: low-melting metals, refractory materials, elements, compounds, conductors and insulators. Additionally, it is possible to sputter in the presence of a reactive gas or a gas mixture so that for example, oxides can be created during the deposition. In general, sputtered layers have a higher adhesive strength on a substrate than evaporated layers. All these properties make sputtering deposition a powerful tool for the preparation of heavy-ion targets. In the following, we characterize the focused ion-beam method, as described by Sletten and Knudsen [1], which is in most cases a good answer to the problem of depositing refractory materials with a high yield. Subsequently, we introduce the complementary magnetron sputtering technique which has become available only recently in our lab.
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.153
ARTICLE IN PRESS B. Kindler et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
223
2. Focused ion-beam sputtering In Fig. 1, a drawing of our focused ion-beam apparatus is shown. It is a Sletten-type from Danfysik, as already described by Folger et al. [2]. From a duoplasmatron ion-source with an Ar partial pressure of 2–3 10 2 mbar, an ion-current of about 1 mA is generated. By means of an Einzellens-arrangement, the Ar ions are extracted into high vacuum and focused to a diameter smaller than 2 mm. With 10 kV, they are accelerated onto the cathode where the target material in the form of a metal bead or a pressed tablet is situated inside the beam stop made of high-purity carbon. The substrates are arranged concentrically around the sputter target as depicted in Fig. 2. The gap between the sputter target and the substrates can be varied between 8 and 50 mm with about 15 mm being a standard distance. In this geometry, six of our standard frames with a 10 mm hole can be deposited in one run with a satisfactory uniformity over the diameter of the beam spot. The form of the cathode influences the directivity of the sputter yield. Since we always work with a vertical incidence beam with the substrates arranged sideways, we prefer a spherical sputter cathode instead of one with a flat surface where more material is sputtered in the vertical direction.
Fig. 2. One possible target arrangement around the sputter source.
The whole assembly can be moved in the X and Y directions for an optimum alignment of the beam relative to the sputter target. Also the substrate holder can be rotated, thus providing a better uniformity of the deposited layers.
3. Pros and cons of the method
Fig. 1. Sketch of the Sletten-type focused ion-beam apparatus.
Since sputtering is a purely ballistic process, material can be deposited independently of its vapor pressure or melting temperature. Sputtered layers adhere in general well to the substrate. The non-sticking quality of the substances like Cd, that in thermal evaporation processes are often problematic, are uncritical in sputtering processes. It is also a ‘‘cold’’ process which does not strain the backing material thermally. This allows for a close deposition geometry which makes the process extremely efficient. With the small beam focus, a deposition with a minimum of starting material is possible. Because of the small beam focus and the extremely poor sputtering yield of carbon, there is only slight danger of contamination of the target with carbon crucible material. The fact that the deposition takes place in a high vacuum
ARTICLE IN PRESS 224
B. Kindler et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
environment helps obtaining clean and well adhering target layers. Most of the advantages of the method also involve certain disadvantages. The ballistic impact has consequences on the choice of the release agent for backing-free targets. Especially betaine–sucrose layers do not always dissolve properly after sputter deposition. In fact we use the method primarily for backed targets or on Cu substrates which can be etched after deposition. The close geometry of the setup leads to a reasonable uniformity only for small target areas. The small beam focus results in low deposition rates which in turn lead to extremely long deposition times for thick or large layers. In view of the two latter arguments, the deposition of large-area targets with this technique is not recommended. Also with increasing deposition time, the danger of target contamination increases.
4. Preparation and sputter yields for some selected isotopes In the following, we will give a few examples of targets made with focused ion-beam sputtering in recent years. We used a standard target-to-substrate distance of about 15 mm for all the targets described here. If only a small amount of starting material is available, we can reduce this distance to about 8 mm therefore, accepting a considerable inhomogeneity of the target layer thickness. Particularly for isotopes of Ru and Hf, we experienced that for thicker layers on thin backings, a deposition in several steps is absolutely necessary. Otherwise the deposited layer flakes off or the backing foil is damaged. For 96Ru, we made targets of 2.15 mg/cm2 on a backing of 1 mg/cm2 92Mo in seven runs, depositing about 300 mg/cm2 each time with 3–5 days rest between the successive runs. We obtained a yield of 3% per cm2 target area. For 178Hf on B35 mg/cm2 C, we used a pressed tablet of the oxide as the sputter target. For targets with a thickness of 1 mg/cm2, we performed nine runs, depositing in each step 100–150 mg/cm2 with 2–3 days rest in between. In this process, we got a yield of 4% per cm2 target area.
The isotopes 186W with a thickness of 280 mg/ cm2 on 10 mg/cm2 Cu and 232Th with a thickness of 610 mg/cm2 on 2.3 mg/cm2 Be could each be deposited in only one step with a yield of 3.6% and 6% per cm2, respectively.
5. Magnetron sputtering A sputter coater from BOC Edwards has been available for 2 years in the GSI target laboratory. There are two 1-in. and two 3-in. planar magnetron sputtering sources available of which three can be mounted simultaneously in the machine. In Fig. 3 shows the inside of the chamber with a 1in. source mounted in front and a 3-in. source located in the rear. In Fig. 4, the principle of such a planar magnetron is depicted. The magnetic field created from permanent magnets below the sputter target together with the vertical electric field, confines the low-pressure plasma to a torus above the sputter cathode. Magnetrons can be operated either in the DC mode with a maximum power of 500 W or in the RF mode with a maximum power of 300 W.
Fig. 3. View inside the Edwards 500 DC and RF sputter coater with a 1-in. source mounted in the front and a 3-in. source in the rear.
ARTICLE IN PRESS B. Kindler et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
Fig. 4. Principle of a planar magnetron source [3]: The magnetic fields drawn above in combination with a vertical E-field confine the low-pressure plasma to a torus above the sputter target that serves as cathode. N, north; S, south.
As a rule of thumb, the minimum distance between the sputter target and the substrate should be of the same order as the diameter of the sputter source used, namely 1-in. or 3-in., to avoid damage of the substrate by the plasma. This rather large gap implies only a medium efficiency in terms of material consumption. A maximum reasonable distance is about 10 cm for a 1-in. source and about 15 cm for a 3-in. source. We next describe some examples of targets successfully prepared with this method, with which we gained our first experience. 5.1. Cadmium For nuclear chemistry experiments, we had a request for the deposition of Cd on 6 mm Tibacking in a 2 cm2 banana-shape. Cd is a difficult candidate to evaporate since it deposits only on the coldest surfaces which means in general the watercooled parts of the chamber and the pump baffle. With magnetron sputtering, we produced targets with 250 and 650 mg/cm2 without problems. The sputtering distance was about 60 mm. 5.2. Tantalum For the heavy-element program at SHIP, we were asked to produce two mountings, each with eight targets of 181Ta with a target area of about 18.5 cm2 and a thickness of 400 mg/cm2 on a C backing of 40 mg/cm2.
225
Since Ta is a refractory material, thermal evaporation which is the standard method for the SHIP targets, was not applicable in this case. Ta can only be evaporated with an electron gun but for the banana-shaped SHIP targets, a large distance between the crucible and the target was necessary to get a tolerable uniformity of the layer. But anyhow, the stress on these large backings was so high that even 45 mg/cm2 of carbon did not survive. Since 181Ta has a natural abundance of 99.988%, it was possible to use a standard plate of Ta as sputtering source and make a first test for the production of SHIP targets with magnetron sputtering. The setup used in this case is shown in Fig. 3 with the Ta target mounted in the 1-in. source in the front and the evaporation wheel mounted on the rotation axis at the top of the chamber. Eight targets were deposited in one run with the sputtering done in two steps for a total of about 45 min and a target-tosubstrate distance of 55 mm. The targets showed a good uniformity and a satisfactory durability during irradiation with a heavy ion beam. In other cases, magnetron sputtering would possibly not be so easy, since for experiments with low reaction cross-sections, it is necessary to use enriched material. To get this in the form of a metal sheet at least 1-in. in diameter with a sufficient thickness would be difficult and expensive. Also the material consumption is rather high. Nevertheless this new method opens up a perspective for heavy-ion targets of materials considered until now as impossible. 5.3. Cobalt and nickel We had a first try at depositing magnetic materials with this method. On 50 mm Ta backings, 100 mg/cm2 Co or 200 mg/cm2 Ni were to be deposited with a target of 45 mm in diameter. The requirements for the layers were high adhesive strength, a variation of less than 710% from the desired thickness and less than 75% in thickness across the diameter. Therefore, magnetron sputtering was the most promising method to do the job.
ARTICLE IN PRESS B. Kindler et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 222–226
226
1
250 245 240 235 230 225 220 215 210 205 200 2
3
4
5
1
2
3
4
thickness in µg/cm²
6. Summary average 237 µ/cm² deviation +/- 9.94 µ/cm² +/- 4.19 %
5
cm
Fig. 5. Distribution of the weight per unit area of a sputtered Ni-layer (left) and the corresponding tolerances (right).
We found that it was not practicable to use standard sputter targets which have a thickness of 3–5 mm. The magnetic susceptibility of Co and Ni interfered with the magnetic field of the permanent Sm–Co magnets situated below the sputtering target (see Fig. 4) so that it was impossible to ignite a stable plasma. We finally succeeded by stacking several thin Co or Ni foils with a thickness not more than 250–300 mm each and replaced the upper foil after some time. The sputtering distance was 65 mm in this case. The bar diagram in Fig. 5 shows the thickness variation for one of the test targets which had a somewhat higher thickness than the required ones. With a deviation of 74.19%, we were within the tolerance we sought.
We have shown that focused ion-beam sputtering is a powerful method for high-melting-point substances when only small amounts of starting material are available. For small target areas, a satisfactory uniformity with low material consumption can be had. For larger target areas, where focused ion-beam sputtering is not recommended, magnetron sputtering can be an alternative for refractory or non-sticking materials and for all cases where a good adhesion of the target layer to the substrate is required. For expensive material, magnetron sputtering is not a good option since the material consumption is rather high.
Acknowledgements The authors thank Gabi Otto and Achim Zschau for the photographs.
References [1] G. Sletten, P. Knudsen, Nucl. Instr. and Meth. 102 (1972) 459. [2] H. Folger, J. Klemm, M. Muller, . IEEE Trans. Nucl. Sci. NS-30 (2) (1983) 1568. [3] http://www.lesker.com