REVIEW OF SCIENTIFIC INSTRUMENTS 85, 043303 (2014)
University of Lodz an electron spectrometer—A new conversion-electron spectrometer for “in-beam” measurements J. Perkowski,1,a) J. Andrzejewski,1 Ł. Janiak,1 J. Samorajczyk,1 T. Abraham,2 Ch. Droste,3 2,4 ´ ´ E. Grodner,3 K. Hadynska-Kl ek, ˛ 2 M. Kisielinski, M. Komorowska,2 M. Kowalczyk,2,3 J. 2,4 2,3 2 Kownacki, J. Mierzejewski, P. Napiorkowski, A. Korman,4 J. Srebrny,2 A. Stolarz,2 2,5 ´ and M. Zielinska 1
Faculty of Physics and Applied Computer Science, University of Lodz, Lodz 90-236, Poland Heavy Ion Laboratory, University of Warsaw, Pasteura 5A, 02-093 Warsaw, Poland 3 Faculty of Physics, University of Warsaw, Warsaw 00-681, Poland 4 ´ The National Centre for Nuclear Research, Andrzeja Sołtana 7, 05-400 Otwock, Swierk, Poland 5 CEA Saclay, Route Nationale, 91400 Gif-sur-Yvette, France 2
(Received 30 January 2014; accepted 29 March 2014; published online 15 April 2014) The designed and constructed at the University of Lodz an electron spectrometer is devoted to “in-beam” measurements. The apparatus is characterized by high efficiency up to 9%, good energy resolution (FWHM = 5 keV at 482 keV) and, what is very important good suppression of delta electrons, positrons, and photons emitted by the targets. This achievement was obtained using a combination of magnetic field in two different layouts: perpendicular and parallel to the axis of the spectrometer being orthogonal to the beamline. The conversion-electron spectrometer coupled to the EAGLE array was successfully tested in an “in-beam” measurement. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4870899] INTRODUCTION
Conversion-electron spectroscopy plays an important role in the study of excited states populated in nuclear reactions induced by heavy ions. Gamma-electron coincidence measurements allow the internal conversion coefficients and, in consequence, the transition multipolarities to be determined. Combined gamma-electron spectroscopy can make a significant contribution to such areas of nuclear physics as1 –nuclear isomerism (fission, K-isomers), –tests of nuclear model predictions (nuclear shapes, particle coupling schemes, very high spin states), –compressibility of nuclear matter and nuclear monopole degrees of freedom, –study of beta-decay (log ft systematics), –validity of conservation laws. The most important requirements for spectrometers in “in-beam” measurements of electrons are the following:1 a) high transmission of electrons between target and detector over the widest possible energy range, b) good energy resolution, c) easy mechanical coupling of the electron spectrometer and a gamma array without a significant loss of detection efficiency, d) efficient elimination of delta electrons and positrons, e) shielding of the silicon detector from X and γ rays emitted by the target.
a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2014/85(4)/043303/7/$30.00
Taking into account configuration of the magnetic fields of electron spectrometers may be divided in two categories: –devices with the magnetic field vector perpendicular to the flat trajectories of electrons (e.g., mini-orange spectrometers),1, 3–6 –devices with the magnetic field parallel to the velocity vector of the electrons (e.g., spectrometers with long magnetic lenses).1, 3, 6 The electrons moving towards the detector follow helical trajectories. Both types of spectrometers have been successfully used in “in beam” experiments,1, 3, 7, 8 however, they showed some deficiencies. Devices of the first type allow positrons and delta electrons to be rejected and the γ -ray background to be reduced, but they have too high electron-energy selectivity that may often be a negative feature. In the case of solenoid spectrometers, electrons are accepted over a wide energy range, although it is necessary to use a baffle or an electrostatic potential barrier produced by a special electrode placed between the target and the detector to eliminate delta electrons. However, such filters do not eliminate high energy positrons.6 The γ -ray background, which is rather low due to the large target-to-detector distance, can be additionally reduced if a lead block is placed in between. We combined both types of geometry of magnetic fields in our old6 and new conversion-electron spectrometers. In both constructions permanent magnets were used for selection and transportation of electrons. Our first spectrometer9, 13 for e-γ measurements was successfully used in several experiments,9–11 but its efficiency for “in-beam” studies was not satisfactory. We, therefore, constructed a new spectrometer where the advantages of both magnetic field configurations were again exploited incorporating at the same time several new solutions.
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© 2014 AIP Publishing LLC
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Rev. Sci. Instrum. 85, 043303 (2014)
DESIGN OF THE SPECTROMETER AND SIMULATIONS
The design of the University of Lodz an electron spectrometer (ULESE) is based on using magnetic fields in different layouts: perpendicular and parallel to the axis of spectrometer, orthogonal to the beam line. The target is placed in the position where the magnetic field vector of four squareshaped plate magnets is generally perpendicular to the initial direction of emitted electrons or positrons (Fig. 1). Trajectories of conversion electrons emitted from both sides of the target (forward and backward) are bent in the direction of the detector; whereas positrons are bent in the opposite direction. The electrons are then transmitted towards Si-detector due to set of six magnets in the form of coaxial rings, which produce a longitudinal magnetic field. Elimination of delta electrons is achieved by a combination of the magnetic field geometry and the special shape of the target holder. The design of this spectrometer was optimised thanks to simulations of the magnetic field and electron trajectories performed using the CST PARTICLE STUDIO software.12 A schematic view of the spectrometer together with a few simulated electron trajectories of 250 keV electrons is presented →
in Fig. 1. The B field can be modified by changing the shape and position of the permanent magnets. The magnetic selector consists of four permanent magnets made of sintered Nd-Fe-B alloy in the shape of square plates with dimensions 40 mm × 40 mm × 3.5 mm and 40 × 14 × 5 mm each. These magnets induce a field perpendicular to beam axis and at the same time perpendicular to the symmetry axis of spectrometer. The result of the magnetic field simulation by the CST code is presented in Fig. 2. Only the values of the main component of the magnetic field (BY ) along the axis of the apparatus are shown. The inhomogenity of the magnetic field ensures that electrons over a wide en→
FIG. 2. Simulation of the magnetic field perpendicular to the beam path (BY ) →
performed by the CST code. Colors correspond to the strength of the B component.
longitudinal field electrons are transported from the target area to the detector. Values of the longitudinal component of the magnetic field (BZ ) at the surface cut along the axis of the spectrometer are presented in Fig. 3. The simulations of the magnetic field were compared to values measured by a FH-55 teslameter manufactured by the “Magnet-Physik” company. The experimental value of the →
longitudinal B is about 50 mT on the axis of the spectrometer, and agrees with simulations performed by the CST code. The experiment and simulation have shown that the strength of magnetic field changes slightly along the axis of the spectrometer (see Fig. 3). The magnetic field inside the target area is highly inhomogeneous. Nevertheless, the perpendicular component of the magnetic field (BY ) was also measured along the beam path. The results of this measurements and simulations are presented in Fig. 4 for three slightly different positions of four selector magnets in the form of square-shaped plates. It is visible that the measured values of the magnetic field agree with the simulations.
ergy range are bent in the direction of the longitudinal B area. Trajectories of low energy electrons and delta electrons are strongly bent in the field and so they do not reach the detector but instead they are stopped in a specially formed aluminum target holder placed along the symmetry axis of the apparatus. A magnetic field parallel to the axis of the spectrometer (BZ ) is produced by a set of six permanent magnets arranged in the form of coaxial rings. These rings are also made of sintered Nd-Fe-B and are each 25 mm high with inner/outer diameters equal to 120 and 150 mm, respectively. In this
CONSTRUCTION AND TESTING OF THE MAGNETIC SPECTROMETER
FIG. 1. The visualisation of the idea of the ULESE spectrometer prepared using the CST PARTICLE STUDIO software. For simplicity only a few trajectories of electrons at energy of 250 keV are shown.
FIG. 3. The longitudinal component of the magnetic field (Bz ) calculated by the CST code. Only values at the surface cut along the axis of the spectrom-
A picture of the ULESE spectrometer is presented in Fig. 5. The housing of the spectrometer is in the form of a cylindrical chamber of 120 mm in diameter and 300 mm in total length. The distance between the target (the source of the electrons) and the detector is 200 mm. The six permanent magnet coaxial rings are placed outside the housing. Standard preamplifiers (A1422H) manufactured by C.A.E.N. with main circuit board are used for preliminary amplification of
→
eter are shown. Colors correspond to the strength of the B component.
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FIG. 4. The strength of the perpendicular BY component of the selecting field (see Fig. 2) along the beam x-axis (x = 0 correspond to the target/source position). The curves correspond to simulations carried out for three distances between the flat permanent magnets and the beamline. The results of the measurements are also shown.
signals from the silicon detector. The construction of the circuit board ensures the shortest electrical connection between the detector and the A1422H chips. A picture of the detection part of the spectrometer is shown in Fig. 6. A standard single side 16-strip silicon detector (W1 type, 5 cm × 5 cm) of 1.5 mm thickness manufactured by Micron Semiconductor Ltd was used for the measurement of the electron energies. A two-step cooling system with Peltier modules was applied to cool the detector. This system allows reaching a minimum temperature of about −20 ◦ C. Two Peltier modules 267 W each are connected in series (see Fig. 6). The first cooling module is placed between a copper plate with the detector fixed to it and a similar copper plate used as a heat radiator. The second Peltier module is located between the conveyer and the spectrometer housing. The heat is then transferred to an outer radiator consisting of a system of channels with circulating water. The stability of the temperature of the water used as coolant ensures the temperature stability of the detectors with an accuracy of 1 ◦ C. The temperatures of each step of the cooling system are controlled by sensors. The energy resolution of all 16-strips of the Si-detector measured at room temperature and a temperature of −20 ◦ C by using a 207 Bi calibration source is shown in
FIG. 5. The outer view of the conversion-electron spectrometer showing the six permanent magnet coaxial rings and preamplifiers.
Rev. Sci. Instrum. 85, 043303 (2014)
FIG. 6. The detection part of the spectrometer with the 16-strip standard silicon detector.
Fig. 7. The energy resolution for 482 keV electrons was improved by about a factor of ∼4 after cooling the detector. Efficiency of the spectrometer for different configurations of the magnetic field was measured using internal conversion electrons from a 207 Bi (478 kBq) and 133 Ba (48 kBq) sources. The calibration sources were located at the target position. Sample spectra of internal conversion electrons from 207 Bi and 133 Ba registered by one strip of the silicon detector are presented in Fig. 8. About 30 different configurations of the magnetic field in the target area were checked. Four arbitrarily chosen experimentally obtained spectrometer efficiencies for different geometrical configurations of the magnetic selector using 207 Bi are shown in Fig. 9. It should be noted that the efficiency is very sensitive to small displacements (∼1 mm) of the selector magnets (40 mm × 14 mm × 5 mm). A comparison of the efficiency of the chosen setup of the magnet selector in the “in-beam” test experiment (labelled as filled squares) with results of the simulations performed
FIG. 7. The energy resolution of the silicon detector at two temperatures, measured for the 482 keV electron line from a 207 Bi calibration source.
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FIG. 8. Internal conversion electron spectra from calibration sources: 133 Ba and 207 Bi (the 570 keV line only) registered in two measurements by a single strip of the cooled silicon detector.
by the CST code is given in Fig. 10. The three curves exactly correspond to simulations of the magnetic field shown in Fig. 4. The effect of electrons backscattering on the surface of the silicon detector was not included in the simulations. The active thickness of the detector (1.5 mm) appears insufficient to register the full energy of 1 MeV electrons. The following conclusions should be pointed out: –small displacement of the selector magnets causes insignificant changes of the magnetic field but significant changes of the efficiency of the spectrometer. –the new spectrometer has an order of magnitude higher efficiency for 300 keV and about 2 times higher for 500 keV electrons in comparison to the old one.9 –the efficiency of the new magnetic spectrometer can easily be changed, which was not the case for the old one9 (see Fig. 10). THE “IN-BEAM” EXPERIMENT
Rev. Sci. Instrum. 85, 043303 (2014)
FIG. 10. Comparison of the efficiencies of the spectrometer for chosen setups of the selector magnet (filled circles) and simulations performed with the CST code (three curves). The curve styles correspond to the magnetic field presented in Fig. 4, the numbers indicate the distance between the magnetic selector of the selector and the beam axis.
array2 consisted of 15 Compton-suppressed HPGe detectors was performed at the Heavy Ion Laboratory of the University of Warsaw. The EAGLE spectrometer together with the conversion-electron spectrometer is presented in Fig. 11. The following reaction channel providing a high yield of 184 Pt nuclei was studied in the test measurement:14, 15 14
N +175 Lu →184 Pt + 5n.
The 14 N beam at an energy of 90 MeV and intensity of 0.4/0.8 enA was delivered by the U-200P cyclotron. In the experiment natural lutetium was used as a target as 97% abundance of 175 Lu isotope is sufficient for production of 184 Pt and collecting satisfactory statistics. The target with thickness of 2.5 mg/cm2 was prepared by rolling the Lu foil between the stainless steel sheets. The Lu foil was backed by a gold layer (∼1 mg/cm2 ) to stop the products of the reaction and to avoid emission of conversion-electrons and gammas emitted in-flight from excited 184 Pt nuclei.16 Therefore, electrons as well as gamma rays were emitted from a well
A test “in-beam” experiment with the new conversionelectron spectrometer coupled to the EAGLE multidetector
FIG. 9. Efficiencies of the spectrometer obtained using a 207 Bi calibration source for four different configurations of magnetic selector.
FIG. 11. Picture of the EAGLE multidetector gamma-ray array at the Heavy Ion Laboratory, University of Warsaw2 with the conversion-electron spectrometer.
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defined spot. The Au material was deposited on the target by the procedure of evaporation–condensation in a high vacuum. The value of the energy loss of electrons is from 2.1 to 1.1 keV/(mg/cm2 ) at the energy range of 0.1–0.6 MeV.17 The chosen thickness of the target is a compromise between the energy loss of the electrons in the target and efficiency of 184 Pt production. The measurement was carried out in electron-gamma and gamma-gamma coincidence modes during the 2 ms “inbeam” time period. The width of the macrostructure pulses of the cyclotron beam was 6 ms. Part of the 184 Pt level scheme18, 21 is shown in Fig. 12. The total gamma and electron spectra 184 Pt collected during 24 h of the test measurement as well as the ones gated on the 273 keV gamma transition are presented in Figs. 13 and 14, respectively. The gamma and conversion electron lines registered for transitions from the rotational band in 184 Pt can be easily recognized in the gated as well as in the total spectra. The value of the internal conversion coefficient can be determined experimentally from the ratio of the number of emitted electrons (Ne ) and γ rays (Nγ ) for a given transition
FIG. 13. The gamma projection spectrum and the spectrum gated on the 273 keV gamma transition from 184 Pt labelled as solid and dotted lines, respectively.
using the following formula:10, 19 α=
εγ Ie Ne = × . Iγ Nγ εe
(1)
In the formula, εe and εγ represent the detection efficiency for electrons and photons. The yields of electrons and gammas should be taken from the spectra obtained by applying the same gate on both γ −γ and γ -e matrices. The ratio εγ /εe is determined from experimental data for transitions with well known multipolarity. The efficiencies ratio curve shown in Fig. 15 were obtained using an internal calibration based on the γ transitions for the decays of excited levels in 184 Pt. The following lines with well-known multipolarity or known internal conversion coefficients were used in the analysis: 163, 273, 362, 432, 476, 498, 522, and 555 keV. The GF3 program from the RadWare package20 was used for the fitting of spectra. This code is a standard tool to analyze gamma-ray spectra but also provides a peak shape function suitable to describe non-gaussian electron peaks. In this
FIG. 12. Parial a level scheme of gamma spectra.
184 Pt.
All transitions are visible in the FIG. 14. The same as in Fig. 13, but for electrons.
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sition is E2/M1 one with a mixing parameter δ 2 (E2/M1) = 1.3 (9). The E3 multipolarity, although α K (exp) ≈ α K (theory) (E3) is ruled out because of the low intensity of the L—electron peak. The E2/M1 multipolarity of the 314.3 keV line confirms the spin and parity 16+ of the band head level at 3596.3 keV in the scheme proposed in Ref. 18.
SUMMARY
A new magnetic conversion-electron spectrometer for “in-beam” measurements was designed and constructed at the University of Lodz. The characteristic features of the spectrometer are as follows: –relatively small dimensions, –detection efficiency of electrons up to 9% at an energy of 300 keV, for the geometry of magnetic field applied in the present experiment, –elimination of positrons, –reduction of the X-ray background, –elimination of delta electrons, –effective two-step cooling of the silicon detector by Peltier modules, –energy resolution of the silicon detector
University of Lodz an electron spectrometer—A new conversion-electron spectrometer for “in-beam” measurements J. Perkowski,1,a) J. Andrzejewski,1 Ł. Janiak,1 J. Samorajczyk,1 T. Abraham,2 Ch. Droste,3 2,4 ´ ´ E. Grodner,3 K. Hadynska-Kl ek, ˛ 2 M. Kisielinski, M. Komorowska,2 M. Kowalczyk,2,3 J. 2,4 2,3 2 Kownacki, J. Mierzejewski, P. Napiorkowski, A. Korman,4 J. Srebrny,2 A. Stolarz,2 2,5 ´ and M. Zielinska 1
Faculty of Physics and Applied Computer Science, University of Lodz, Lodz 90-236, Poland Heavy Ion Laboratory, University of Warsaw, Pasteura 5A, 02-093 Warsaw, Poland 3 Faculty of Physics, University of Warsaw, Warsaw 00-681, Poland 4 ´ The National Centre for Nuclear Research, Andrzeja Sołtana 7, 05-400 Otwock, Swierk, Poland 5 CEA Saclay, Route Nationale, 91400 Gif-sur-Yvette, France 2
(Received 30 January 2014; accepted 29 March 2014; published online 15 April 2014) The designed and constructed at the University of Lodz an electron spectrometer is devoted to “in-beam” measurements. The apparatus is characterized by high efficiency up to 9%, good energy resolution (FWHM = 5 keV at 482 keV) and, what is very important good suppression of delta electrons, positrons, and photons emitted by the targets. This achievement was obtained using a combination of magnetic field in two different layouts: perpendicular and parallel to the axis of the spectrometer being orthogonal to the beamline. The conversion-electron spectrometer coupled to the EAGLE array was successfully tested in an “in-beam” measurement. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4870899] INTRODUCTION
Conversion-electron spectroscopy plays an important role in the study of excited states populated in nuclear reactions induced by heavy ions. Gamma-electron coincidence measurements allow the internal conversion coefficients and, in consequence, the transition multipolarities to be determined. Combined gamma-electron spectroscopy can make a significant contribution to such areas of nuclear physics as1 –nuclear isomerism (fission, K-isomers), –tests of nuclear model predictions (nuclear shapes, particle coupling schemes, very high spin states), –compressibility of nuclear matter and nuclear monopole degrees of freedom, –study of beta-decay (log ft systematics), –validity of conservation laws. The most important requirements for spectrometers in “in-beam” measurements of electrons are the following:1 a) high transmission of electrons between target and detector over the widest possible energy range, b) good energy resolution, c) easy mechanical coupling of the electron spectrometer and a gamma array without a significant loss of detection efficiency, d) efficient elimination of delta electrons and positrons, e) shielding of the silicon detector from X and γ rays emitted by the target.
a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2014/85(4)/043303/7/$30.00
Taking into account configuration of the magnetic fields of electron spectrometers may be divided in two categories: –devices with the magnetic field vector perpendicular to the flat trajectories of electrons (e.g., mini-orange spectrometers),1, 3–6 –devices with the magnetic field parallel to the velocity vector of the electrons (e.g., spectrometers with long magnetic lenses).1, 3, 6 The electrons moving towards the detector follow helical trajectories. Both types of spectrometers have been successfully used in “in beam” experiments,1, 3, 7, 8 however, they showed some deficiencies. Devices of the first type allow positrons and delta electrons to be rejected and the γ -ray background to be reduced, but they have too high electron-energy selectivity that may often be a negative feature. In the case of solenoid spectrometers, electrons are accepted over a wide energy range, although it is necessary to use a baffle or an electrostatic potential barrier produced by a special electrode placed between the target and the detector to eliminate delta electrons. However, such filters do not eliminate high energy positrons.6 The γ -ray background, which is rather low due to the large target-to-detector distance, can be additionally reduced if a lead block is placed in between. We combined both types of geometry of magnetic fields in our old6 and new conversion-electron spectrometers. In both constructions permanent magnets were used for selection and transportation of electrons. Our first spectrometer9, 13 for e-γ measurements was successfully used in several experiments,9–11 but its efficiency for “in-beam” studies was not satisfactory. We, therefore, constructed a new spectrometer where the advantages of both magnetic field configurations were again exploited incorporating at the same time several new solutions.
85, 043303-1
© 2014 AIP Publishing LLC
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Perkowski et al.
Rev. Sci. Instrum. 85, 043303 (2014)
DESIGN OF THE SPECTROMETER AND SIMULATIONS
The design of the University of Lodz an electron spectrometer (ULESE) is based on using magnetic fields in different layouts: perpendicular and parallel to the axis of spectrometer, orthogonal to the beam line. The target is placed in the position where the magnetic field vector of four squareshaped plate magnets is generally perpendicular to the initial direction of emitted electrons or positrons (Fig. 1). Trajectories of conversion electrons emitted from both sides of the target (forward and backward) are bent in the direction of the detector; whereas positrons are bent in the opposite direction. The electrons are then transmitted towards Si-detector due to set of six magnets in the form of coaxial rings, which produce a longitudinal magnetic field. Elimination of delta electrons is achieved by a combination of the magnetic field geometry and the special shape of the target holder. The design of this spectrometer was optimised thanks to simulations of the magnetic field and electron trajectories performed using the CST PARTICLE STUDIO software.12 A schematic view of the spectrometer together with a few simulated electron trajectories of 250 keV electrons is presented →
in Fig. 1. The B field can be modified by changing the shape and position of the permanent magnets. The magnetic selector consists of four permanent magnets made of sintered Nd-Fe-B alloy in the shape of square plates with dimensions 40 mm × 40 mm × 3.5 mm and 40 × 14 × 5 mm each. These magnets induce a field perpendicular to beam axis and at the same time perpendicular to the symmetry axis of spectrometer. The result of the magnetic field simulation by the CST code is presented in Fig. 2. Only the values of the main component of the magnetic field (BY ) along the axis of the apparatus are shown. The inhomogenity of the magnetic field ensures that electrons over a wide en→
FIG. 2. Simulation of the magnetic field perpendicular to the beam path (BY ) →
performed by the CST code. Colors correspond to the strength of the B component.
longitudinal field electrons are transported from the target area to the detector. Values of the longitudinal component of the magnetic field (BZ ) at the surface cut along the axis of the spectrometer are presented in Fig. 3. The simulations of the magnetic field were compared to values measured by a FH-55 teslameter manufactured by the “Magnet-Physik” company. The experimental value of the →
longitudinal B is about 50 mT on the axis of the spectrometer, and agrees with simulations performed by the CST code. The experiment and simulation have shown that the strength of magnetic field changes slightly along the axis of the spectrometer (see Fig. 3). The magnetic field inside the target area is highly inhomogeneous. Nevertheless, the perpendicular component of the magnetic field (BY ) was also measured along the beam path. The results of this measurements and simulations are presented in Fig. 4 for three slightly different positions of four selector magnets in the form of square-shaped plates. It is visible that the measured values of the magnetic field agree with the simulations.
ergy range are bent in the direction of the longitudinal B area. Trajectories of low energy electrons and delta electrons are strongly bent in the field and so they do not reach the detector but instead they are stopped in a specially formed aluminum target holder placed along the symmetry axis of the apparatus. A magnetic field parallel to the axis of the spectrometer (BZ ) is produced by a set of six permanent magnets arranged in the form of coaxial rings. These rings are also made of sintered Nd-Fe-B and are each 25 mm high with inner/outer diameters equal to 120 and 150 mm, respectively. In this
CONSTRUCTION AND TESTING OF THE MAGNETIC SPECTROMETER
FIG. 1. The visualisation of the idea of the ULESE spectrometer prepared using the CST PARTICLE STUDIO software. For simplicity only a few trajectories of electrons at energy of 250 keV are shown.
FIG. 3. The longitudinal component of the magnetic field (Bz ) calculated by the CST code. Only values at the surface cut along the axis of the spectrom-
A picture of the ULESE spectrometer is presented in Fig. 5. The housing of the spectrometer is in the form of a cylindrical chamber of 120 mm in diameter and 300 mm in total length. The distance between the target (the source of the electrons) and the detector is 200 mm. The six permanent magnet coaxial rings are placed outside the housing. Standard preamplifiers (A1422H) manufactured by C.A.E.N. with main circuit board are used for preliminary amplification of
→
eter are shown. Colors correspond to the strength of the B component.
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FIG. 4. The strength of the perpendicular BY component of the selecting field (see Fig. 2) along the beam x-axis (x = 0 correspond to the target/source position). The curves correspond to simulations carried out for three distances between the flat permanent magnets and the beamline. The results of the measurements are also shown.
signals from the silicon detector. The construction of the circuit board ensures the shortest electrical connection between the detector and the A1422H chips. A picture of the detection part of the spectrometer is shown in Fig. 6. A standard single side 16-strip silicon detector (W1 type, 5 cm × 5 cm) of 1.5 mm thickness manufactured by Micron Semiconductor Ltd was used for the measurement of the electron energies. A two-step cooling system with Peltier modules was applied to cool the detector. This system allows reaching a minimum temperature of about −20 ◦ C. Two Peltier modules 267 W each are connected in series (see Fig. 6). The first cooling module is placed between a copper plate with the detector fixed to it and a similar copper plate used as a heat radiator. The second Peltier module is located between the conveyer and the spectrometer housing. The heat is then transferred to an outer radiator consisting of a system of channels with circulating water. The stability of the temperature of the water used as coolant ensures the temperature stability of the detectors with an accuracy of 1 ◦ C. The temperatures of each step of the cooling system are controlled by sensors. The energy resolution of all 16-strips of the Si-detector measured at room temperature and a temperature of −20 ◦ C by using a 207 Bi calibration source is shown in
FIG. 5. The outer view of the conversion-electron spectrometer showing the six permanent magnet coaxial rings and preamplifiers.
Rev. Sci. Instrum. 85, 043303 (2014)
FIG. 6. The detection part of the spectrometer with the 16-strip standard silicon detector.
Fig. 7. The energy resolution for 482 keV electrons was improved by about a factor of ∼4 after cooling the detector. Efficiency of the spectrometer for different configurations of the magnetic field was measured using internal conversion electrons from a 207 Bi (478 kBq) and 133 Ba (48 kBq) sources. The calibration sources were located at the target position. Sample spectra of internal conversion electrons from 207 Bi and 133 Ba registered by one strip of the silicon detector are presented in Fig. 8. About 30 different configurations of the magnetic field in the target area were checked. Four arbitrarily chosen experimentally obtained spectrometer efficiencies for different geometrical configurations of the magnetic selector using 207 Bi are shown in Fig. 9. It should be noted that the efficiency is very sensitive to small displacements (∼1 mm) of the selector magnets (40 mm × 14 mm × 5 mm). A comparison of the efficiency of the chosen setup of the magnet selector in the “in-beam” test experiment (labelled as filled squares) with results of the simulations performed
FIG. 7. The energy resolution of the silicon detector at two temperatures, measured for the 482 keV electron line from a 207 Bi calibration source.
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FIG. 8. Internal conversion electron spectra from calibration sources: 133 Ba and 207 Bi (the 570 keV line only) registered in two measurements by a single strip of the cooled silicon detector.
by the CST code is given in Fig. 10. The three curves exactly correspond to simulations of the magnetic field shown in Fig. 4. The effect of electrons backscattering on the surface of the silicon detector was not included in the simulations. The active thickness of the detector (1.5 mm) appears insufficient to register the full energy of 1 MeV electrons. The following conclusions should be pointed out: –small displacement of the selector magnets causes insignificant changes of the magnetic field but significant changes of the efficiency of the spectrometer. –the new spectrometer has an order of magnitude higher efficiency for 300 keV and about 2 times higher for 500 keV electrons in comparison to the old one.9 –the efficiency of the new magnetic spectrometer can easily be changed, which was not the case for the old one9 (see Fig. 10). THE “IN-BEAM” EXPERIMENT
Rev. Sci. Instrum. 85, 043303 (2014)
FIG. 10. Comparison of the efficiencies of the spectrometer for chosen setups of the selector magnet (filled circles) and simulations performed with the CST code (three curves). The curve styles correspond to the magnetic field presented in Fig. 4, the numbers indicate the distance between the magnetic selector of the selector and the beam axis.
array2 consisted of 15 Compton-suppressed HPGe detectors was performed at the Heavy Ion Laboratory of the University of Warsaw. The EAGLE spectrometer together with the conversion-electron spectrometer is presented in Fig. 11. The following reaction channel providing a high yield of 184 Pt nuclei was studied in the test measurement:14, 15 14
N +175 Lu →184 Pt + 5n.
The 14 N beam at an energy of 90 MeV and intensity of 0.4/0.8 enA was delivered by the U-200P cyclotron. In the experiment natural lutetium was used as a target as 97% abundance of 175 Lu isotope is sufficient for production of 184 Pt and collecting satisfactory statistics. The target with thickness of 2.5 mg/cm2 was prepared by rolling the Lu foil between the stainless steel sheets. The Lu foil was backed by a gold layer (∼1 mg/cm2 ) to stop the products of the reaction and to avoid emission of conversion-electrons and gammas emitted in-flight from excited 184 Pt nuclei.16 Therefore, electrons as well as gamma rays were emitted from a well
A test “in-beam” experiment with the new conversionelectron spectrometer coupled to the EAGLE multidetector
FIG. 9. Efficiencies of the spectrometer obtained using a 207 Bi calibration source for four different configurations of magnetic selector.
FIG. 11. Picture of the EAGLE multidetector gamma-ray array at the Heavy Ion Laboratory, University of Warsaw2 with the conversion-electron spectrometer.
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defined spot. The Au material was deposited on the target by the procedure of evaporation–condensation in a high vacuum. The value of the energy loss of electrons is from 2.1 to 1.1 keV/(mg/cm2 ) at the energy range of 0.1–0.6 MeV.17 The chosen thickness of the target is a compromise between the energy loss of the electrons in the target and efficiency of 184 Pt production. The measurement was carried out in electron-gamma and gamma-gamma coincidence modes during the 2 ms “inbeam” time period. The width of the macrostructure pulses of the cyclotron beam was 6 ms. Part of the 184 Pt level scheme18, 21 is shown in Fig. 12. The total gamma and electron spectra 184 Pt collected during 24 h of the test measurement as well as the ones gated on the 273 keV gamma transition are presented in Figs. 13 and 14, respectively. The gamma and conversion electron lines registered for transitions from the rotational band in 184 Pt can be easily recognized in the gated as well as in the total spectra. The value of the internal conversion coefficient can be determined experimentally from the ratio of the number of emitted electrons (Ne ) and γ rays (Nγ ) for a given transition
FIG. 13. The gamma projection spectrum and the spectrum gated on the 273 keV gamma transition from 184 Pt labelled as solid and dotted lines, respectively.
using the following formula:10, 19 α=
εγ Ie Ne = × . Iγ Nγ εe
(1)
In the formula, εe and εγ represent the detection efficiency for electrons and photons. The yields of electrons and gammas should be taken from the spectra obtained by applying the same gate on both γ −γ and γ -e matrices. The ratio εγ /εe is determined from experimental data for transitions with well known multipolarity. The efficiencies ratio curve shown in Fig. 15 were obtained using an internal calibration based on the γ transitions for the decays of excited levels in 184 Pt. The following lines with well-known multipolarity or known internal conversion coefficients were used in the analysis: 163, 273, 362, 432, 476, 498, 522, and 555 keV. The GF3 program from the RadWare package20 was used for the fitting of spectra. This code is a standard tool to analyze gamma-ray spectra but also provides a peak shape function suitable to describe non-gaussian electron peaks. In this
FIG. 12. Parial a level scheme of gamma spectra.
184 Pt.
All transitions are visible in the FIG. 14. The same as in Fig. 13, but for electrons.
043303-6
Perkowski et al.
Rev. Sci. Instrum. 85, 043303 (2014)
sition is E2/M1 one with a mixing parameter δ 2 (E2/M1) = 1.3 (9). The E3 multipolarity, although α K (exp) ≈ α K (theory) (E3) is ruled out because of the low intensity of the L—electron peak. The E2/M1 multipolarity of the 314.3 keV line confirms the spin and parity 16+ of the band head level at 3596.3 keV in the scheme proposed in Ref. 18.
SUMMARY
A new magnetic conversion-electron spectrometer for “in-beam” measurements was designed and constructed at the University of Lodz. The characteristic features of the spectrometer are as follows: –relatively small dimensions, –detection efficiency of electrons up to 9% at an energy of 300 keV, for the geometry of magnetic field applied in the present experiment, –elimination of positrons, –reduction of the X-ray background, –elimination of delta electrons, –effective two-step cooling of the silicon detector by Peltier modules, –energy resolution of the silicon detector
