Curved crystal spectrometer for high-resolution in-beam spectroscopy of X-rays and low-energy gamma rays

Nuclear Instruments and Methods in Physics Research A245 (1986) 393-401 North-Holland, Amsterdam

393

CURVED CRYSTAL SPECTROMETER FOR HIGH-RESOLUTION IN-BEAM SPECTROSCOPY OF X-RAYS AND LOW-ENERGY GAMMA RAYS

G.L. BORCHERT, J. BOJOWALD, A. ERCAN, H. L A B U S , Th. R O S E a n d O . W . B . S C H U L T Institut ff4r Kernphysik, Kernforschungsanlage Jfilich, 5170 J~lich, FRG

Received 18 November 1985

A DuMond-type curved crystal spectrometer has been installed at a beam line of the isochronous cyclotron JULIC. The instrument serves for high resolution spectroscopy of X-rays and y-rays from charged particle induced reactions. It can also be used for life time studies of excited nuclear states and for off-line work.

1. Introduction In-beam y-ray spectroscopy from charged particle induced reactions is usually performed with Ge or Si detectors because of their good energy resolution and reasonable detection efficiency. Even at modest beam intensities they permit coincidence and multiparameter studies. In combination with NaI and BGO scintillation detectors big detection systems have been built covering a large fraction of the solid angle, like the Crystal Ball [1], TESSA [2] or OSIRIS [3]. They allow coincidence and multiplicity studies of nuclear levels with very high spin with high efficiency but at modest energy resolution. Ultimate energy resolution and accuracy can be achieved with curved crystal spectrometers which have been used with few exceptions [4,5] mainly for studies of slow neutron induced reactions so far. These systems, like G A M S at the ILL [6], are especially well suited for photons with energies from - 3 0 keV to - 1 MeV which are of major interest for nuclear structure studies [7]. For higher energies their resolution and detection efficiency decreases rapidly, and above 2 MeV germanium detectors are usually preferable. Because of the small solid angle of a crystal spectrometer detailed spectroscopy requires very intense sources. These can easily be produced at high flux reactors. Crystal spectrometers have rarely been used for v-ray work at particle accelerators except for studies of lowenergy X-rays. The main reason is the relatively small source strength. Compared with (n, T) reactions, the beam intensity at an accelerator is mostly - 1 /~A, which is two orders of magnitude below the equivalent of the neutron flux at the ILL. Furthermore, the reaction cross sections of - 1 b are mostly less than the slow neutron capture cross sections which are often two or three orders of magnitude larger. Finally, the charged 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

particle beam intensity is often limited by target heating. Therefore, the use of curved crystal diffractometers for the spectroscopy of photons from charged particle induced reactions is quite limited. In order to provide the possibility to carry out high-resolution spectroscopy in the X-ray region and for low-energy y-rays, the curved crystal spectrometer at J~lich [8], which had been used previously for precision y-ray measurements [9,10], has been installed at a beam line of the cyclotron J U L I C [11].

2. The apparatus The curved crystal spectrometer has been described in detail elsewhere [8]. Fig. 1 shows its main components and the geometry of the focussing diffraction spectrometer. The Bragg angle ~ is measured with high accuracy with an optical interferometric system [8]. The optical detection and control system of the interference fringes has been extended to an array of twelve diodes in a magnified interference field. The diffracted y-beam is usually recorded with a NaI scintillation counter (7.6 cm diameter × 7.6 cm). The better energy resolution, good timing property and detection probability for low-energy photons of germanium detectors make these attractive as an alternative to the scintillation counter. Because of the better energy resolution a narrower window can be set on the full-energy peak. This implies background reduction. Thus we also use a high purity Ge detector (HP-Ge) with 5.2 cm diameter x 1.2 cm sensitive volume as indicated in fig. 1. A special support has been built for the spectrometer, which is now installed in a small room separated from the source area by a 2 m thick shielding wall made of concrete (see fig. 2). G a m m a radiation or X-rays

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from the target at the source position S reach the curved crystal C after penetration of a collimator K that contains lead d i a p h r a g m s in order to suppress b a c k g r o u n d radiation. The source S is m o u n t e d in a cubic reaction chamber, a p h o t o g r a p h of which is shown in fig. 3. This c h a m b e r can be moved in the direction S - C of the -y-beam so that source alignment on the focal circle is maintained (see fig. 1). The target is m o u n t e d in a

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U-profile support fabricated out of graphite with minimal thickness in the region where the cylotron b e a m hits the target. This support keeps the target flat. It can be rotated a r o u n d the vertical axis so that the target width is minimal as seen from the spectrometer. Provisions were made to tilt the target a r o u n d the axis S - C so that the line source S is oriented parallel to the diffraction planes of the curved crystal C. The reaction c h a m b e r has a tube extension opposite to the direction

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Fig. 3. Photograph of the source holder as seen from a point between K and K' (see fig. 2). Photons leaving the source towards the spectrometer penetrate a thin AI window in the center of one face of the source holder cube. The circular window allows observation of the source with a TV camera. The other components permit precise alignment of the source holder and of the source with respect to the curved crystal spectrometer.

395

of the y-beam for reduction of neutrons scattered through the y-ray collimator K. A m o n i t o r Ge(Li) detector is located at position M in fig. 2, immediately below the b e a m from S to C. A gap in the collimator diaphragms permits the transmission of p h o t o n s from S to M. The technical details of the spectrometer can be seen in fig. 4. Fig. 5 shows the block diagram of the electronic c o m p o n e n t s of the pulse measuring system. The photons reflected by the curved crystal and detected with the NaI or the H P G e detector produce signals that are fed, after preamplification, into an " e n e r g y " b r a n c h for d e t e r m i n a t i o n and discrimination of the p h o t o n energy a n d into a " t i m e " b r a n c h for the m e a s u r m e n t of the time differences between the cyclotron b e a m burst and the p h o t o n detection by means of a time-to-amplitude converter (TAC). The pulse transformer at the cyclotron rf pulse input helps to avoid problems with the ground potentials of the different systems. Timing single channel analysers (TSCA) select the proper signals in the time and energy spectrum, e.g. in the time spectrum the p r o m p t signals and in the energy spectrum the signals corresponding to the full-energy peak of the investigated p h o t o n transition. Via the coincidence unit ( C O I N C ) and the linear gates only events with the correct time and energy correlation are fed to the analysing system. Each high-energy pulse saturating the amplifiers generates ( G G = gate pulse generator) an

Fig. 4. Photograph of the curved cyrstal spectrometer as seen from the lower right corner of the spectrometer room (fig. 2). The monitor Ge(Li) detector is located close to the exit of the collimator. The crystal clamping block is centered on the compact precision bearing. On the right half follow the Soller collimator, the lead shield of the detector and the HP Ge detector on its support.

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anti-coincidence signal and blocks the system for - 15 p,s. The pulses from the monitor detector are amplified and fed into two TSCAs with equal window widths. The windows are set on the line of a prompt photon transition and on the background near this line, respectively. The output pulses are fed into the inputs of an u p / d o w n counter the content of which is thus proportional to the actual reaction rate in the target. The possibility of selecting the time window allows the determination of lifetimes of highly excited nuclear states and permits the reduction of background caused by 7-rays from activation in the target which are random in time, and by neutrons, because their time of flight is long compared with that of photons and with the time interval of 34-49 ns between the cyclotron beam bursts. The hardware of the spectrometer position control is installed in a 9HE Europe frame containing three Siemens SMP boards (Processor, Memory and DAC) seven different single boards and three different double boards developed by our electronic group, and three Wiener SN-50 primarily switched power modules. The rather

complex control system consists of five main parts: - The signal processing and error-free counting of the interference fringes even during power failure. - The angle control of the bent crystal consists of a dc motor coarse control to perform large angle variations and an underlying fine control by a piezo-electric column which permits fast but small displacements of + 15 fringes and fast angle stabilization on a selected phase of a single fringe during the whole measurement period. The automatic setting and control of the collimator and radiation detector with the lead shield which has to be rotated through twice the angle of the crystal. The manual control system consisting of a panel with digi-switches to preset the fringe counter and preselect the next position and a large L C D display for the actual position indication. - The computer control interface for data transfer to and from a P D P l l / 1 0 via two special parallel I / O boards which have to meet the intrinsic requirements of several existing P D P l l programs. Compared with the older version we could reduce the settling time for the crystal rotation by an order of -

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lator the software could be realized fast and comfortably. The spectrometer is operated fully under the control of a P D P l l / 1 0 computer that has been incorporated into the electronical control system. In this way large flexibility has been achieved regarding the measurement program. The data are stored on disquettes and can be transfered to a VAX 780 for further analysis.

magnitude. In addition, the accuracy of the collimator angle was improved by a factor of 10. Much better operational convenience, maintainability and error-free fringe counting were obtained during operational periods of several months. The software for the INTEL 8085 processor was developed on our INTEL-DS246 software development system. By writing the sources in P L / M - 8 0 and debugging the code with the ICE85 emu-

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Energy (keV) Fig. 6. Spectrum of the 114.7 and 115.5 keV 7 lines. The middle part shows the data recorded with the curved crystal spectrometer (CCS) at Jiilich in the first order of reflection with a measuring time of 40 s per point and using the N a l counter as detector. The fwhm is - 134 eV. The resolution is inversely proportional to the order n of reflection if the source width is constant. The lower part of the figure shows the second order spectrum of the 115 keV doublet in larEu measured with minimized width of the source in the new source holder. The photons were detected with the HP Ge detector and the measuring time was 80 s per point. For the (110) plane of quartz the reflection power in the 100 keV region of the second order is - 75% of the first order reflectivity so that the actual measurements are usually performed in the second or higher order of reflection. In the upper part of the figure a high resolution Ge(Li) spectrum of the same energy region is shown for comparison (fwhm - 690 eV).

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3. Measurements and results

m e n t of the source in two directions to minimize the effective source width seen by the spectrometer. For the reduction of the b a c k g r o u n d the source holder was designed to contain as little as possible material that could scatter neutrons towards the spectrometer. Furthermore, the H P G e detector was installed for recording diffracted p h o t o n s with low energies. The spectrum of the 114.7 a n d 115.5 keV lines in fig. 6c shows the very good resolution achieved in the second order of reflection a n d the i m p r o v e m e n t in peak to b a c k g r o u n d [14]. The 275 keV doublet was measured again in the fifth order of reflection, b u t now with a resolution of A E - 1 0 7 eV so that the doublet spacing could be determined to be (121 + 7) eV (see fig. 7). As a second case, parts of the 181Ta(p, 3ny)179W spectrum were studied [15]. The target consisted of a 0.1 m m thick foil. It was b o m b a r d e d with a current of 3 / t A of 26 MeV protons. For normalization of the reaction rate the intensity of the strong 189 keV 7-line was monitored. The K a 1 line of Ta was measured at positiive a n d negative Bragg angle for absolute energy calibration. The data are plotted in fig. 8. The observed reflections show p r o n o u n c e d Lorentzian shapes as expected for the X-rays. The measured fwhm of 55 eV is as expected from folding the natural line width of 47 eV

The first on-line experiment with the crystal spectrometer aimed at the investigation of the 275 keV doublet [12] in 146EH [13]. A target of enriched 147Sm was b o m b a r d e d with a 22 M e V p r o t o n b e a m of - 3 ~A, inducing mainly the (p, 2n) reaction. The measurem e n t of the 114.7 a n d 115.5 keV lines depicted in fig. 6b clearly demonstrates the high resolving power of the spectrometer already in the first order of reflection a n d u n d e r conditions where the source has not even been optimized for best resolution. As the coincidence data suggested [12] a spacing of the c o m p o n e n t s of the 275 keV doublet of less t h a n 200 eV, these transitions were measured in different orders n of reflection up to n = 7. There the reflections became very weak. Because of partial feeding from the isomeric state, data were also recorded with windows on the p r o m p t time peak and between the cyclotron b e a m bursts [13]. In this run a resolution of 210 eV was o b t a i n e d for the p h o t o n s of 275 keV measured in the fifth order of reflection. In order to achieve better resolution, the curvature of the crystal was improved so that the reflection width could be reduced to - 3 arcsec. In addition, a new source holder was built that allowed the remote adjust-

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recorded during measuring periods of 700 s per point using the HP Ge detector and with a 6 ns window set on the cyclotron beam burst. The data points were fitted under the constraint of identical shape parameters for the positive and negative Bragg reflections. The dashed lines and the full lines correspond to the fit values of the individual components and the complex, respectively.

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[16] (corresponding with an angular width of - 7 . 4 arcsec) with the instrumental width of - 5.4 arcsec. The latter yields a fwhm of 185 eV for the second order of reflection of the 189 keV transition in 179W (see fig. 9). The instrumental width is in fact d o m i n a t e d by the source width, which is - 4 . 5 arcsec, because of its thickness of 0.1 mm. For an extremely thin source the

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width of the second order reflection of the 189 keV line could have been as narrow as - 3 arcsec corresponding to - 103 eV. In fig. 10 are depicted reflections of other lines. The 136 keV doublet would have been impossible to resolve with a germanium detector. C o m p a r e d with earlier experiments [4,5,13] the peak to background ratio and the resolution were improved considerably. The energies of a few prominent 3,-transitions in 179W are listed in table 1. For comparison the results of an earlier experiment [4] are also shown. To convert our relative energies to the absolute energy scale and in order to circumvent the problems that seem to be connected with the W K a 1 energy we have used the high precision data of Kessler et al. [18] for the K c h lines of Er, Au and Pb to calculate an average conversion factor from their absolute energies and the corre-

Table 1 Gamma transitions in 179W Present experiment

Ref. [4]

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119.913 189.053 308.951 312.725 63.859 135.775 136.185 137.439 142.129

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Fig. 10. Part of the y-ray spectrum from the ]8]Ta(p, 3n)179W reaction measured with the curved crystal spectrometer in the second order of reflection and recorded with the HP Ge detector and with a time window of 10 ns set on the prompt time peak. The measuring time was - 5 min per point. A "/-transition with 135.02 keV and an intensity similar to that of the 135.8 keV line [4] could not be detected in the present measurement [15]. The arrow marks the position where the line would have been expected. The line widths are ~ 97 eV. With a very narrow source the minimum line width would be around 54 eV in the second order of reflection.

s p o n d i n g wavelengths given by Bearden [19]. We have then normalized our data using the same conversion factor and Bearden's Ta K a I wavelength. The last five lines in table 1 were measured only at negative Bragg angles. For these we have used the Kfl a transition as a secondary standard for internal calibration.

4. S u m m a ~ The curved crystal spectrometer which was previously in use for very precise measurements of -/-ray energies has been set up at one of the b e a m lines of the J U L I C cyclotron. E q u i p p e d with c o m p u t e r control, a H P G e counter for optimal detection of p h o t o n s with lower energies, and a monitor Ge(Li) detector for correction of reaction rate changes, the spectrometer permits very high resolution studies of X-rays and of low-energy 7-lines following charged particle induced reactions. The spectrometer can also be used for measurements of radioactive samples or other off-beam studies. Very recently it has served for studies of X-ray intensity shifts [17].

Acknowledgment We are especially grateful to Mr. K.P. Wieder for his assistance during the measurements.

References [1] V. Metag et al. MP I H-1984-V35 (1984). 12] P.J. Nolan et al., submitted to Nucl. Instr. and Meth. [3] R.M. Lieder, H. J~iger, A. Neskakis, T. Venkova and C. Michel, Nucl. Instr. and Meth. 220 (1984) 363. [4] B.J. Meijer, J. Konijn, B. Klank, J.H. Jett and R.A. Ristinen, Z. Phys. A275 (1975) 79. [5] J. Kern, J.-C1. Dousse, M. Gasser, B. Perny and Ch. Rheme, Proc. 5 Int. Symp. on Capture y Ray Spectroscopy, Knoxville (1984). [6] H.R. Koch, H.G. B~Srner, J.A. Pinston, W.F. Davidson, J. Faudou, R. Roussille and O.W.B. Schult, Nucl. Instr. and Meth. 175 (1980) 401. [7] T. v. Egidy, F. G~Snnenwein and B. Maier, Neutron-Capture Gamma-Ray Spectroscopy and Related Topics 1981, Conference Series Number 62, The Institute of Physics, Bristol and London (1982). [8] G.L. Borchert, W. Scheck and O.W.B. Schult, Nucl. Instr. and Meth. 124 (1975) 107.

G.L. Borchert et al. / Curved crystal spectrometer [9] G.L. Borchert, W. Scheck and K.P. Wieder, Z. Naturforsch. 31a (1975) 274. [10] G.L. Borchert, W. Scheck and O.W.B. Schult, 5th Int. Conf. on Atomic Masses and Fundamental Constants, Paris 1975, Proc. AMC05 (Plenum, New York, London, 1976) p. 42. [11] L. Aldea, W. Br~iutigam, R. Brings, C. Mayer-BSricke, J. Reich and P. Wucherer, Proc. 9th Int. Conf. on Cyclotrons and Their Applications (1981) p. 103. [12] A. Ercam R. Broda, M. Piiparinen, Y. Nagai, R. Pengo and P. Kleinheinz, Z. Phys. A295 (1980) 197. [13] J. Bojowald, G.L. Borchert, A. Ercan, O.W.B. Schult, E. Siefert and K.P. Wieder, IKP Annual Report (1980) p. 162, JiaI-Spez 99 (1981).

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[14] G.L. Borchert, A. Ercan, Th. Rose, O.W.B. Schult and K.P. Wieder, IKP Annual Report (1982) p. 125, Ji~l-Spez 202 (1983). [15] G.L. Borchert, Th. Rose, R. Salziger, O.W.B. Schult and K.P. Wieder, IKP Annual Report (1984) p. 377, Jial-Spez 305 (1985). [16] G.C. Nelson, W. John and B.G. Saunders, Phys. Rev. 187 (1969) 1. [17] Th. Rose, G.L. Borchert and O.W.B. Schult, Z. Naturforsch. 39a (1984) 924. [18] E.G. Kessler Jr., R.D. Deslattes, D. Girard, W. Schwitz, L. Jacobs and O. Rennet, Phys. Rev. A26 (1982) 2696. [19] J.A. Bearden, Rev. Mod. Phys. 31 (1967) 78.