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A bent crystal spectrometer geometry with a shadow source
NUCLEAR
INSTRUMENTS
AND
METHODS
14I
(1977) 3 9 1 - 3 9 2 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
A BENT CRYSTAL S P E C T R O M E T E R G E O M E T R Y W I T H A S H A D O W SOURCE* W I L L I A M W A D E SAPP, Jr.
Laboratory for High Energy Physics, Swiss Federal Institute o f Technology, Ziirich, c/o SIN, C H 5234 villigen, Switzerland Received 2 November 1976 We describe a new " g e o m e t r y " for bent crystal spectrometers which provides another variable for optimizing their use at accelerators, it has several advantages over existing geometries and no new disadvantages.
At the SIN meson factory and at the Leningrad 1 GeV synchrotron precision measurements of low energy ( E < 1 0 0 k e V ) muonic and pionic X-rays are being performed with bent crystal spectrometers1'2). Similar measurements are planned to begin soon at LAMPF3). These experiments are predated by the original work of Shafer4), who used a crystal spectrometer of DuMond geometry to determine the pion mass. In all of these cases the experimenters have chosen a spectrometer design, of either DuMond or Cauchois type, which they felt provided a good match to the beam properties at their respective accelerators. Inevitably compromises must be made for physical or technical reasons which often result in a less than optimum experiment from the point of view of data accumulation rates, background rates, comparison with standard sources, etc. We present here a modified geometry that, to our knowledge, is new and offers several advantages over the standard D u M o n d and Cauchois geometries at accelerator facilities. The added flexibility may also prove useful in conventional measurements with radioactive sources and at reactors where the techn;cal constraints are generally not so severe as at accelerator installations. The principle of this new technique is schematically indicated in fig. l a. It effectively enables a spectrometer of DuMond geometry to function with an extended Cauchois-type source. At the focus of the curved crystal is a high density absorber whose dimensions are, for example, the same as that of a source which one would design for a normal DuMond spectrometer. At some distance beyond the absorber is the source, usually an extended one. Rotating the crystal with the absorber fixed one observes a reduction in the detector counting rate as the focus of the spectrometer (for the energy in question) sweeps over the absorber.
* This work was partially supported by SIN.
Fig. I b shows the angle spectrum for an ideal spectrometer using an absorber with a rectangular profile. One could, of course, fix the crystal and move the absorber. A straightforward extension of this concept is the use of more than one absorber, appropriately spaced, to measure more than one energy simultaneously. On the other hand, if only one absorber is used it would be
o
SPECTROMETER
I I ]
(a)
I
el
e2
(b) Fig. I. (a) Use o f absorber (A) at the focus of the curved crystal (C) in a normal D u M o n d spectrometer permits the use of an extended source (S). The counting rate N v measured by detector (D) as a function of angle (0) would be as shown in (b) for the case of an ideal absorber with rectangular profile. 01 and 02 are both determined by the energy of the g a m m a rays emitted from the source. 01 is relatively poorly defined because the source is out of focus; 02 is well defined.
392
w . w . SAPP
geometrically equivalent to use the classical slit arrangement will generally give a better signal-to-noise ratio than that obtained with a single absorber because much of the direct radiation will be absorbed in the jaws of the slit. An exception to this general remark occurs if the relatively massive slit itself is a source of significant background, as could be the case if it is also irradiated by the accelerator beam. We list briefly (and not exhaustively) what we, as accelerator users, perceive as advantages for this geometry. With respect to the normal DuMond geometry: a) Increased counting rate for pionic X-rays if one employs a proton/pion target technique as described by Maruschenko et al.2). This is because the pion stopping density for thin targets increases with target thickness. b) Relative insensitivity to source motion and position. If the crystal reflectivity and lattice spacing were constant over the entire crystal, then, of course, the system would be completely insensitive to these effects. This insensitivity permits, for example, the trivial removal of an arbitrarily intense calibration source and its associated background. c) Flexibility of target (source) design together with the possible elimination of the contribution of finite source depth to the system resolution. With respect to the normal Cauchois geometry: a) Increased counting rates in general because of added design flexibility in which crystal size is no longer coupled to beam size. In particular, if the minimum beam dimensions (corresponding to the maximum specific induced activity) are smaller than those of the crystal, then the absorber (or slit) geometry will give a
counting rate greater than that of the Cauchois geometry (but less than that of the DuM ond geometry). b) Reduction of beam associated background which comes from the source vicinity in that the shielding can be near the source and need have only a very small opening for the slit or about the absorber(s). We are currently using this techn;que to provide precision calibration and target location information with the SIN bent crystal spectrometer (DuMond) °) which is installed at the SIN superconducting muon channel. The current experiment of muonic X-ray measurements is being conducted by a University of F r i b o u r g - E T H Z collaboration. I would like to thank my colleagues for many stimulating and helpful discussions. Suggestions and comments from W. Beer, H.J. Leisi, J. Kern and B. Aas were particularly useful and especially appreciated. References ~) Experimentiervorschlag fi.ir das SIN Nr. R-71-01.1, H . J . Leisi (Spokesman), SIN Jahresbericht (1975); R. Eichler, B. Aas, W. Beer, 1. Beltrami, P. Ebersold, Th. v. Ledebur, H . J . Leisi, W . W . Sapp, J. Kern and W. Schwitz, to be published. 2) V. 1. M a r u s h e n k o , A. F. Mesentsev, A . A . Petrunin, S . G . Scovniakov and A. I. Smirnov, J E T P Lett. 23 (1976) 80 (in Russian). 3) D. L. Lu (Spokesman), L A M P F Research Proposal Nr. 155. 4) R. E. Sharer, Phys, Rev. 163 (1967) 1451. 2) A. E. Sandstr6m, Encyclopedia of physics (Springer Verlag, Berlin 1957) vol. 30, pp. 78-245. 6) O. Piller, W. Beer and J. Kern, Nucl. Instr. and Meth. 107 (1973) 61; W. Schwitz and J. Kern, Proc. 2nd Int. Syrup. on Neutron capture gamma-ray spectroscopy, Petten (1974) p. 697.
INSTRUMENTS
AND
METHODS
14I
(1977) 3 9 1 - 3 9 2 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
A BENT CRYSTAL S P E C T R O M E T E R G E O M E T R Y W I T H A S H A D O W SOURCE* W I L L I A M W A D E SAPP, Jr.
Laboratory for High Energy Physics, Swiss Federal Institute o f Technology, Ziirich, c/o SIN, C H 5234 villigen, Switzerland Received 2 November 1976 We describe a new " g e o m e t r y " for bent crystal spectrometers which provides another variable for optimizing their use at accelerators, it has several advantages over existing geometries and no new disadvantages.
At the SIN meson factory and at the Leningrad 1 GeV synchrotron precision measurements of low energy ( E < 1 0 0 k e V ) muonic and pionic X-rays are being performed with bent crystal spectrometers1'2). Similar measurements are planned to begin soon at LAMPF3). These experiments are predated by the original work of Shafer4), who used a crystal spectrometer of DuMond geometry to determine the pion mass. In all of these cases the experimenters have chosen a spectrometer design, of either DuMond or Cauchois type, which they felt provided a good match to the beam properties at their respective accelerators. Inevitably compromises must be made for physical or technical reasons which often result in a less than optimum experiment from the point of view of data accumulation rates, background rates, comparison with standard sources, etc. We present here a modified geometry that, to our knowledge, is new and offers several advantages over the standard D u M o n d and Cauchois geometries at accelerator facilities. The added flexibility may also prove useful in conventional measurements with radioactive sources and at reactors where the techn;cal constraints are generally not so severe as at accelerator installations. The principle of this new technique is schematically indicated in fig. l a. It effectively enables a spectrometer of DuMond geometry to function with an extended Cauchois-type source. At the focus of the curved crystal is a high density absorber whose dimensions are, for example, the same as that of a source which one would design for a normal DuMond spectrometer. At some distance beyond the absorber is the source, usually an extended one. Rotating the crystal with the absorber fixed one observes a reduction in the detector counting rate as the focus of the spectrometer (for the energy in question) sweeps over the absorber.
* This work was partially supported by SIN.
Fig. I b shows the angle spectrum for an ideal spectrometer using an absorber with a rectangular profile. One could, of course, fix the crystal and move the absorber. A straightforward extension of this concept is the use of more than one absorber, appropriately spaced, to measure more than one energy simultaneously. On the other hand, if only one absorber is used it would be
o
SPECTROMETER
I I ]
(a)
I
el
e2
(b) Fig. I. (a) Use o f absorber (A) at the focus of the curved crystal (C) in a normal D u M o n d spectrometer permits the use of an extended source (S). The counting rate N v measured by detector (D) as a function of angle (0) would be as shown in (b) for the case of an ideal absorber with rectangular profile. 01 and 02 are both determined by the energy of the g a m m a rays emitted from the source. 01 is relatively poorly defined because the source is out of focus; 02 is well defined.
392
w . w . SAPP
geometrically equivalent to use the classical slit arrangement will generally give a better signal-to-noise ratio than that obtained with a single absorber because much of the direct radiation will be absorbed in the jaws of the slit. An exception to this general remark occurs if the relatively massive slit itself is a source of significant background, as could be the case if it is also irradiated by the accelerator beam. We list briefly (and not exhaustively) what we, as accelerator users, perceive as advantages for this geometry. With respect to the normal DuMond geometry: a) Increased counting rate for pionic X-rays if one employs a proton/pion target technique as described by Maruschenko et al.2). This is because the pion stopping density for thin targets increases with target thickness. b) Relative insensitivity to source motion and position. If the crystal reflectivity and lattice spacing were constant over the entire crystal, then, of course, the system would be completely insensitive to these effects. This insensitivity permits, for example, the trivial removal of an arbitrarily intense calibration source and its associated background. c) Flexibility of target (source) design together with the possible elimination of the contribution of finite source depth to the system resolution. With respect to the normal Cauchois geometry: a) Increased counting rates in general because of added design flexibility in which crystal size is no longer coupled to beam size. In particular, if the minimum beam dimensions (corresponding to the maximum specific induced activity) are smaller than those of the crystal, then the absorber (or slit) geometry will give a
counting rate greater than that of the Cauchois geometry (but less than that of the DuM ond geometry). b) Reduction of beam associated background which comes from the source vicinity in that the shielding can be near the source and need have only a very small opening for the slit or about the absorber(s). We are currently using this techn;que to provide precision calibration and target location information with the SIN bent crystal spectrometer (DuMond) °) which is installed at the SIN superconducting muon channel. The current experiment of muonic X-ray measurements is being conducted by a University of F r i b o u r g - E T H Z collaboration. I would like to thank my colleagues for many stimulating and helpful discussions. Suggestions and comments from W. Beer, H.J. Leisi, J. Kern and B. Aas were particularly useful and especially appreciated. References ~) Experimentiervorschlag fi.ir das SIN Nr. R-71-01.1, H . J . Leisi (Spokesman), SIN Jahresbericht (1975); R. Eichler, B. Aas, W. Beer, 1. Beltrami, P. Ebersold, Th. v. Ledebur, H . J . Leisi, W . W . Sapp, J. Kern and W. Schwitz, to be published. 2) V. 1. M a r u s h e n k o , A. F. Mesentsev, A . A . Petrunin, S . G . Scovniakov and A. I. Smirnov, J E T P Lett. 23 (1976) 80 (in Russian). 3) D. L. Lu (Spokesman), L A M P F Research Proposal Nr. 155. 4) R. E. Sharer, Phys, Rev. 163 (1967) 1451. 2) A. E. Sandstr6m, Encyclopedia of physics (Springer Verlag, Berlin 1957) vol. 30, pp. 78-245. 6) O. Piller, W. Beer and J. Kern, Nucl. Instr. and Meth. 107 (1973) 61; W. Schwitz and J. Kern, Proc. 2nd Int. Syrup. on Neutron capture gamma-ray spectroscopy, Petten (1974) p. 697.