- Email: [email protected]
A resolution measurement of an inclined-plane sector magnet made with an intrinsic germanium detector
Nuclear Instruments and Methods 193 (1982) 485-488 North-Holland Publishing Company
485
A RESOLUTION MEASUREMENT OF AN INCLINED-PLANE SECTOR MAGNET MADE WITH AN INTRINSIC GERMANIUM DETECTOR J.W. KNOWLES, W.F. MILLS, R.N. KING, R.J. TOONE, H.L. MALM * Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ 1JO
B.O. PICH, S.YEN and T.E. DRAKE Physics Department, University of Toronto, Toronto, Ontario, Canada M5S 1A 7
Received 26 August 1981
The focusing properties of an inclined-plane electron spectrometer have been measured with a narrow-slit and intrinsic-germanium detector. The aberrational width of 3.2 -+1.0 keV obtained from this measurement is in agreement with ray-trace calculations.
The focal properties of an inclined-plane sector magnet can be measured conveniently, independent of many other factors contributing to the total resolution of an electron spectrometer system, if one makes use of the excellent response of an intrinsicgermanium detector [ 1] to monoenergetic electrons. Resolution measurements were made on the bremsstrahlung monchromator [2] shown in fig. l(a). For this device, electrons of energy E i + ~ are incident on foil R. Some of the electrons which lose energy in R, produce bremsstrahlung which is incident on target T. These elecrrons, with kinetic energies between 2 and 4MeV, are focussed by an inclined-plane sector magnet onto a position-sensitive detector, e.g. onto a multi-wire proportional gascounter (MWPC) and an array of plastic scintillators (SA) [3]. Photons or charged particles emitted by the target, following photo-absorption, are detected in coincidence with pulses from the wire counter and scintillators. The energy resolution of this system, for 10.8 MeV incident and 3.5 MeV scattered electrons from a 5.4 × 10 -3 gm/cm 2 foil R, depends on the many factors listed in columns (1) and (2) or table 1. Some of the more important factors, expressed as partial resolution widths are: aberrational broadening of the sector magnet, with a -+1 cm vertical mask in front of the wire counter, which contributes W1 = 5.2 keV; the spatial sensitivity of the wire counter * Now at Aptec Engineering Ltd., Downsview, Toronto, Ontario, Canada. 0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
which contributes W2 -- 8.0 keV; and the 4.5 mg/cm 2 exit window for electrons in the focal plane which contributes, because of multiple scattering, W4 = 7.0 keV. Other factors, such as imperfect focusing Ws, the energy spread W6 because of energy straggling of electrons in the radiator R and the width W7 of the image of the reference slit [2] also contribute significantly. The total resolution width (fwhm) from all these causes is WT = 13.9 keV. This is approximately the width obtained in a resonance fluorescence experiment using the bremsstrahlung monochromator [2]. By comparison, the total width measurement made with the narrow slit S and the germanium detector, shown in fig. l(b) includes only three effects; the aberrational broadening of magnet M through partial width W1, the pulse-height distribution of partial width W2 shown in fig. 2(b) and the energy distribution of partial width W3 of electrons transmitted by slit S. The other partial widths listed in column (1) of table 1, except for the defocus width, are inherently zero in this method of measurement. The defocus width is made negligibly small by correctly positioning the foil R in the direction of the incident beam so that the slit S is in the focal plane. It is assumed that the slit is in the focal plane when the foil R is positioned to make the width of the germanium pulse-height distribution a minimum as shown in fig. 3. The pulse-height spectrum of the 3.5 MeV electrons shown in fig. 2(b) is obtained with a high purity germanium detector 1.3 cm ~ and with a depletion
J.W. Knowle9 et al / Resolution oj it clined-plane s e c t o r , agnet
486
I
E
iEy
I
T
J
E+AE
62 cm
2 F
/
/SA
/
~/'~
I~Fe
F (a)
(b)
(a) ±L~
S
0 8 mm
8.
~
i
IF~
132 cm
Ge :L ra F~I I 3
cm
I T
Fig. 1. (a) The geometry of the bremsstrahlung monochromator. Electrons of energy E i are incident on a thin nickel foil R where they lose energy and produce bremsstrahlung which proceeds to target T. The slowed down electrons of energy Es -+AE are focussed by inclined-plane magnet M, on to an electron detector including a wire counter (MWPC)and an array of plastic scintillators (SA). (b) The geometry for the germanium resolution measurement. Here a 0.8 mm wide 2 cm high lead slit is located on the tbcal plane F and a germanium detector Ge is placed behind the slit, all within the vacuum system. The Ge is attached to the cold finger of cryostat C.
depth of 1.5 m m [1]. The minimum pulse-height width (fwhm) measured iS.WT = 7.7-+ t.0 keV. In order to obtain from WT the partial width W1 caused by the aberrational effects of magnet M the widths W2 and W3, caused by the germanium pulse-height distribution and by the energy distribution of the incident electrons, respectively, must be subtracted. Partial width (fwhm) W2 = 4.1 -+ 1.0 keV * is obtained from the pulse-height distribution of the 1.17 and 1.33 MeV T-rays of 6°Co measured in the germa-
* Assumed constant between 1.1 and 3.5 MeV.
nium detector and shown in fig. 2(a). Partial width W3 is calculated knowing, for magnet M, the dispersion Ds = 1.13 -+ 0.01 [2], the perpendicular distance, 51 cm, between the slit S and the electron beam incident on the foil R, the width 0.8 + 0.1 mm of slit S and the width 1.0 + 0.1 mm of foil R. For this calculation it is assumed that electrons are scattered from all parts of the foil R with the same probability. The calculated energy distribution of electrons transmitted by slit S is found to be nearly Gaussian in shape with a width (fwhm) W3 = 5.7 + 1.0 keV. Because both W2 and W3 are obtained from distributions which can be approximated by Gaussians, it
J.W. Knowles et al. / Resolution o f inclined-plane sector magnet
487
500
."
oo z
I
I
25t
0 L2
(b) 0
,300
3350
3400
34'50
35'00
5550keV
500
03 F-
Z
250
0
"-"-.- ....,
--'-.-%'.--
0
I
IiO0
-
&..,...
I
I
115o
1200
(a) .
~
| "
....
~
1250
~]
""~"~
.
1300
.
.
.
.
kbV
Fig. 2. The germanium pulse-height spectra of (a), the 1.17 and 1.33 MeV ~/-rays of 6°Co used for calibration and (b) the E s 3.5 MeV electrons transmitted by the lead slit of fig. l(b).
is sufficiently accurate to subtract in quadrature IV2 and IV3 f r o m IVT to o b t a i n IVx. This m e a s u r e m e n t o f the aberrational b r o a d e n i n g ( f w h m ) , W1 = 3.2 + 1.0
k e V , agrees within its limits o f error w i t h the estim a t e d value Ivx = 3.7 k e V o b t a i n e d f r o m ray-trace calculations for m a g n e t M [2]. These calculations
Table 1 Partial resolution widths of 3.5 MeV electrons in an inclined-plane sector magnet Partial width (fwhm)
Resonance fluorescence a) (keV)
Slit + intrinsic germanium detector (keV) 3.7 4.1±1.0 5.7±1.0 0 0 0 0
Aberrational width Detector (pulse-height width) Dispersion width Scattering in 4.5 mg/cm 2 exit window Defocus width Energy loss by electron straggling in radiator Reference slit width
Wl 14/2 W3 W4 W5 W6 W7
5.2 8.0 0.2 7.0 4.0 3.2 4.0
Total width (fwhm)
Wv
13.9
7.9 b)
Measured width (fwhm)
WT
14.1
7.7±0.7keV
a) See ref. 2 for the estimate of partial and total widths in the resonance fluorescence measurement. b) For the germanium measurement, 1¥T is obtained by summing the partial widths in quadrature.
488
J.W. Knowles et al, / Resolution eftinclined-plane sector magnet
t3,--
show that W~ is smaller than in the resonance fluorescence experiment, columm (2) table 1, because the vertical divergence of the electrons scattered by the foil R and detected by the germanium, after passage through slit S, is limited to a vertical scattering angle of -+2.7° by the size 1.3 cm ~ of the gemlanium,
/
This research was made possible by the co-operation of the University of Illinois staff and by the development and operation of MUSL-2 and its experimental areas supported by the National Science Foundation.
References 9--
8--
7--
I
I
I
15
16
17
___
I __ 18
l 19
DISTANCE OF FOIL R FROMMAGNETFACE cm
Fig. 3. The pulse-height width (fwhm) of the 3.5 MeV electrons transmitted by the lead slit as a funeiton of the distance of the foil R from the magnet face.
[1] R.H. Pehl, R.C. Cordi and F.S. Goulding, 1971 Nuclear Science Symposium, in IEEE Trans. Nucl. Sci. NS-19, no. 1 (1972) 265. [2] J.W. Knowles, W.F. Mills, R.N. King, G.E. Lee-Whiting, S. Yen, B.O. Pich, J.C. Kim, T.E. Drake, L.S. Cardman and R.L. Gulbranson, Nucl. Instr. and Meth. 193 (1982) 463. [3] B.O. Pich, S. Yen, T.E. Drake, J.W. Knowtes and R.N. King, to be published.
485
A RESOLUTION MEASUREMENT OF AN INCLINED-PLANE SECTOR MAGNET MADE WITH AN INTRINSIC GERMANIUM DETECTOR J.W. KNOWLES, W.F. MILLS, R.N. KING, R.J. TOONE, H.L. MALM * Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ 1JO
B.O. PICH, S.YEN and T.E. DRAKE Physics Department, University of Toronto, Toronto, Ontario, Canada M5S 1A 7
Received 26 August 1981
The focusing properties of an inclined-plane electron spectrometer have been measured with a narrow-slit and intrinsic-germanium detector. The aberrational width of 3.2 -+1.0 keV obtained from this measurement is in agreement with ray-trace calculations.
The focal properties of an inclined-plane sector magnet can be measured conveniently, independent of many other factors contributing to the total resolution of an electron spectrometer system, if one makes use of the excellent response of an intrinsicgermanium detector [ 1] to monoenergetic electrons. Resolution measurements were made on the bremsstrahlung monchromator [2] shown in fig. l(a). For this device, electrons of energy E i + ~ are incident on foil R. Some of the electrons which lose energy in R, produce bremsstrahlung which is incident on target T. These elecrrons, with kinetic energies between 2 and 4MeV, are focussed by an inclined-plane sector magnet onto a position-sensitive detector, e.g. onto a multi-wire proportional gascounter (MWPC) and an array of plastic scintillators (SA) [3]. Photons or charged particles emitted by the target, following photo-absorption, are detected in coincidence with pulses from the wire counter and scintillators. The energy resolution of this system, for 10.8 MeV incident and 3.5 MeV scattered electrons from a 5.4 × 10 -3 gm/cm 2 foil R, depends on the many factors listed in columns (1) and (2) or table 1. Some of the more important factors, expressed as partial resolution widths are: aberrational broadening of the sector magnet, with a -+1 cm vertical mask in front of the wire counter, which contributes W1 = 5.2 keV; the spatial sensitivity of the wire counter * Now at Aptec Engineering Ltd., Downsview, Toronto, Ontario, Canada. 0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
which contributes W2 -- 8.0 keV; and the 4.5 mg/cm 2 exit window for electrons in the focal plane which contributes, because of multiple scattering, W4 = 7.0 keV. Other factors, such as imperfect focusing Ws, the energy spread W6 because of energy straggling of electrons in the radiator R and the width W7 of the image of the reference slit [2] also contribute significantly. The total resolution width (fwhm) from all these causes is WT = 13.9 keV. This is approximately the width obtained in a resonance fluorescence experiment using the bremsstrahlung monochromator [2]. By comparison, the total width measurement made with the narrow slit S and the germanium detector, shown in fig. l(b) includes only three effects; the aberrational broadening of magnet M through partial width W1, the pulse-height distribution of partial width W2 shown in fig. 2(b) and the energy distribution of partial width W3 of electrons transmitted by slit S. The other partial widths listed in column (1) of table 1, except for the defocus width, are inherently zero in this method of measurement. The defocus width is made negligibly small by correctly positioning the foil R in the direction of the incident beam so that the slit S is in the focal plane. It is assumed that the slit is in the focal plane when the foil R is positioned to make the width of the germanium pulse-height distribution a minimum as shown in fig. 3. The pulse-height spectrum of the 3.5 MeV electrons shown in fig. 2(b) is obtained with a high purity germanium detector 1.3 cm ~ and with a depletion
J.W. Knowle9 et al / Resolution oj it clined-plane s e c t o r , agnet
486
I
E
iEy
I
T
J
E+AE
62 cm
2 F
/
/SA
/
~/'~
I~Fe
F (a)
(b)
(a) ±L~
S
0 8 mm
8.
~
i
IF~
132 cm
Ge :L ra F~I I 3
cm
I T
Fig. 1. (a) The geometry of the bremsstrahlung monochromator. Electrons of energy E i are incident on a thin nickel foil R where they lose energy and produce bremsstrahlung which proceeds to target T. The slowed down electrons of energy Es -+AE are focussed by inclined-plane magnet M, on to an electron detector including a wire counter (MWPC)and an array of plastic scintillators (SA). (b) The geometry for the germanium resolution measurement. Here a 0.8 mm wide 2 cm high lead slit is located on the tbcal plane F and a germanium detector Ge is placed behind the slit, all within the vacuum system. The Ge is attached to the cold finger of cryostat C.
depth of 1.5 m m [1]. The minimum pulse-height width (fwhm) measured iS.WT = 7.7-+ t.0 keV. In order to obtain from WT the partial width W1 caused by the aberrational effects of magnet M the widths W2 and W3, caused by the germanium pulse-height distribution and by the energy distribution of the incident electrons, respectively, must be subtracted. Partial width (fwhm) W2 = 4.1 -+ 1.0 keV * is obtained from the pulse-height distribution of the 1.17 and 1.33 MeV T-rays of 6°Co measured in the germa-
* Assumed constant between 1.1 and 3.5 MeV.
nium detector and shown in fig. 2(a). Partial width W3 is calculated knowing, for magnet M, the dispersion Ds = 1.13 -+ 0.01 [2], the perpendicular distance, 51 cm, between the slit S and the electron beam incident on the foil R, the width 0.8 + 0.1 mm of slit S and the width 1.0 + 0.1 mm of foil R. For this calculation it is assumed that electrons are scattered from all parts of the foil R with the same probability. The calculated energy distribution of electrons transmitted by slit S is found to be nearly Gaussian in shape with a width (fwhm) W3 = 5.7 + 1.0 keV. Because both W2 and W3 are obtained from distributions which can be approximated by Gaussians, it
J.W. Knowles et al. / Resolution o f inclined-plane sector magnet
487
500
."
oo z
I
I
25t
0 L2
(b) 0
,300
3350
3400
34'50
35'00
5550keV
500
03 F-
Z
250
0
"-"-.- ....,
--'-.-%'.--
0
I
IiO0
-
&..,...
I
I
115o
1200
(a) .
~
| "
....
~
1250
~]
""~"~
.
1300
.
.
.
.
kbV
Fig. 2. The germanium pulse-height spectra of (a), the 1.17 and 1.33 MeV ~/-rays of 6°Co used for calibration and (b) the E s 3.5 MeV electrons transmitted by the lead slit of fig. l(b).
is sufficiently accurate to subtract in quadrature IV2 and IV3 f r o m IVT to o b t a i n IVx. This m e a s u r e m e n t o f the aberrational b r o a d e n i n g ( f w h m ) , W1 = 3.2 + 1.0
k e V , agrees within its limits o f error w i t h the estim a t e d value Ivx = 3.7 k e V o b t a i n e d f r o m ray-trace calculations for m a g n e t M [2]. These calculations
Table 1 Partial resolution widths of 3.5 MeV electrons in an inclined-plane sector magnet Partial width (fwhm)
Resonance fluorescence a) (keV)
Slit + intrinsic germanium detector (keV) 3.7 4.1±1.0 5.7±1.0 0 0 0 0
Aberrational width Detector (pulse-height width) Dispersion width Scattering in 4.5 mg/cm 2 exit window Defocus width Energy loss by electron straggling in radiator Reference slit width
Wl 14/2 W3 W4 W5 W6 W7
5.2 8.0 0.2 7.0 4.0 3.2 4.0
Total width (fwhm)
Wv
13.9
7.9 b)
Measured width (fwhm)
WT
14.1
7.7±0.7keV
a) See ref. 2 for the estimate of partial and total widths in the resonance fluorescence measurement. b) For the germanium measurement, 1¥T is obtained by summing the partial widths in quadrature.
488
J.W. Knowles et al, / Resolution eftinclined-plane sector magnet
t3,--
show that W~ is smaller than in the resonance fluorescence experiment, columm (2) table 1, because the vertical divergence of the electrons scattered by the foil R and detected by the germanium, after passage through slit S, is limited to a vertical scattering angle of -+2.7° by the size 1.3 cm ~ of the gemlanium,
/
This research was made possible by the co-operation of the University of Illinois staff and by the development and operation of MUSL-2 and its experimental areas supported by the National Science Foundation.
References 9--
8--
7--
I
I
I
15
16
17
___
I __ 18
l 19
DISTANCE OF FOIL R FROMMAGNETFACE cm
Fig. 3. The pulse-height width (fwhm) of the 3.5 MeV electrons transmitted by the lead slit as a funeiton of the distance of the foil R from the magnet face.
[1] R.H. Pehl, R.C. Cordi and F.S. Goulding, 1971 Nuclear Science Symposium, in IEEE Trans. Nucl. Sci. NS-19, no. 1 (1972) 265. [2] J.W. Knowles, W.F. Mills, R.N. King, G.E. Lee-Whiting, S. Yen, B.O. Pich, J.C. Kim, T.E. Drake, L.S. Cardman and R.L. Gulbranson, Nucl. Instr. and Meth. 193 (1982) 463. [3] B.O. Pich, S. Yen, T.E. Drake, J.W. Knowtes and R.N. King, to be published.