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Photocathodes for BBQ applications
NUCLEAR INSTRUMENTS
AND METHODS
167 (1979) 3 5 9 - 3 6 1 :
©
NORTH-HOLLAND
P U B L I S H I N G CO.
PHOTOCATHODES FOR BBQ APPLICATIONS RICHARD JAMES STAPLETON and ANTHONY GEORGE WRIGHT
EMI Electron Tubes, Bur)" Street, Ruislip, Middlesex, UK HA4 7TA Received 23 August 1979 A direct method for assessing the suitability of photomultipliers for BBQ applications is described, The new rubidium based cathode is shown to give optimum performance in low light level applications•
1. Introduction The conventional methods for collecting light from very large area or volume scintillation counters require many photomultipliers. The cost of the phototubes and the associated lightguides is often prohibitive. The use of BBQ wavelength shifter bars in high energy physics is now becoming well established. This alternative wavelength shifter bar technique is highly economical in the use of photomultipliers. Barish et al.~), for example, have described a 10 m 2 detector which requires only four tubes. The light collection efficiency is at best only a few percent. In addition, the emission spectrum of BBQ light peaks at N 500 nm where the sensitivity of most photocathodes is decreasing rapidly with wavelength. Good resolution or signal to noise performance clearly depends on the degree of match between the photocathode response and the BBQ emission spectrum. Typical photocathode response curves for three photocathode types suitable for use in BBQ applications are shown in fig. 1. The bi-alkali and Sll photocathodes are well known and have been used extensively in large detectors.
q(k)% 2520 '~
' \
•
-.-...\
•
\\X\' ~"\ X "\ " \ -
10 . . . .
5 i !
""4
. . . --. . . . . . .
"..
I
I 350
400
"\
~-50
k
500 Into)
Photocathodes based on rubidium were amongst the earliest in phototubes. Recently, however, new processing techniques have evolved a highly sensitive photocathode which offers enhanced long wavelength response compared with the bi-alkali cathode• Moreover, the noise performance compares favourably with that of the bi-alkali cathode but is an order of magnitude better than that for the typical S11 cathode. 2. Comparison of relative performance A direct means for evaluating the relative performance of various phototubes incorporating the three photocathode types is shown in fig. 2. The scintillator is a thin disc of NE 104 B (50><4 mm), separated by a 1.0mm gap from a light pipe (200xl0><5mm) containing a concentration of 80 mg of BBQ per litre of PVT plastic2). The photomultiplier under test is mounted behind an optical. shutter at a constant distance of 2 mm from one end of the BBQ strip. The strength of the 3H source was chosen to yield -104 photoelectrons s ~ for a typical photocathode. The actual count rate from each photomultiplier tested was taken as a measure of the efficiency of the particular tube for BBQ light. Tubes from the EMI, 12 stage, linear focused series were selected for evaluation. Tube types 9814, 9811 and 9914 (bialkali, Sll and Rb respectively) being fast tubes, are the preferred types for use in accelerator experiments. Also the ability to resolve a single electron peak proved vital in this
\'\ "-..
i
600
~50
~°BBQ 550
Fig. i. Typical spectral response curves: (a) r u b i d i u m ; (b) S I I ; and (c) bi-alkali.
E~baffte Fig. 2. The p h o t o m u l t i p l i e r
I
• ~"NE 3H
test gear.
Shl~fer bar 10:, B
360
R.J.
STAPLETON
application of the photon counting technique. The counting rate obtained from each photomultiplier was determined by integrating the area under the single electron response spectrum as displayed on a multichannel analyser. The background counts, measured by closing the shutter, were subtracted from the initial readings. A typical pulse height distribution is shown in fig. 3. It is possible although not feasible, where large numbers are involved, to calculate the expected yield from a cathode of known spectral sensitivity, r/(2). If the spectral emission function B(2) for BBQ light is known, then the effective quantum efficiency r/ is given by f/ =
;
t/(),) B(2) d2
/f:
B(2)d2.
(2)
The practical advantage of (2) is obvious for grading photomultipliers since only a single quantum efficiency measurement is required. Most manufacturers will provide this but at additional cost. It is therefore desirable to relate ~ to a standard photocathode test parameter, such as the luminous cathode sensitivity, S(/~A lm ~). By definition 3)
S =
I(2) 2r/(2) d2 #Aim- 1
/ *oo
(3)
1.24x680| l(2) V(2)d2 Jo 100 /~
• f * % T /
i "°
/
-,~
,
c
u 50 °
re
/,
B B o
__l I
--I(
e~ >, ~u >
Tin
/j
J
,3 300
~ r/(520 nm).
fo
1-0
(1)
Computation of (1) for the three photocathode response curves of fig. 1 shows that
10 3
A N D A. G. W R I G H T
i
400
500 600 ~, (oral Fig. 4. The area under the l(2),:,r/(Z) curve represents the l u m i n o u s cathode efficiency, S. r/(;.) values were those from fig. l(a). The close agreement between the two curves is noteworthy.
oc
I(2) 2r/(2) d2.
(4)
where 1(2) is the spectral emission in W m ~ from a tungsten lamp at 2856 k, ref. 4, and V(2) is the relative sensitivity of the standard human eye I(2)2r/(2), computed using the curve of fig. l(a), is shown in fig. 4 together with a typical BBQ emission spectrumS). The close agreement between the two curves suggests that S is also an acceptable parameter for BBQ applications. This is verified by plotting the experimental results (count rates) against r/(520 nm) and against S. It is clear from fig. 5(a) and (b) that a linear relationship exists between the photoelectron yield and both parameters S and r/(520 nm). The scatter in the results arises from the uncertainties in the measurement of spectral and luminous sensitivity (about 4-5%), Also, cathodes with the same S value but with different shaped r/(2) curves will not necessarily respond equally to BBQ light. TABLE l Background counts ( > 0 . 2 5 photoelectrons equivalent, 20°C1.
V¸
0.0
• /
•
~ ~°°
05 10 1.5 phofoetecfrons eqmvatenf
2.0
Fig. 3. A typical single electron response curve. The shaded area is a measure o f the sensitivity o f the photomultiplier to BBO light.
Tube type
Range (s
1)
9814 {bi-alk.) 9914 (RbCs) 9811 (Sll)
50-1000 100-2000 3 0 0 - 2 × 104
Typical(s 200 500 5000
1)
PttOFOCAFttODES
FOR BBQ A P P L I C A F I O N S
2O
Rb
Photomultiplier background rate is a parameter which ranges over many orders of magnitude, even for tubes of the same type. The processing and the operating history are important factors which bear on the background performance of an individual tube. The figures quoted in table 1 refer to unselected production samples of about l OOtubes of each type.
I
o $11 • B~a{k ,,~ 1_C
L
÷ - 1~'- oo ÷
o
o x
o
2 ¸
•
c
D
*e~
i
÷
o
5
10
20
15
q(520)
%
20
+Rb {b)
o $11 • BiaLk T 12
+¢9 o O
%
÷ ÷o
×
o I
:.:.,
4
o
361
3. Conclusions This work indicates that the rubidium-based photocatbode offers real advantages over the hialkali and Sll variants by virtue of the good spectral match to the BBQ emission spectrum and its relatively low background count rate. The Sll photocathode is also well suited to BBQ applications but its relatively high background rate makes it unsuitable where light levels are feeble. The bialkali cathode is only 60% efficient compared with the rubidium and SI1 photocathodes although its lower background rate may be important in certain low light level experiments.
i
I
i References
I 25
50
~)S
O0
S(j~ALrn -~} Fig. 5. The experimental count rates as a function of: (a) pl(520nm): and (b) S(/zAIm l). The linear relationships establish that either o[" these parameters is suitable for grading purposes.
ul 2) 3) 4)
B. Barish et al., IEEE, NS25, i (1978) 532. Nuclear Enterprises, Edinburgh. EMI Photomultiplier Catalogue P001/E79, p. 2. M. Pivovonsky and M.R. Nagel, Black belly radiation tabh's IMacMillan, London, 1961) p. 84. ') V. Eckart ct al., Nucl. Instr. and Meth. 155 (1978) 389.
AND METHODS
167 (1979) 3 5 9 - 3 6 1 :
©
NORTH-HOLLAND
P U B L I S H I N G CO.
PHOTOCATHODES FOR BBQ APPLICATIONS RICHARD JAMES STAPLETON and ANTHONY GEORGE WRIGHT
EMI Electron Tubes, Bur)" Street, Ruislip, Middlesex, UK HA4 7TA Received 23 August 1979 A direct method for assessing the suitability of photomultipliers for BBQ applications is described, The new rubidium based cathode is shown to give optimum performance in low light level applications•
1. Introduction The conventional methods for collecting light from very large area or volume scintillation counters require many photomultipliers. The cost of the phototubes and the associated lightguides is often prohibitive. The use of BBQ wavelength shifter bars in high energy physics is now becoming well established. This alternative wavelength shifter bar technique is highly economical in the use of photomultipliers. Barish et al.~), for example, have described a 10 m 2 detector which requires only four tubes. The light collection efficiency is at best only a few percent. In addition, the emission spectrum of BBQ light peaks at N 500 nm where the sensitivity of most photocathodes is decreasing rapidly with wavelength. Good resolution or signal to noise performance clearly depends on the degree of match between the photocathode response and the BBQ emission spectrum. Typical photocathode response curves for three photocathode types suitable for use in BBQ applications are shown in fig. 1. The bi-alkali and Sll photocathodes are well known and have been used extensively in large detectors.
q(k)% 2520 '~
' \
•
-.-...\
•
\\X\' ~"\ X "\ " \ -
10 . . . .
5 i !
""4
. . . --. . . . . . .
"..
I
I 350
400
"\
~-50
k
500 Into)
Photocathodes based on rubidium were amongst the earliest in phototubes. Recently, however, new processing techniques have evolved a highly sensitive photocathode which offers enhanced long wavelength response compared with the bi-alkali cathode• Moreover, the noise performance compares favourably with that of the bi-alkali cathode but is an order of magnitude better than that for the typical S11 cathode. 2. Comparison of relative performance A direct means for evaluating the relative performance of various phototubes incorporating the three photocathode types is shown in fig. 2. The scintillator is a thin disc of NE 104 B (50><4 mm), separated by a 1.0mm gap from a light pipe (200xl0><5mm) containing a concentration of 80 mg of BBQ per litre of PVT plastic2). The photomultiplier under test is mounted behind an optical. shutter at a constant distance of 2 mm from one end of the BBQ strip. The strength of the 3H source was chosen to yield -104 photoelectrons s ~ for a typical photocathode. The actual count rate from each photomultiplier tested was taken as a measure of the efficiency of the particular tube for BBQ light. Tubes from the EMI, 12 stage, linear focused series were selected for evaluation. Tube types 9814, 9811 and 9914 (bialkali, Sll and Rb respectively) being fast tubes, are the preferred types for use in accelerator experiments. Also the ability to resolve a single electron peak proved vital in this
\'\ "-..
i
600
~50
~°BBQ 550
Fig. i. Typical spectral response curves: (a) r u b i d i u m ; (b) S I I ; and (c) bi-alkali.
E~baffte Fig. 2. The p h o t o m u l t i p l i e r
I
• ~"NE 3H
test gear.
Shl~fer bar 10:, B
360
R.J.
STAPLETON
application of the photon counting technique. The counting rate obtained from each photomultiplier was determined by integrating the area under the single electron response spectrum as displayed on a multichannel analyser. The background counts, measured by closing the shutter, were subtracted from the initial readings. A typical pulse height distribution is shown in fig. 3. It is possible although not feasible, where large numbers are involved, to calculate the expected yield from a cathode of known spectral sensitivity, r/(2). If the spectral emission function B(2) for BBQ light is known, then the effective quantum efficiency r/ is given by f/ =
;
t/(),) B(2) d2
/f:
B(2)d2.
(2)
The practical advantage of (2) is obvious for grading photomultipliers since only a single quantum efficiency measurement is required. Most manufacturers will provide this but at additional cost. It is therefore desirable to relate ~ to a standard photocathode test parameter, such as the luminous cathode sensitivity, S(/~A lm ~). By definition 3)
S =
I(2) 2r/(2) d2 #Aim- 1
/ *oo
(3)
1.24x680| l(2) V(2)d2 Jo 100 /~
• f * % T /
i "°
/
-,~
,
c
u 50 °
re
/,
B B o
__l I
--I(
e~ >, ~u >
Tin
/j
J
,3 300
~ r/(520 nm).
fo
1-0
(1)
Computation of (1) for the three photocathode response curves of fig. 1 shows that
10 3
A N D A. G. W R I G H T
i
400
500 600 ~, (oral Fig. 4. The area under the l(2),:,r/(Z) curve represents the l u m i n o u s cathode efficiency, S. r/(;.) values were those from fig. l(a). The close agreement between the two curves is noteworthy.
oc
I(2) 2r/(2) d2.
(4)
where 1(2) is the spectral emission in W m ~ from a tungsten lamp at 2856 k, ref. 4, and V(2) is the relative sensitivity of the standard human eye I(2)2r/(2), computed using the curve of fig. l(a), is shown in fig. 4 together with a typical BBQ emission spectrumS). The close agreement between the two curves suggests that S is also an acceptable parameter for BBQ applications. This is verified by plotting the experimental results (count rates) against r/(520 nm) and against S. It is clear from fig. 5(a) and (b) that a linear relationship exists between the photoelectron yield and both parameters S and r/(520 nm). The scatter in the results arises from the uncertainties in the measurement of spectral and luminous sensitivity (about 4-5%), Also, cathodes with the same S value but with different shaped r/(2) curves will not necessarily respond equally to BBQ light. TABLE l Background counts ( > 0 . 2 5 photoelectrons equivalent, 20°C1.
V¸
0.0
• /
•
~ ~°°
05 10 1.5 phofoetecfrons eqmvatenf
2.0
Fig. 3. A typical single electron response curve. The shaded area is a measure o f the sensitivity o f the photomultiplier to BBO light.
Tube type
Range (s
1)
9814 {bi-alk.) 9914 (RbCs) 9811 (Sll)
50-1000 100-2000 3 0 0 - 2 × 104
Typical(s 200 500 5000
1)
PttOFOCAFttODES
FOR BBQ A P P L I C A F I O N S
2O
Rb
Photomultiplier background rate is a parameter which ranges over many orders of magnitude, even for tubes of the same type. The processing and the operating history are important factors which bear on the background performance of an individual tube. The figures quoted in table 1 refer to unselected production samples of about l OOtubes of each type.
I
o $11 • B~a{k ,,~ 1_C
L
÷ - 1~'- oo ÷
o
o x
o
2 ¸
•
c
D
*e~
i
÷
o
5
10
20
15
q(520)
%
20
+Rb {b)
o $11 • BiaLk T 12
+¢9 o O
%
÷ ÷o
×
o I
:.:.,
4
o
361
3. Conclusions This work indicates that the rubidium-based photocatbode offers real advantages over the hialkali and Sll variants by virtue of the good spectral match to the BBQ emission spectrum and its relatively low background count rate. The Sll photocathode is also well suited to BBQ applications but its relatively high background rate makes it unsuitable where light levels are feeble. The bialkali cathode is only 60% efficient compared with the rubidium and SI1 photocathodes although its lower background rate may be important in certain low light level experiments.
i
I
i References
I 25
50
~)S
O0
S(j~ALrn -~} Fig. 5. The experimental count rates as a function of: (a) pl(520nm): and (b) S(/zAIm l). The linear relationships establish that either o[" these parameters is suitable for grading purposes.
ul 2) 3) 4)
B. Barish et al., IEEE, NS25, i (1978) 532. Nuclear Enterprises, Edinburgh. EMI Photomultiplier Catalogue P001/E79, p. 2. M. Pivovonsky and M.R. Nagel, Black belly radiation tabh's IMacMillan, London, 1961) p. 84. ') V. Eckart ct al., Nucl. Instr. and Meth. 155 (1978) 389.