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Time-zero detector based on microchannel plates and a
friable dielectric emitter
S.M. Lukyanov, M. Lewitowicz, Yu.E. Penionzhkevich, G.G. Chubarian, D.
Bazin, D. Guillemaud-Mueller, A.C. Mueller, M.G. Saint-Laurent
To cite this version:
S.M. Lukyanov, M. Lewitowicz, Yu.E. Penionzhkevich, G.G. Chubarian, D. Bazin, et al.. Time-zero
detector based on microchannel plates and a friable dielectric emitter. [Research Report] GANIL.
1986, pp.1-12. �in2p3-00833813�
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TIME-ZERO DETECTOR BASED ON MICROCHANNEL \
PLATES AND A FRIABLE DIELECTRIC EMITTER A
by
S.M. Lukyanov
1), M. Lewitowicz
2), Yu.E.
Penionzhke-vich
1), C.G. Chubarian
3), D. Bazin"), D.
Guillemaud-Mueller"), A.C. Guillemaud-Mueller"), M.G. Saint-Laurent").
1
)
JINR, Dubna, USSR
2
) Polish Atomic Agency, Warsaw, Poland
3
) Yerevan Institute of Physics, Yerevan, USSR
*) GANIL, Caen, France
S.M. Lukyanov , M. Lewitowicz ' , Yu.E. Penionzhkevich ,
C.G. Chubarian ' , D. Bazin , D. GuillemaudMueller ' , A.C. Mueller ' ,
•« >
M.G. Saint-Laurent ' .
TIME-ZERO DETECTOR BASED ON'MICROCHANNEL PLATES AND A FRIABLE
DIELECTRIC EMITTER
1) JINR, Dubna, USSR
2) Polish Atomic Agency, Warsaw, Poland
3) Yerevan Institute of Physics, Yerevan, USSR
r
I - INTRODUCTION
In the recent years the t i m e o f f l i g h t technique was widely used to determine the mass of heavyion reaction products. One of the reasons f o r t h a t i s the p o s s i b i l i t y o f using the transmission detectors based on microchannel plates (MCP) f o r measuring the t i m e o f f l i g h t of passing p a r t i c l e s '1
" ' " ' . The main advantages of such detectors are a high time r e s o l u t i o n ( i n t r i n s i c time r e s o l u t i o n of about 70100 p s ) , the r e l i a b i l i t y and simple design.
A MCP i s used i n timezero detectors as a f a s t m u l t i p l i e r of secondary electrons, emitted by charged p a r t i c l e s passing through a f o i l and then isochronously c o l l e c t e d w i t h a m a g n e t i c ' ,1
' or e l e c t r o s t a t i c '2
' systems on the MCP's mounted i n a chevron c o n f i g u r a t i o n !
The t y p i c a l m u l t i p l i c a t i o n f a c t o r f o r a c o n f i g u r a t i o n w i t h two MCP i s 1 06
1 07 .
For the measurements of f i s s i o n fragments or f i s s i o n l i k e products ( w i t h an energy ~ 2 MeV/n e m i t t e r s ) from t h i n (1030 (ig/cm^) organic or carbon f o i l s give an e f f i c i e n c y of about 100 %. For highenergy r e a c t i o n p r o d u c t s , however, these e m i t t e r s are inadaquate because o f the very low (<1) secondary e l e c t r o n emission c o e f f i c i e n t '6
' . For example, f o r e w i t h an energy o f 7.5 MeV/n the e f f i c i e n c y o f d e t e c t i o n was only a few percent f o r a t r 1 n i t r o c e l l u l o s e e m i t t e r '6
' .
One o f the most e f f e c t i v e and simplest way to increase the d e t e c t i o n e f f i c i e n c y i s , In our view, to use e m i t t e r s w i t h c o n t r o l o f secondary
n a/ e l e c t r o n emission (CSEE) by means o f f r i a b l e d i e l e c t r i c s ' .
The CSSE 1s the process of e l e c t r o n emission from the l a y e r of a f r i a b l e d i e l e c t r i c i n the strong e l e c t r i c f i e l d produced between two electrodes placed on both sides of the l a y e r .
The f r i a b l e d i e l e c t r i c layers are produced by evaporation in a vacuum of C s l , KC1 or LiF sal ta onto a support which is used, at the same time as one of the e l e c t r o d e s . The evaporation i s provided in an i n e r t gas
at a pressure of several t o r r s , whereas layers of magnesium oxide ) are
obtained by d e f l a g r a t i o n of p o l y c r y s t a l l me magnesium in t h i n a i r and by d e p o s i t i n g i t s oxide onto a support. Depending on the evaporation conditions (the d e f l a g r a t i o n temperature, the distance between magnesium and c o l l e c t i n g supports, a i r pressure, deposition t i m e , etc) i t is possible to obtain
layers w i t h a thickness from 30 ^g/cm2
t o several mg/cm2
w i t h a r e l a t i v e
density p/p. ranging from 0.3 to serveral percent (p<, being the density of a
MgO monocrystal ) . As a r e s u l t of t h a t evaporation process, a f r i a b l e d i e l e c t r i c e m i t t e r w i t h the channel s t r u c t u r e presented in f i g . 1 , i s produced. The i n v e s t i g a t i o n s of CSEE show t h a t f o r f r i a b l e d i e l e c t r i c s the c o e f f i c i e n t of secondary e l e c t r o n emission i n an e l e c t r i c f i e l d o f E > 3XI01
*
Y/cm i s more than 102
times greater than t h a t f o r normal s o l i d s ' *7
' . This f a c t can be explained as f o l l o w s : the emission of electrons occurs from the e n t i r e volume o f a f r i a b l e d i e l e c t r i c and then t h e i r number i s m u l t i p l i e d i n t h e i n t r i n s i c channels i n the s t r o n g e l e c t r i c f i e l d . In such an I n t e r p r e t a t i o n t h e CSEE process i s v e r y s i m i l a r t o the e l e c t r o n m u l t i p l i c a t i o n i n a M C P/ 9 /
.
I I DESCRIPTION OF THE TIMEZERO OETECTOR
The p r i n c i p l e of operation and design features o f our detector are analogous t o those presented i n r e f . '2
' . F i g . 2 shows a schematic view of the d e t e c t o r . The detector consists of f i v e parts : a CSEE e m i t t e r w i t h a MgO l a y e r , an a c c e l e r a t i o n harp, an e l e c t r o s t a t i c m i r r o r , two MCP's i n a chevron c o n f i g u r a t i o n and a f l a t anode. A MgO d i e l e c t r i c has been chosen because o f i t s high nonhygroscopic p r o p e r t i e s ( i n c o n t r a s t t o other
d i e l e c t r i c s such as C s l , '<C1 and Li F ) and, at the same time, of i t s very good e l e c t r o n emission c h a r a c t e r i s t i c s . A layer of MgO was evaporated onto a nickel g r i d with a t h i n (~ 10 ug/cin2) t r i n i t r o c e l l u l o s e f o i l as a backing. In order to obtain a high m u l t i p l i c a t i o n c o e f f i c i e n t at an a c c e l e r a t i o n of emitted electrons to an energy of about I XeV, an e l e c t r i c f i e l d o f 5 > 10" V/cm i s applied between the g r i d s u p p o r t of MgO and the a c c e l e r a t i n g harp. A f t e r bending i n the e l e c t r o s t a t i c m i r r o r through 90 the e l e c t r o n s were c o l l e c t e d on the MCP's where t h e i r number i s m u l t i p l i e d by 1 06
1 07 times. Output pulses are obtained from a f l a t anode placed under the MCP's. The main c h a r a c t e r i s t i c s of our detector are presented un table 1 .
«
I I I EXPERIMENTAL TESTS WITH A HEAVYION BEAM 1) Experimental setupA t e s t o f the d e t e c t o r has been performed w i t h a heavyion beam from the GANIL a c c e l e r a t o r . 1S()8+ io ns w i t h an energy of 50 MeY/n bombarded a gold t a r g e t (thickness ~ 100 mg/cm2
). The r e a c t i o n products were selected by the LISE magnetic s p e c t r o m e t e r '1 0 /
. The timezero detector was mounted a f t e r the seconu d i p o l e magnet o f the spectrometer ( F i g . 3 ) . At about 6 m downstream a 1 mm t h i c k t£ semiconductor detector was placed at the achromatic refocusing p o i n t of the spectrometer. In order to sustain a not too high counting r a t e f o r the AE detector the spectrometer was set to a magnetic r i g i d i t y corresponding to the transmission o f ( h e l i u m l i k e ) I S O6
* . The spectrometer was U n i t e d t o a momentum acceptance of ap/p = 0.2 %. The output pulse from the timezero detector was d i r e c t e l y fed to a 7174 Enertec constant f r a c t i o n d i s c r i m i n a t o r (CFD). The time signal derived from the aE d e t e c t o r by means o f a f a s t p r e a m p l i f i e r and another constant f r a c t i o n d i s c r i m i n a t o r served as a START i n p u t f o r a 566 ORTEC t i m e t o a m p l i t u d e converter (TAC) which was stopped by the delayed t i m i n g signal of the
time-zero detector.
The start and stop input had been reversed in order to not untimely
trigger the TAC on background pulses from the MCPS. The output p,:l ses from
the TAC were analysed by a SI LENA (Cato) multichannel analyser.
2) Results
The e f f i c i e n c y of the detector depends of course s t r o n g l y on the d i s c r i m i n a t i o n t h r e s h o l d . For the actual oxygen beam, representing the "worst case" of a l i g h t ion of high energy ( i . e . small energy l o s s ) , an e f f i c i e n c y of 70 % was found f o r a d i s c r i m i n a t o r threshold of 70 mV, decreasing to 50 " f o r a s e t t i n g o f 150 mV. «
The r e s u l t of the time r e s o l u t i o n measurement i s shown i n F i g . 4 a ) , where a FWHM of 360 ps i s obtained. This value i s c o n s i s t e n t w i t h a c o n v o l u t i o n of several c o n t r i b u t i o n s which have to be taken i n t o account :
i ) the t i m e o f f l i g h t o f the oxygen p a r t i c l e s over the 6 m distance amounts to 65 ns which, since Ap/p = 0.2 % lead to a width of —130 ps of the beam i t s e l f , i i ) the i n t r i n s i c width of the timezero detector (~ 100 ps) i i i ) the "walk" of the constant f r a c t i o n d i s c r i m i n a t o r . This c o n t r i b u t i o n i s r e l a t i v e l y large since the i n p u t pulses cover a wide span. From the operating manual of the CFD a value of 270 ps i s found f o r an i n p u t between several v o l t s and the t h r e s h o l d s e t t i n g ,
i v ) t h e w i d t h induced by the A£ detector t i m i n g chain (~ 150 ps)
F i g . 4b) shows f o r comparaison, the r e s o l u t i o n which i s obtained by stopping the TAC by the radiofrequency pulse of GANIL a c c e l e r a t o r i n l i e u o f the signal from the timezero d e t e c t o r . A FWHM o f 680 ps i s observed.
This value is still relatively good and associated to the very satisfactory
operation of th.e new ECR ion source for gaseous materials. For metallic ions
however, produced by the PIG source, values of up to 2 ns have been found,
accompagnied by occasionnai phase shifts of several ns. Here the use of a
time-zero detector of the proposed type will apport a substancial
improvement.
The test of the detector will be persued with heavier beams in order
to obtain a complete picture of its potential and performance.
1. A.M.Zebelman et al. Nucl. Instr.and Moth., v.141, (1977), p.439
2. F.Duscn et al. N'ucl. Ins t r . and Meth., v. 171, ; 1930 ) , p . 7 1
.
3. A.Oeu et al. Nucl.Instr. and Moth., v.179 (1931), p.253.
4. K.Kotte, G.Ortlepp, F.Stary, 5.M.Lukyanov ,
Workshop on heavy ions experiments, Varna, 1934,
JI.NR D7-34-736, Dubna 1934.
5. U
.0
.Draitriev et al. Sov. Journal Devices and Methods
of Experiments, N 2', 1931,p.7.
6 .
R.L.Kavalov et al. Gov. Journal Oeviaes and. .lethous
of Experiments, M 3, 1932, p.46.
7. N.P.Lorikan et al. N'uol.Inotr. and Methods, v.122, 1974, p.377
3. N .iN .Trof imchuk et al. Sov. Journal of Experimental and
Theoretical Physics, v.69, N 3, 1975, p.639.
9. A..\.Arvanov et al. Soviet Journal of Radio-engineering
and electronics, v.27, M 1, 1952, p.163.
TABLE 1
I - Secondary electron emitter
g r i d s u p p o r t : Ni w i r e s , 92 ο t r a n s p a r e n c y , l a y e r of t r i n i t r o c e l l u l o s e with thickness ~ 10 ug/cm2
. f r i a b l e d i e l e c t r i c : MgO w i t h r e l a t i v e density P/D < l%
(p« = 3.55 g / c m3
) , thickness > 100 ag/cm . a c c e l e r a t i n g harp : Bebronze wires w i t h 0 90 |iin, step between w i r e s , lmrn. . e l e c t r i c f i e l d , E =>10" V/cm
I I E l e c t r o s t a t i c m i r r o r
. two p a r a l l e l harps (same as a c c e l e r a t i n g h a r p ) , distance between them : 7.5 mm. I I I Electron m u l t i p l i e r . 2 MCP i n a chevron c o n f i g u r a t i o n , 0 56 mm each, distances MCPMCP and HCPanode 1 mm each, . anode : 0 60 mm f l a t , . m u l t i p l i c a t i o n f a c t o r 1 0s 1 07 a t high v o l t a g e , 2.2 2.5 kv.
Figure Captions
F i g . 1 . Electron microscope photograph of the f r i a b l e d i e l e c t r i c layer (MgO) evaporated onto a g r i d s u p p o r t . F i g , 2 . Schematic view of the timezero d e t e c t o r . F i g . 3. Experimental s e t u p . F i g . 4 . T i m e o f f l i g h t spectra f o r ' 6 o6 + ions measured i n the systems : a) timezero detectorsemiconductor AE detector, b) timezero detector CSS2 accelerator radio frequency
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