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Method of an attempt to detect the lyman-α radiation from positronium
NUCLEAR
INSTRUMENTS
AND
METHODS
85
(I97O) 5 3 - 5 9 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
M E T H O D OF AN A T T E M P T TO DETECT THE LYMAN-~ R A D I A T I O N F R O M P O S I T R O N I U M L.W. FAGG
Nuclear Phystcs Dtrtston, Naval Research Laboratory, Washington, D.C. 20390, U S A Received 22 April 1970 This report descrJbes the method used m an unsuccessful attempt to detect the Lyman-~ radiation from the p o s l t r o m u m atom A description of the experimental apparatus and procedure Is presented The essential feature of the techmque is the isolation
of the desired event by means of a " q u a d r u p l e " coincidence between the Lyman-~ photon and the three ortho-pos~tromum anmhllatlon quanta The results are discussed along with suggestions for improvements for future attempts
1. Introduction
2.6 eV). It is m this energy interval that posltronlum formation does not have to compete with excitation of the argon atoms by the positron. Since what is needed in the present experiment is posltronium formation at least in the first excited state, whose ionization potential is only 1.7 eV, it means there is no "excited state" Ore gap in argon, and such formation must compete wxth argon excitation. This is also true of the other noble gases. Despite the competition given by argon excitation, the attempts mentioned above were made. All have been unsuccessful. An additional major handicap suffered in these attempts, which used only an ultraviolet spectrometer, is that bremsstrahlung and degraded atomic radiation can yield radiahon of the same wave length (2430 ~) as the Lyman-c~ and mask the effect being sought. In an effort to partially overcome the latter handicap, it was decided in the present experiment to isolate the actual event to be detected by requiring detection of the three-quantum annihilation from the orthoposltronlum essentially simultaneously with that of the Lyman-~ photon. This entailed the use of a triple coincidence scintillation counter arrangement to detect the three annihilation quanta roughly 10-v sec after emission of the Lyman-ct radiation which was detected by a fourth counter sensitive to ultraviolet radiation. Thus, roughly speaking, there was required a quadruple coincidence with a time resolution of the order of the ortho-posltronium half-hfe. Of course, because of the solid angles and counter efficlencles involved, the additional triple coincidence requirement would considerably diminish the intensity m an experiment whose feasibility is already marginal from an intensity point of view. To compensate for this intensity loss, it was decided to depart from the use of a spectrometer to detect the Lyman-~ radiation and use instead a series of three ultravtolet filters which passed radlatton at 2330, 2430 and 2530 A. In effect a
Most prewous attempts~-3) to detect the Lyman-~ radiation from positromum have been undertaken using positronlum formed by positrons slowing down m SF6 or a noble gas, usually argon. The exception has been the effort of Brock and Strleb 4) who depended on such formation m a surface layer of a sample of gold. Once posltronium is formed it is three times more hkely to exist m the ortho-posltromum state (three photon decay in the ground state, l a x 10 -7 sec halfllfe) than in the para-posltromum state. Thus one of the major reasons why a noble gas has been used by most workers is that it causes little ortho-positronium quenching, that is, spin flip conversion to the paraposltromum state (two-photon decay). Although the approach could have been taken to quench deliberately and work with para-posltronium, it was decided in this lmtial attempt to use a triplecoincidence technique with ortho-posltromum (see discussion below) despite the intensity loss resulting from the solid-angle limitation involved in using a thwd scintillation counter. This disadvantage was at least partially balanced by the fact that working with para-positromum would have resulted in a contribution to the accidental rate from the more intense twophoton in-flight annihilation radiation. In any event in this experiment the ortho-posltromum approach utdlzmg a noble gas, namely argon, was used. A crucial parameter upon which the probability of positromum formation in a gas depends is the energy width of the Ore gap. This is the energy interval whose upper hmlt is the first excitation potential of the atoms of the gas in which the posltronium is formed and whose lower limit is the difference between the gas atom and posltromum ionization potentials, respectively (e.g. with ionization potentials for argon and positronlum of 15.8 and 6 8 eV, respectively, and a first excitation potential for argon of 11.6 eV, the Ore gap for ground-state positronlum formation is 53
54
L . W . FAGG
three point spectrum would be obtained. If the intensity of the 2430 A point was found to be greater than the other two, then this could be considered as evidence for detection of the Lyman-~ line. Since there would undoubtedly be considerable broadening of this line, due to the gas pressure, the 100 A spacing was judged suitable. With such an apparatus it was estimated that no
more than 25 events per week could be detected using a 5 mCl positron source. This estimate is based on an upper limit given by Duff and Heymann 3) for the percentage of positrons in a noble gas at high pressure which form positronia emitting Lyman-~ photons. In view of the uncertainties in the estimate, it was decided to proceed with the experiment. The following sections describe in some detail the apparatus and
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Fig. 2. Partly cross-sectional & a g r a m (m the h o r i z o n t a l plane) of the p o s t t r o n l u m gas c h a m b e r s h o w i n g the r o t a t i n g filter m o u n t i n g and one of the a n m h i l a t i o n scintillators The vacuum and pressure t u b i n g on the rxght side of the & a g r a m are not m cross secUon. The center an d left end of the p o r t i o n of the gas c h a m b e r c o n t a i n i n g the m~rrors are s how n with t hi nne r walls to allow passage of the annihllaUon q u a n t a to the three scmtfllaUon counters Section A A is the position of the plane of fig. 3, n o r m a l to the plane of th~s figure. Fig. 4 gives a front view of the r o t a t i n g filter m o u n t i n g whose plane ]s also n o r m a l to that of this figure
LYMAN-~
RADIATION
electronics used, the experimental procedure for the data accumulation, and the results. A dlscusston is also given of possible improvements which might lead to successful detection in the future.
2. Experimental apparatus When positrons are slowed down in a non-quenching gas the number of three quantum annihilations rises with pressure. Then a levehng off occurs, followed by a decrease 5) as shown in fig. 1 for the case of S F 6. A pressure of 400 psig of argon was chosen m this experiment since it was calculated that this would correspond to operation on the "plateau" of the curve in fig. 1. The view of the apparatus which best illustrates the course of the positron and that of the resulting Lyman-c~ radmtion is that of a partial cross section of the gas chamber presented in fig. 2. A 5 mCi 22Na source was fixed on the source mounting. The source material was deposited in a thin layer, 6 mm in diameter, covered with an acrylic film to isolate the sample from the gas. The film was, however, thin enough to allow positrons to pass through without significant energy loss. The position of the source was set so that the average range of the positrons terminated in the center of the chamber tube
FROM POSITRONIUM
55
at the focal point of the thin aluminum-coated parabolic mirror. It was in this region that the positronla were expected to be formed and the Lyman-~ radiation to be emitted. The purpose of the parabolic mirror is to gather as many of the Lyman-c~ photons as possible and send them to the right, down the tubing to be reflected by a second aluminum mirror. This is a plane mirror at 45 ° to these incident photons and passes them down the second portion of the tubing at right angles to the original direction. The photons then pass through the quartz window, one of the UV filters, and finally to the UV photomultlpher for detection. This rather indirect path was adopted to allow the square piece of lead to be introduced to diminish the amount of direct radiation from the source reaching the UV photomultiplier and the quartz. Fluorescence from the quartz would otherwise produce a large unwanted background. The lead collar plus additional specially shaped pieces of lead not shown in the figure reduced the direct radiation to the annihilation scintillators as well as to personnel. The parabolic mirror was made quite thin (6.3/m0 in order to allow those positrons emitted in the left
LEAD LEAD~ ALUMINUM MIRROR LEAD
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SECTION A A Fig 3 Cross-sectional view showing section A A of fig 2 and illustrating the a r r a n g e m e n t o f the three annihilation scintillators separated by lead absorbers.
56
L.W.
h a n d p o r t i o n o f the source area to pass to the focal region o f the mirror. This was necessary since the focal p o i n t is only 8 m m f r o m the intersection o f the m i r r o r surface a n d the tube axis. A l s o the p a r a b o l i c m i r r o r as well as the tube walls in this region had to be thin to allow the a n n i h i l a t i o n q u a n t a to pass to the three scintillation counters. The scintillators o f these counters are shown in fig. 3 which is section A A o f fig. 2 a n d in a plane n o r m a l to that o f fig. 2. They are 120 ° a p a r t and are o f p d o t B, 5.95 cm long and 5 cm in diameter. A s e p a r a t i o n o f 120 ° was chosen for convenience a n d s y m m e t r y since the threeq u a n t u m a n n i h i l a t i o n p h o t o n s in such a case are equal in energy, 340 keV. As can be seen, lead is also placed between the scintillators to diminish interscmtillator scatterlngs which would cause false triple coincidences. Pilot B scintillators were used p r i m a r i l y because o f their fast scintillation time ~ 10 . 8 sec. Thls means that a triple coincidence event can be m a r k e d in time r a t h e r precisely relative to the slower lifetime o f the o r t h o - p o s i t r o n i u m , i.e. ,~ 10 . 7 sec. This is useful if one wishes to regulate with any accuracy the n u m b e r o f half-hves o f the o r t h o - p o s i t r o m u m t h a t elapse before the three-quantum annihilation occurs. However, the pilot B has the d i s a d v a n t a g e o f a relatively low g a m m a ray counting efficiency, estimated to be a b o u t 30% for 340 keV g a m m a rays. If an inorganic scintillator such as N a l were used, the efficiency for the same scintillator size would a p p r o a c h 100%. On the other hand, the cost o f the electronics necessary to a p p r o p r i a t e l y clip the longer pulses resulting from the
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FAGG
c o n s i d e r a b l y longer scmtillatlon time ~ 10 -6 sec was, at the time, considered prohibitive. The three U V filters were Inserted in a r o t a t i n g m o u n t i n g which has five positions, one for each o f the filters, an o p a q u e p o s i t i o n (blocking all UV and lower energy photons), and an open one. This is illustrated in fig. 4 which shows a front view (with the UV p h o t o m u l t i p l i e r removed) of the m o u n t i n g perpendicular to the plane o f fig. 2. A l t h o u g h the average transmission o f the filters was a b o u t 10% at m a x i m u m , the large increase in the solid angle o f the r a d m t l o n received by the detector, c o m p a r e d to that received by a spectrometer, m o r e than c o m p e n s a t e s for this loss.
3. Electronic system The three a n m h l l a t l o n counters used 14-stage A m p e r e x 56 A V P p h o t o m u l t i p l i e r tubes. D u e to the high singles counting rates expected in the a n n i h i l a t i o n counters p r e c a u t i o n s h a d to be t a k e n to m a k e the d y n o d e voltages, especially the ones near the anode, i n d e p e n d e n t o f c o u n t i n g rate. Thus the current in the resistor chain o f the p h o t o m u l t i p h e r s was m a d e as large as practically feasible, a b o u t 2 mA. The resultant heating necessitated the use o f an air cooling system to keep the t e m p e r a t u r e o f the tube bases relatively constant. F o r the ultraviolet c o u n t e r receiving the Lyman-c~ r a d i a t i o n , a very low noise EM R 541F p h o t o m u l t l p h e r was used. This is a sapphire window tube with a high w o r k function p h o t o c a t h o d e o f C s - T e so t h a t cooling to reduce noise was unnecessar2¢. The q u a n t u m efficiency o f this tube was m a x i m u m (7.5%) at a b o u t 2500 A. which is j u s t the o p e r a t i n g region o f this experiment. The pulses p r o d u c e d by the counters were h a n d l e d by electronic logic circuitry consisting a l m o s t entirely o f Chronetlcs equipment. A pulse from the U V c o u n t e r is p r e p a r e d and amplified by a p r e - a m p h f i e r and a X10 amplifier as can be seen in fig. 5. It then passes to a discriminator, thence to a gate generator. The latter device could p r o d u c e a gate pulse o f variable d u r a t i o n which " o p e n e d " the rest o f the electronics, l e. the circuitry associated with the three a n n i h i l a t i o n counters so t h a t a triple coincidence could be recorded. Thus in principle an event consisting o f the emission o f a Lyman-c~ p h o t o n followed by a 3 - q u a n t u m annihilation could be isolated and detected. Since the lifetime for the 3 - q u a n t u m a n n i h i l a t i o n is a b o u t 1 4 x 10 - v sec, the gate pulse d u r a t i o n was usually set between 1 and 2 x 10 . 7 sec. A p p r o p r i a t e delay had to be introduced in the signal lines from the three annihilation counters in
LYMAN-~ RADIATION FROM POSITKONIUM
57
ANN,H,LAT,ON ICL,P COUNTR/OLAY 1
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FAST SCALER
-UV COUNTER
Fig 5. Block dmgram of the electrontc apparatus used to detect the desired quadruple coincidences order to time-align the three counters with respect to the UV counter so that the beginning of the gate pulse was indeed time-zero for the events desired. Also some additional relative delay among the three counters had to be introduced as shown in fig. 5 to account mostly for time differences in the discriminators in the three channels. The pulses from the annlhdatlon counters were clipped using shorted cables (2 nsec round trip transit time) and after passmg through the delay cables were sent to discriminators. After again being clipped they are passed to a fan-in or linear adder circuit, thence through a X 0 2 attenuator and a discriminator and finally to a fast scaler. The combination of the hnear adder, X 0.2 attenuator and the discriminator effectively constituted a classic Rossi type triple coincidence circuit, which for any comblnatmn of double coincidences was found to have a 4 nsec resolving time. The 541F UV tube was usually operated at about 3100 V. The high voltage of each of the annihilation counters was independently controllable and their voltages were set so that the discriminators passed the upper two-thirds of the Compton dlstrlbutmn produced by 340 keV photons. This meant their voltages were usually in the vicinity of 2000 V. A lower discriminator setting would have passed too many pulses arising mostly from low energy photons degraded from scattered high energy radiation such as the 1.28MeV gamma rays and 511keV annihdatmn quanta from the 22Na source. Such high singles countmg rates would have produced a prohibitive amount of pulse pile-up resulting in an unacceptably high accidental coincidence rate. Even with the existing settings the singles resulted in a duty cycle for discriminators of approximately 2%. Of course, it
was the additional requirement of a quadruple coincidence with the UV counter that reduced the resultant counting rate to the low values that were observed.
4. Experimental procedure Previous to each run the time alignment of the triple coincidence clrcmt was checked by taking delay curves in which the delay of each of the three counters in turn was varied. Runs usually were conducted continuously over a period of one or two weeks in approximately eight hour segments. An eight hour segment was devoted to each of the three UV filters and to the "opaque" configuration. The latter run was of course used as a background run. Separate runs were made with both 22Na and 13VCs sources. The purpose of 137Cs runs was again essentrolly for background determination purposes, to insure that any real effects observed were due to positrons uniquely. The energy of the fl- particle from ~3VCs is quite comparable to the fl* particle from 22Na, 500 keV. Also the average energy of the 22Na gamma rays is about equal to that of the 661 keV photon from 137Cs. Thus except for the difference in charge of the electron the sources were reasonably similar, the significant difference being that in most 22Na decays three gamma rays were ultimately emitted compared to only one in the 13VCs. At the end of each eight hour run the discriminator level of the discriminator receiving pulses from the linear adder was checked with a precision pulse generator. Before, after, and occasionally during a run the Compton pulse height spectrum of each of the annihilation counters was checked with an oscilloscope for constancy in voltage height.
58
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FAGG
5. Results and discussion
lengths on either side. As can be seen m fig. 6, thxs is not the case. If anything, there is an indication o f an increase at 2430 in the 137Cs a n d not in the 22Na. However, a straight h o r i z o n t a l line can p r o b a b l y still be d r a w n t h r o u g h the 13VCs curve since the n u m b e r o f counts involved is still small enough to allow p r o b a b l e uncertainties s o m e w h a t larger than that Indicated by the square r o o t criterion. W i t h the Z2Na results, roughly 25 desired events per week could have gone undetected as a result o f statistical uncertainties. The upper limit calculation o f 25 per week given at the end o f section 1 was based on a 5 m O source (the fact that the two rates just given are the same is entirely fortuitous). An estimate that only 40% o f the source was effective due to the 22Na source leak, reduces the latter rate to a b o u t 10. Thus, ff this n u m b e r is correct, observation o f the desired effect was ~mposslble under the circumstances. Thin also means that the value estimated from the present w o r k o f 0.25% as an upper limit for the percentage o f positrons which ultimately p r o d u c e a L y m a n - ~ p h o t o n is higher than those given by some earlier workers1'3). O f course, if the source had not leaked, this n u m b e r would be lower.
Both the 22Na a n d 137Cs d a t a for each o f the four settings (the three filters a n d the " o p a q u e " setting) were reduced to counts p e r h o u r by simply d i w d i n g the total c o u n t a c c u m u l a t e d at a given setting by the total t~me spent at t h a t setting. The " o p a q u e " counting rate was then subtracted f r o m each o f the three filter counting rates. These three counting rates were then n o r m a l i z e d using a n u m b e r p r o p o r t i o n a l to the integral o f each o f the filter transmission curves. U n f o r t u n a t e l y due to a leak in the 22Na source dtscussed in the next section, the validity o f a c o m p a r i s o n o f the 22Na and 137Cs spectra is s o m e w h a t diluted. In p a r t thin is because the leak prevented the a c c u m u l a t i o n o f enough counts with the 22Na source to achieve the statistical accuracy possible in the 137Cs runs. In fact the numbers, p a r t i c u l a r l y in the case o f the case o f the 22Na o r d i n a t e at 2430 A, are small enough to t h r o w serious d o u b t on the use o f the square r o o t uncertainty criterion. Nevertheless, these spectra are presented in fig. 6. The e r r o r bars shown are based solely on the counting statistics using the square r o o t uncertainty criterion. O f course, as mentioned earlier, the result h o p e d for in this experiment was to observe a higher counting rate m the 22Na spectrum at 2430 A than at the wave
Several shortcomings can be found with the a p p a ratus in its present state which m a y possibly explain
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Flg. 6. "Three point" ultra violet spectra (2330, 2430 and 2530 A) showing the relative counting rates of quadruple coincidences for 22Na and 137Cs.
LYMAN-~
RADIATION
the lack of success. These faults and suggested improvements for a subsequent attempt are discussed in the next section.
6. Future improvements The most serious problem encountered in this experiment was the leak in the 22Na source mentioned in the previous section. At some time during the 22Na run the acrylic coating suffered a rupture resulting in a loss of roughly 80% of the source material from the deposit area. This radioactivity spread throughout the gas chamber. Whereas a considerable part of this lost material was probably still useful in producing the events to be detected and was localized near the source, most of it probably was not, effectively diminishing the potency of the source. In future work thin metallic coatings may be preferable in order to avoid the deterioration in organic films caused by radiation damage. The minimization of air pockets between the deposit and the film would probably also be helpful since bursting could occur when the gas chamber is first evacuated. For succeeding experiments a smaller resolving time than 4 nsec would be very desirable in order to reduce the high accidental rate. Also, ff the expense can be sustained, a future attempt would be greatly aided by the use of Na! crystals for the annihilation counters along with the associated pulse chpplng and shaping electronics needed to compensate for the slow scintillation time of the crystal. Although the suggestion may be somewhat conjectural, future efforts might be also helped with the use of xenon instead of argon. Such a substitution would not yield improvement from the standpoint of the Ore gap, since, as mentioned earlier this gap does
FROM POSITRONIUM
59
not exist for the first excited state. However, it is felt that the use of xenon whose ionization potential is more comparable to that of the positronium would improve the chances of its formation. This is because posltronium formed from positrons of lower energy would generally have a better chance of remaining stable. Finally, if there have been very recent improvements of UV optical technology making available filters of smaller bandwidth and higher transmission at maximum, naturally they would be desirable to use. Despite the lack of success of this first attempt, it was deemed fitting to write this report of a completed phase of this effort with the hope that other experimenters might benefit from the experience. The author wishes to express his appreciation to Mr. E. C. Jones, Jr. who was most helpful in constructing much of the apparatus, especially the electronics. He wishes to thank Miss S. Numrlch for her help in the data accumulation and data treatment. He ts grateful for valuable discussions with Prof. S. Berko, Dr. T. F. Godlove and Dr. K. M. Murray. The use of the EMR 541F photomultlplier tube furnished by Mrs. 1. Packer of the Space Science Division of N R L is also gratefully acknowledged.
References 1) v . w . Hughes, J. Appl Phys. 28 (1957) 16 o) W. R Bennett, Jr., W. T h o m a s , V W. H u g h e s and C S. W u , Bull. A m Phys Soc. 6 ( 1 9 6 1 ) 4 9 3) B. G Duff" and F . F . H e y m a n n , Proc Roy Soc. L o n d o n A272 (1963) 363 4) R L Brock and J. R. Strelb, Phys Rev. 109 (1958) 399. 5) S DeBenedettl and R. T. Siegel, Phys Rev 94 (1954) 955.
INSTRUMENTS
AND
METHODS
85
(I97O) 5 3 - 5 9 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
M E T H O D OF AN A T T E M P T TO DETECT THE LYMAN-~ R A D I A T I O N F R O M P O S I T R O N I U M L.W. FAGG
Nuclear Phystcs Dtrtston, Naval Research Laboratory, Washington, D.C. 20390, U S A Received 22 April 1970 This report descrJbes the method used m an unsuccessful attempt to detect the Lyman-~ radiation from the p o s l t r o m u m atom A description of the experimental apparatus and procedure Is presented The essential feature of the techmque is the isolation
of the desired event by means of a " q u a d r u p l e " coincidence between the Lyman-~ photon and the three ortho-pos~tromum anmhllatlon quanta The results are discussed along with suggestions for improvements for future attempts
1. Introduction
2.6 eV). It is m this energy interval that posltronlum formation does not have to compete with excitation of the argon atoms by the positron. Since what is needed in the present experiment is posltronium formation at least in the first excited state, whose ionization potential is only 1.7 eV, it means there is no "excited state" Ore gap in argon, and such formation must compete wxth argon excitation. This is also true of the other noble gases. Despite the competition given by argon excitation, the attempts mentioned above were made. All have been unsuccessful. An additional major handicap suffered in these attempts, which used only an ultraviolet spectrometer, is that bremsstrahlung and degraded atomic radiation can yield radiahon of the same wave length (2430 ~) as the Lyman-c~ and mask the effect being sought. In an effort to partially overcome the latter handicap, it was decided in the present experiment to isolate the actual event to be detected by requiring detection of the three-quantum annihilation from the orthoposltronlum essentially simultaneously with that of the Lyman-~ photon. This entailed the use of a triple coincidence scintillation counter arrangement to detect the three annihilation quanta roughly 10-v sec after emission of the Lyman-ct radiation which was detected by a fourth counter sensitive to ultraviolet radiation. Thus, roughly speaking, there was required a quadruple coincidence with a time resolution of the order of the ortho-posltronium half-hfe. Of course, because of the solid angles and counter efficlencles involved, the additional triple coincidence requirement would considerably diminish the intensity m an experiment whose feasibility is already marginal from an intensity point of view. To compensate for this intensity loss, it was decided to depart from the use of a spectrometer to detect the Lyman-~ radiation and use instead a series of three ultravtolet filters which passed radlatton at 2330, 2430 and 2530 A. In effect a
Most prewous attempts~-3) to detect the Lyman-~ radiation from positromum have been undertaken using positronlum formed by positrons slowing down m SF6 or a noble gas, usually argon. The exception has been the effort of Brock and Strleb 4) who depended on such formation m a surface layer of a sample of gold. Once posltronium is formed it is three times more hkely to exist m the ortho-posltromum state (three photon decay in the ground state, l a x 10 -7 sec halfllfe) than in the para-posltromum state. Thus one of the major reasons why a noble gas has been used by most workers is that it causes little ortho-positronium quenching, that is, spin flip conversion to the paraposltromum state (two-photon decay). Although the approach could have been taken to quench deliberately and work with para-posltronium, it was decided in this lmtial attempt to use a triplecoincidence technique with ortho-posltromum (see discussion below) despite the intensity loss resulting from the solid-angle limitation involved in using a thwd scintillation counter. This disadvantage was at least partially balanced by the fact that working with para-positromum would have resulted in a contribution to the accidental rate from the more intense twophoton in-flight annihilation radiation. In any event in this experiment the ortho-posltromum approach utdlzmg a noble gas, namely argon, was used. A crucial parameter upon which the probability of positromum formation in a gas depends is the energy width of the Ore gap. This is the energy interval whose upper hmlt is the first excitation potential of the atoms of the gas in which the posltronium is formed and whose lower limit is the difference between the gas atom and posltromum ionization potentials, respectively (e.g. with ionization potentials for argon and positronlum of 15.8 and 6 8 eV, respectively, and a first excitation potential for argon of 11.6 eV, the Ore gap for ground-state positronlum formation is 53
54
L . W . FAGG
three point spectrum would be obtained. If the intensity of the 2430 A point was found to be greater than the other two, then this could be considered as evidence for detection of the Lyman-~ line. Since there would undoubtedly be considerable broadening of this line, due to the gas pressure, the 100 A spacing was judged suitable. With such an apparatus it was estimated that no
more than 25 events per week could be detected using a 5 mCl positron source. This estimate is based on an upper limit given by Duff and Heymann 3) for the percentage of positrons in a noble gas at high pressure which form positronia emitting Lyman-~ photons. In view of the uncertainties in the estimate, it was decided to proceed with the experiment. The following sections describe in some detail the apparatus and
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.25
Density of SF 6 In gm /cm 3 Fig
1 The rate of 3 - q u a n t u m a n m h i l a t t o n s vs the density of SF6. This figure Is from ref 5.
541F UV
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Fig. 2. Partly cross-sectional & a g r a m (m the h o r i z o n t a l plane) of the p o s t t r o n l u m gas c h a m b e r s h o w i n g the r o t a t i n g filter m o u n t i n g and one of the a n m h i l a t i o n scintillators The vacuum and pressure t u b i n g on the rxght side of the & a g r a m are not m cross secUon. The center an d left end of the p o r t i o n of the gas c h a m b e r c o n t a i n i n g the m~rrors are s how n with t hi nne r walls to allow passage of the annihllaUon q u a n t a to the three scmtfllaUon counters Section A A is the position of the plane of fig. 3, n o r m a l to the plane of th~s figure. Fig. 4 gives a front view of the r o t a t i n g filter m o u n t i n g whose plane ]s also n o r m a l to that of this figure
LYMAN-~
RADIATION
electronics used, the experimental procedure for the data accumulation, and the results. A dlscusston is also given of possible improvements which might lead to successful detection in the future.
2. Experimental apparatus When positrons are slowed down in a non-quenching gas the number of three quantum annihilations rises with pressure. Then a levehng off occurs, followed by a decrease 5) as shown in fig. 1 for the case of S F 6. A pressure of 400 psig of argon was chosen m this experiment since it was calculated that this would correspond to operation on the "plateau" of the curve in fig. 1. The view of the apparatus which best illustrates the course of the positron and that of the resulting Lyman-c~ radmtion is that of a partial cross section of the gas chamber presented in fig. 2. A 5 mCi 22Na source was fixed on the source mounting. The source material was deposited in a thin layer, 6 mm in diameter, covered with an acrylic film to isolate the sample from the gas. The film was, however, thin enough to allow positrons to pass through without significant energy loss. The position of the source was set so that the average range of the positrons terminated in the center of the chamber tube
FROM POSITRONIUM
55
at the focal point of the thin aluminum-coated parabolic mirror. It was in this region that the positronla were expected to be formed and the Lyman-~ radiation to be emitted. The purpose of the parabolic mirror is to gather as many of the Lyman-c~ photons as possible and send them to the right, down the tubing to be reflected by a second aluminum mirror. This is a plane mirror at 45 ° to these incident photons and passes them down the second portion of the tubing at right angles to the original direction. The photons then pass through the quartz window, one of the UV filters, and finally to the UV photomultlpher for detection. This rather indirect path was adopted to allow the square piece of lead to be introduced to diminish the amount of direct radiation from the source reaching the UV photomultiplier and the quartz. Fluorescence from the quartz would otherwise produce a large unwanted background. The lead collar plus additional specially shaped pieces of lead not shown in the figure reduced the direct radiation to the annihilation scintillators as well as to personnel. The parabolic mirror was made quite thin (6.3/m0 in order to allow those positrons emitted in the left
LEAD LEAD~ ALUMINUM MIRROR LEAD
/
1
-"ALUMINUM COATED PARABOLICMIRROR
LEAD
SECTION A A Fig 3 Cross-sectional view showing section A A of fig 2 and illustrating the a r r a n g e m e n t o f the three annihilation scintillators separated by lead absorbers.
56
L.W.
h a n d p o r t i o n o f the source area to pass to the focal region o f the mirror. This was necessary since the focal p o i n t is only 8 m m f r o m the intersection o f the m i r r o r surface a n d the tube axis. A l s o the p a r a b o l i c m i r r o r as well as the tube walls in this region had to be thin to allow the a n n i h i l a t i o n q u a n t a to pass to the three scintillation counters. The scintillators o f these counters are shown in fig. 3 which is section A A o f fig. 2 a n d in a plane n o r m a l to that o f fig. 2. They are 120 ° a p a r t and are o f p d o t B, 5.95 cm long and 5 cm in diameter. A s e p a r a t i o n o f 120 ° was chosen for convenience a n d s y m m e t r y since the threeq u a n t u m a n n i h i l a t i o n p h o t o n s in such a case are equal in energy, 340 keV. As can be seen, lead is also placed between the scintillators to diminish interscmtillator scatterlngs which would cause false triple coincidences. Pilot B scintillators were used p r i m a r i l y because o f their fast scintillation time ~ 10 . 8 sec. Thls means that a triple coincidence event can be m a r k e d in time r a t h e r precisely relative to the slower lifetime o f the o r t h o - p o s i t r o n i u m , i.e. ,~ 10 . 7 sec. This is useful if one wishes to regulate with any accuracy the n u m b e r o f half-hves o f the o r t h o - p o s i t r o m u m t h a t elapse before the three-quantum annihilation occurs. However, the pilot B has the d i s a d v a n t a g e o f a relatively low g a m m a ray counting efficiency, estimated to be a b o u t 30% for 340 keV g a m m a rays. If an inorganic scintillator such as N a l were used, the efficiency for the same scintillator size would a p p r o a c h 100%. On the other hand, the cost o f the electronics necessary to a p p r o p r i a t e l y clip the longer pulses resulting from the
---~OPENING FOR541UV /~'~HOTOMULTIPLIE R FILTERS~ \ y / ,
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Fig. 4. A front view of the rotating filter mounting (with the 541F UV photomultlpher removed and looking from the top of fig. 2 toward the quartz window) A new filter position was obtained by rotating the dial pointer knob to the approprmte marker
FAGG
c o n s i d e r a b l y longer scmtillatlon time ~ 10 -6 sec was, at the time, considered prohibitive. The three U V filters were Inserted in a r o t a t i n g m o u n t i n g which has five positions, one for each o f the filters, an o p a q u e p o s i t i o n (blocking all UV and lower energy photons), and an open one. This is illustrated in fig. 4 which shows a front view (with the UV p h o t o m u l t i p l i e r removed) of the m o u n t i n g perpendicular to the plane o f fig. 2. A l t h o u g h the average transmission o f the filters was a b o u t 10% at m a x i m u m , the large increase in the solid angle o f the r a d m t l o n received by the detector, c o m p a r e d to that received by a spectrometer, m o r e than c o m p e n s a t e s for this loss.
3. Electronic system The three a n m h l l a t l o n counters used 14-stage A m p e r e x 56 A V P p h o t o m u l t i p l i e r tubes. D u e to the high singles counting rates expected in the a n n i h i l a t i o n counters p r e c a u t i o n s h a d to be t a k e n to m a k e the d y n o d e voltages, especially the ones near the anode, i n d e p e n d e n t o f c o u n t i n g rate. Thus the current in the resistor chain o f the p h o t o m u l t i p h e r s was m a d e as large as practically feasible, a b o u t 2 mA. The resultant heating necessitated the use o f an air cooling system to keep the t e m p e r a t u r e o f the tube bases relatively constant. F o r the ultraviolet c o u n t e r receiving the Lyman-c~ r a d i a t i o n , a very low noise EM R 541F p h o t o m u l t l p h e r was used. This is a sapphire window tube with a high w o r k function p h o t o c a t h o d e o f C s - T e so t h a t cooling to reduce noise was unnecessar2¢. The q u a n t u m efficiency o f this tube was m a x i m u m (7.5%) at a b o u t 2500 A. which is j u s t the o p e r a t i n g region o f this experiment. The pulses p r o d u c e d by the counters were h a n d l e d by electronic logic circuitry consisting a l m o s t entirely o f Chronetlcs equipment. A pulse from the U V c o u n t e r is p r e p a r e d and amplified by a p r e - a m p h f i e r and a X10 amplifier as can be seen in fig. 5. It then passes to a discriminator, thence to a gate generator. The latter device could p r o d u c e a gate pulse o f variable d u r a t i o n which " o p e n e d " the rest o f the electronics, l e. the circuitry associated with the three a n n i h i l a t i o n counters so t h a t a triple coincidence could be recorded. Thus in principle an event consisting o f the emission o f a Lyman-c~ p h o t o n followed by a 3 - q u a n t u m annihilation could be isolated and detected. Since the lifetime for the 3 - q u a n t u m a n n i h i l a t i o n is a b o u t 1 4 x 10 - v sec, the gate pulse d u r a t i o n was usually set between 1 and 2 x 10 . 7 sec. A p p r o p r i a t e delay had to be introduced in the signal lines from the three annihilation counters in
LYMAN-~ RADIATION FROM POSITKONIUM
57
ANN,H,LAT,ON ICL,P COUNTR/OLAY 1
L_ Njl
I
I
FAST SCALER
-UV COUNTER
Fig 5. Block dmgram of the electrontc apparatus used to detect the desired quadruple coincidences order to time-align the three counters with respect to the UV counter so that the beginning of the gate pulse was indeed time-zero for the events desired. Also some additional relative delay among the three counters had to be introduced as shown in fig. 5 to account mostly for time differences in the discriminators in the three channels. The pulses from the annlhdatlon counters were clipped using shorted cables (2 nsec round trip transit time) and after passmg through the delay cables were sent to discriminators. After again being clipped they are passed to a fan-in or linear adder circuit, thence through a X 0 2 attenuator and a discriminator and finally to a fast scaler. The combination of the hnear adder, X 0.2 attenuator and the discriminator effectively constituted a classic Rossi type triple coincidence circuit, which for any comblnatmn of double coincidences was found to have a 4 nsec resolving time. The 541F UV tube was usually operated at about 3100 V. The high voltage of each of the annihilation counters was independently controllable and their voltages were set so that the discriminators passed the upper two-thirds of the Compton dlstrlbutmn produced by 340 keV photons. This meant their voltages were usually in the vicinity of 2000 V. A lower discriminator setting would have passed too many pulses arising mostly from low energy photons degraded from scattered high energy radiation such as the 1.28MeV gamma rays and 511keV annihdatmn quanta from the 22Na source. Such high singles countmg rates would have produced a prohibitive amount of pulse pile-up resulting in an unacceptably high accidental coincidence rate. Even with the existing settings the singles resulted in a duty cycle for discriminators of approximately 2%. Of course, it
was the additional requirement of a quadruple coincidence with the UV counter that reduced the resultant counting rate to the low values that were observed.
4. Experimental procedure Previous to each run the time alignment of the triple coincidence clrcmt was checked by taking delay curves in which the delay of each of the three counters in turn was varied. Runs usually were conducted continuously over a period of one or two weeks in approximately eight hour segments. An eight hour segment was devoted to each of the three UV filters and to the "opaque" configuration. The latter run was of course used as a background run. Separate runs were made with both 22Na and 13VCs sources. The purpose of 137Cs runs was again essentrolly for background determination purposes, to insure that any real effects observed were due to positrons uniquely. The energy of the fl- particle from ~3VCs is quite comparable to the fl* particle from 22Na, 500 keV. Also the average energy of the 22Na gamma rays is about equal to that of the 661 keV photon from 137Cs. Thus except for the difference in charge of the electron the sources were reasonably similar, the significant difference being that in most 22Na decays three gamma rays were ultimately emitted compared to only one in the 13VCs. At the end of each eight hour run the discriminator level of the discriminator receiving pulses from the linear adder was checked with a precision pulse generator. Before, after, and occasionally during a run the Compton pulse height spectrum of each of the annihilation counters was checked with an oscilloscope for constancy in voltage height.
58
L.W.
FAGG
5. Results and discussion
lengths on either side. As can be seen m fig. 6, thxs is not the case. If anything, there is an indication o f an increase at 2430 in the 137Cs a n d not in the 22Na. However, a straight h o r i z o n t a l line can p r o b a b l y still be d r a w n t h r o u g h the 13VCs curve since the n u m b e r o f counts involved is still small enough to allow p r o b a b l e uncertainties s o m e w h a t larger than that Indicated by the square r o o t criterion. W i t h the Z2Na results, roughly 25 desired events per week could have gone undetected as a result o f statistical uncertainties. The upper limit calculation o f 25 per week given at the end o f section 1 was based on a 5 m O source (the fact that the two rates just given are the same is entirely fortuitous). An estimate that only 40% o f the source was effective due to the 22Na source leak, reduces the latter rate to a b o u t 10. Thus, ff this n u m b e r is correct, observation o f the desired effect was ~mposslble under the circumstances. Thin also means that the value estimated from the present w o r k o f 0.25% as an upper limit for the percentage o f positrons which ultimately p r o d u c e a L y m a n - ~ p h o t o n is higher than those given by some earlier workers1'3). O f course, if the source had not leaked, this n u m b e r would be lower.
Both the 22Na a n d 137Cs d a t a for each o f the four settings (the three filters a n d the " o p a q u e " setting) were reduced to counts p e r h o u r by simply d i w d i n g the total c o u n t a c c u m u l a t e d at a given setting by the total t~me spent at t h a t setting. The " o p a q u e " counting rate was then subtracted f r o m each o f the three filter counting rates. These three counting rates were then n o r m a l i z e d using a n u m b e r p r o p o r t i o n a l to the integral o f each o f the filter transmission curves. U n f o r t u n a t e l y due to a leak in the 22Na source dtscussed in the next section, the validity o f a c o m p a r i s o n o f the 22Na and 137Cs spectra is s o m e w h a t diluted. In p a r t thin is because the leak prevented the a c c u m u l a t i o n o f enough counts with the 22Na source to achieve the statistical accuracy possible in the 137Cs runs. In fact the numbers, p a r t i c u l a r l y in the case o f the case o f the 22Na o r d i n a t e at 2430 A, are small enough to t h r o w serious d o u b t on the use o f the square r o o t uncertainty criterion. Nevertheless, these spectra are presented in fig. 6. The e r r o r bars shown are based solely on the counting statistics using the square r o o t uncertainty criterion. O f course, as mentioned earlier, the result h o p e d for in this experiment was to observe a higher counting rate m the 22Na spectrum at 2430 A than at the wave
Several shortcomings can be found with the a p p a ratus in its present state which m a y possibly explain
7x 16J
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I 2450
2550
Flg. 6. "Three point" ultra violet spectra (2330, 2430 and 2530 A) showing the relative counting rates of quadruple coincidences for 22Na and 137Cs.
LYMAN-~
RADIATION
the lack of success. These faults and suggested improvements for a subsequent attempt are discussed in the next section.
6. Future improvements The most serious problem encountered in this experiment was the leak in the 22Na source mentioned in the previous section. At some time during the 22Na run the acrylic coating suffered a rupture resulting in a loss of roughly 80% of the source material from the deposit area. This radioactivity spread throughout the gas chamber. Whereas a considerable part of this lost material was probably still useful in producing the events to be detected and was localized near the source, most of it probably was not, effectively diminishing the potency of the source. In future work thin metallic coatings may be preferable in order to avoid the deterioration in organic films caused by radiation damage. The minimization of air pockets between the deposit and the film would probably also be helpful since bursting could occur when the gas chamber is first evacuated. For succeeding experiments a smaller resolving time than 4 nsec would be very desirable in order to reduce the high accidental rate. Also, ff the expense can be sustained, a future attempt would be greatly aided by the use of Na! crystals for the annihilation counters along with the associated pulse chpplng and shaping electronics needed to compensate for the slow scintillation time of the crystal. Although the suggestion may be somewhat conjectural, future efforts might be also helped with the use of xenon instead of argon. Such a substitution would not yield improvement from the standpoint of the Ore gap, since, as mentioned earlier this gap does
FROM POSITRONIUM
59
not exist for the first excited state. However, it is felt that the use of xenon whose ionization potential is more comparable to that of the positronium would improve the chances of its formation. This is because posltronium formed from positrons of lower energy would generally have a better chance of remaining stable. Finally, if there have been very recent improvements of UV optical technology making available filters of smaller bandwidth and higher transmission at maximum, naturally they would be desirable to use. Despite the lack of success of this first attempt, it was deemed fitting to write this report of a completed phase of this effort with the hope that other experimenters might benefit from the experience. The author wishes to express his appreciation to Mr. E. C. Jones, Jr. who was most helpful in constructing much of the apparatus, especially the electronics. He wishes to thank Miss S. Numrlch for her help in the data accumulation and data treatment. He ts grateful for valuable discussions with Prof. S. Berko, Dr. T. F. Godlove and Dr. K. M. Murray. The use of the EMR 541F photomultlplier tube furnished by Mrs. 1. Packer of the Space Science Division of N R L is also gratefully acknowledged.
References 1) v . w . Hughes, J. Appl Phys. 28 (1957) 16 o) W. R Bennett, Jr., W. T h o m a s , V W. H u g h e s and C S. W u , Bull. A m Phys Soc. 6 ( 1 9 6 1 ) 4 9 3) B. G Duff" and F . F . H e y m a n n , Proc Roy Soc. L o n d o n A272 (1963) 363 4) R L Brock and J. R. Strelb, Phys Rev. 109 (1958) 399. 5) S DeBenedettl and R. T. Siegel, Phys Rev 94 (1954) 955.