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Lamb shift polarimeter of the slow proton beam
N U C L E A R I N S T R U M E N T S AND METHODS
I35 (I976)
497-5OI;
© N O R T H - H O L L A N D P U B L I S H I N G CO.
LAMB SHIFT POLARIMETER OF THE SLOW P R O T O N BEAM Y U . A. PLISS and L. M. SOROKO
Laboratory o f Nuclear Problems, Joint Institute for Nuclear Research, Dubna, U.S.S.R. Received 15 March 1976 The Lamb shift polarimeter of the slow proton beam for the polarized ion source of the strong current phasotron of the Laboratory of Nuclear Problems, Joint Institute for Nuclear Research, is described. The proton polarization of the polarized ion source has been found to be P = -(0.594-0.04). The polarimeter enables one to investigate the depolarization of protons and deuterons during their injection and acceleration.
I. Introduction
The currently used technique for the polarization measurement of ions emerging from the polarized ion source (PIS) involves the acceleration of particles up to the energy above the threshold of the nuclear reaction and the measurement of the axial asymmetry of nuclear reaction products. For example, the tensor polarization of deuterons has been measured in the T(d,n)~ reaction at a deuteron energy of about 100keW). To measure the vector polarization of deuterons or protons the beam energy must be increased still further. The discussion of this technique has been given elsewhere2). The updated requirements to the accuracy of spin polarization measurements on the one hand, and the need for the precise investigation of the process of spin depolarization during the acceleration of particles on the other hand, can be satisfied simultaneously by measuring the spin polarization of ions directly at the H S exit. As the energy of ions extracted from PIS is equal to 1-10keV, that is, much lower than the threshold of any nuclear reaction, the unique way is to else atomic processes, sensitive to the spin polarization of the atomic nucleus. The metastable atoms formed in the charge exchange of polarized ions offer a promising way. These atoms can be effectively produced and easily transported from one part of PIS to the other. The idea of using the metastable atoms for measuring the spin polarization of the atomic nucleus is not r~Lew3-6). The background of this idea is the relation l:,etween the populations of the hyperfine substates of the 2 $1/2 metastable state in the external magnetic and electric fields, on the one hand, and the spin polarization of the nucleus, protons or deuterons, of these metastable atoms, on the other hand. However, up to new nobody has built a device based on this principle. The Lamb shift polarimeter (LSP) of the slow proton
beam has been designed and built at the Laboratory of Nuclear Problems, JINR. It consists of a cesium vapour target, Helmholtz magnetic coils, two Geiger counters sensitive to Lyman photons of 1216 ~, an electromagnet to analyse the proton beam of energy of about l keV, and the vacuum system. The typical counting rates amount to about 102 counts/s which correspond to a relative error of measurement of _+0.01 x/T, where T is the measurement time in hours. The LSP can also analyse the spin polarization of the slow deuteron beam. The LSP is intended to optimize the working conditions of various parts of the P|S of the Laboratory of Nuclear Problems, J I N R , such as a radio-frequency transition unit, a Penning ionizer and also to investigate the depolarizing processes during polarized ion injection and acceleration. 2. Theory There are two possible ways to convert the spin polarization of the atomic electron to the spin polarization of the atomic nucleus. The first one, quantummechanical, is accomplished via the adiabatic transition of the atoms from the region of the strong magnetic field into the region of the weak magnetic field, where the ionizer runs (Pmax<½-). The second technique includes the radio-frequency transitions and the ionization of atoms in the strong magnetic field (P ..... < 1). The last technique gives a possibility to change the proton spin polarization alternatively from P2 = P .... to P1 = 0, and, thus, to measure the spin polarization by means of atomic transitions between the substates of the metastable atoms. The structure of the hyperfine substates of hydrogen atoms in the 2 S u 2 and 2P1/2 states is given in fig. 1 in accordance with notations used by Lamb and Ruther-
498
YU.
A. P L I S S
AND
fordV). In the absence of any field, either electric or magnetic, the lifetimes of the metastable 2S~/z and 2P1/2 states are equal to r s = + s and rp = 1.6 x 10 - 9 S, respectively. The external electric field induces the Stark effect, and the lifetime of the metastable state diminishes greatly. At an electric field of E < 100 V/cm and a magnetic field B = 0, the lifetime of the 2S1/2 state is equal to "Cs =
--
× 10 - 6 s .
L. M. S O R O K O
The 1 and 2 levels and, respectively, the 3 and 4 levels cannot be separated by means of our photon counters and their populations must be grouped in pairs. The counting rates of two Geiger counters of the LSP, NI and N 2 can be considered as the indicators of the populations of a and fl levels which, in turn, have been produced in the cesium cell in a weak magnetic field. Namely,
(1) N1 = q l N ( f l ) = ½ q l [ l - ½ P ( l
The magnetic field induces a more severe effect on the lifetime of the metastable state than the electric one. Namely, the lifetimes of e and fl levels in the magnetic field B ~ 574 G, when these substates cross each other, are equal to r s = 1 . 1 3 x E - 2 [ ( 5 7 4 + B ) 2 + 7 1 6 ] x 10-9 s,
(2)
where E is the transversal electric field, V/cm, the plus sign refers to the a-level, and the minus sign refers to the fl-level. The lifetime of the c~-level is 1850 times longer than that of the fl-level. The relative populations of four levels: 1, 2, 3 and 4, depend on the spin polarization, P, of protons in the metastable atoms as follows: N(1) - I + P 4 '
N2=q2
N(3) 1-P 4 '
m I
40+x 2
,
(4)
N;-N~
P1 = 201 - - ,
N~-
(5)
P2 = 2 Cx - - , N2
(3)
m~
1000
0<
'
N2 -N;
where x = B/Bc, B is the magnetic field inside the cesium cell, and Bc = 63.4 G for hydrogen. '9 MH,e
2
where q l and q2 are the factors which depend on the temperature of the cesium cell, the beam intensity, the efficiency of the counters and other conditions. It has been supposed that the q-factors should vary in time only slowly. The two counting runs with radiofrequency transition unit " o n " , N +, N f , or "off", N t , N~, determine the two independent values of proton polarization
N(2)= ~1[ 1 - P ~/(l~x x 2i] ,
N(4) = ~ 1[ l + n x / ( 1 - +xx zi] ,
N ( ~ ) = ½q211 + ½ P ( l
40+
'~2
5O0
[]0 -500
~N -150(
Fig. 1. Hyperfine structure of the 2S1/z and 2P~/2 levels of hydrogen atoms in the magnetic field.
where
Cl = (t
____L__x )-1 4(1 + x 2)
In view of the time variations of the ql and q2 factors, these two values must be averaged in accordance with their statistical errors, APt and AP2: p = (AP1) z P2 + (AP2) 2 P, (AP1) 2 + (AP2) 2
(6)
The advantage of our counting technique consists in the fact that it is not necessary to measure the photon polarization similar to ref. 3. The polarized proton beam at the exit of the ionizer with a strong magnetic field has a longitudinal polarization. The proton velocity vector rotates over an angle of 90 ° in the analysing magnet. Due to the anomalous magnetic moment of protons the longitudinal component of the proton spin becomes ElL = P cos 90°(g/2--1) = P cos 161 ° = - P C 2 ,
499
LAMB S H I F T P O L A R I M E T E R
where C2 = 0.947. The transverse polarization does not play any role in the pick-up processes inside the cesium cell.
Due to crossing of the 2Sx/2 and 2P1/2 levels in the magnetic field B ~ 574 G as a result of the Stark effect in the electric field on the electrodes (15), the metastable atomic levels 2St/2 undergo the decay accompanied by the emission of Lyman photons 2 = 1216A. These photons are detected by the first photon counter (16). The second quenching electrodes (17) were placed downstream in the region of weak magnetic field. The second photon counter (18) views this quenching region. During the tuning runs the Faraday cup (19) can be interposed now and again in the beam to measure the total beam intensity. The operating part of the cesium cell has the shape of a cylinder 20 cm long and with an inner diameter of 2 cm. The restricting diaphragms at the entrance and at the exit of the cell having an inner diameter of 1.1 cm were designed to withstand the diffusion of cesium atoms of the operating part of the cesium cell. The oven (8) containing a 5 g cesium sample in the form of the break-seal vial (9) was connected with the operating volume through the heatable stainless steel valve (7). The oven together with the valve is heated separately from the operating volume. Two electric heaters were made of NiCr wire wounded on the tubes. The ends of the operating part of the cesium cell and the valve (10) are water-cooled. The temperature of the cesium cell is measured by a thermocouple on the digital voltmeter. To prevent cesium oxidation during the time between experiments argon was used as ballast gas.
3. The experimental systems A schematic view of the experimental system is shown in fig. 2. There are two principal parts: the polarized ion source (PIS) and the Lamb shift polarimeter (LSP), the PIS is shown only partly. The sextupole magnet (I) separates the ground states of the hydrogen atom according to their spin polarization. The hydrogen atomic beam H ° at room temperature velocities passes through the radiofrequency transition unit (2) which includes the permanent magnet with the transverse magnetic field B ~ 12 G growing downstream and the inductance coil excited by the hf-generator at a frequency v = 11.5 MHz and t]he amplitude of the rf magnetic field H ~ 1 G. The ionizer (3) using a high vacuum Penning discharge in the region of the strong longitudinal magnetic field, B ~ 1.5 kG, produces the proton beam of about 1 keV accompanied by various heavier ions. The ion beam emerging from the ionizer (3) crosses t]he analysing electric magnet (4). The protons of interest are deflected over an angle of 90 °, then focused by the einzel lens (5) and passed through the cesium cell (6). The rest of the charged components is deflected by the electrostatic field fed to the plates (12). At the entrance of the Helmholtz coils (13) the beam consisting of the fast atoms partly in the metastable 2Sx/2 state, partly in the ground state, are restricted by the diaphragm (14). 17 18
19
16
13
4. The experimental details In the course of the preliminary experiments we have found that the oven (8) and the valve (7) should be
4
9
3
Nt /n/.n.-~ 3-103
10
Ar~_...~ 11
~
-,-~ 2.103
Fig. 2. Schematic diagram of the experimental systems: (1) sextupole magnet, (2) rf transition unit, (3) ionizer, (4) analysing magnet, (5) einzel lens, (6) cesium vapour cell, (7) heatable valve, (8) cesium oven, (9) cesium vial, (10) unheatable valve, (11) argon vessel, (12) deflecting plates, (13) Helmholtz coils, (14) diaphragm, (15) 1st quenching plates, (16) 1st photon counter, (17) 2nd quenching plates, (18) 2nd photon counter, (19) Faraday cap.
103
o
1~o
. . . .
+~ t'~
Fig. 3. Counting rate of the first photon counter vs the cesium cell temperature.
500
YU.
A.
PLISS
AND
heated at different temperatures, namely, ~ 2 5 0 ° C at the valve (7), and ~ 120°C at the cesium cell. The temperature stable state is reached after an hour of heating. The counting rate of the first counter versus the temperature of the cesium cell is given in fig. 3. As can be seen, the small variations in temperature stongly affect the counting rate. There is a maximum in this curve, after which the counting rate diminishes considerably. To reduce the effect of random variations of temperature on the counting rates we made measurements simultaneously with two counters. The counting rates of the first and second photon counters versus the magnetic field in the centre of the Helmholtz coils are shown in fig. 4 for the case of the unpolarized beam. It is seen that the efficient quenching of the /?-levels of the 2S1/2 metastable state begins with a magnetic field of about 400 G. When the magnetic field is above ~540 G, the effective quenching volume moves upstream, the first counter stops to view this region and therefore its counting rate diminishes. The second photon counter responds to a-levels of the 2Stn state quenched by the strong electric field. The second curve in fig. 4 shows that with very high magnetic fields of about 500 G, the counting rate of the second photon counter for the unpolarized proton beam is equal to about ½ of the initial counting rate at B = 0 . To measure vacuum ultraviolet photons of 2 = 1216 A the window of the Geiger photon counter was made of a thin LiF crystal. The measured dead time of the Geiger counter is equal to about 1 ms. The intensity of the proton beam at the entrance of the cesium cell was typically about 10 9 protons/s. The intensity of metastable atoms amounts to 108 atoms/s and this induces the counting rate of 100-300 pulses/s.
L. M. S O R O K O
The following measurement procedure has been accepted: 1) The magnetic field in the centre of the Helmholtz coil was taken to be B = 500 G. The electric fields at the deflecting plates and at the electric field at the first quenching electrode were 20-30 V/cm and 10V/cm, respectively. The electric field at the second quenching electrode was 100 V/era. 2) The maximum proton intensity at the exit of the analysing electromagnet was set up. 3) The background counting rates, N b and N b, from two photon counters were measured. The "background" conditions correspond to the absence of the hydrogen atomic beam at the exit of the sextupole electromagnet, but to the presence of the Penning discharge in the ionizer. 4) The counting rates N~- and N 2, corresponding to the unpolarized proton beam, that is with open hydrogen gas supply and discharge in the dissociator and with the radiofrequency transition unit "off", were measured. 5) The counting rates N + and N~, corresponding to the polarized proton beam, that is with the radiofrequency transition unit " o n " , were measured. 5. Results Typical results of the measurements with a running time of 3 T = 300 s are the following: N + = 21020,
N~- = 23420,
N b = 13140,
N2 =
N2 =
Nb =
7930,
6980,
3570.
After the corrections for the dead time of the Geiger counter we have: N +(corr) = 25940, N ~ (corr) = 29560, Nb(corr) = 14990, N+(corr)= 8560, N~-(corr)= 7467, Nb(corr)= 3698. The polarizations P1 and P2 are equal to ( - P1)C1 C2 = 0.50_+0.04, (-P2)CIC2 0.58_+0.07. The average of these two mutually statistically consistent values is equal to ( - Po)CI C2 = 0.525 4- 0.035 and finally to
Nt
=
2.10z
2103
Po = - (0.59___0.04). 10:
1~
i
i
Fig. 4. C o u n t i n g H e l m h o l t z coils.
i
i
soo
i
8.; o ~o
~
i
8,o
r a t e v s t h e m a g n e t i c field in t h e c e n t r e o f t h e
As the running time is equal to 5 rain, the statistical error of the measurement is equal to 4-0.01 v/T, where T is the total measurement time in hours. The use of the photon counters with open photomultiplier would permit substantially to increase the counting rates and thus to diminish the necessary running time considerably.
LAMB SHIFT POLARIMETER
The authors wish to express their gratitude to V. G. Zinov for helpful discussions and to V. M. Grebeniuk for the assistance with the electronics. References 1) A. Galonsky, H.B. Willard and T.A. Welton, Phys. Rev. Lett. 2 (1959) 349.
501
2) G. G. Ohlsen, Rep. Progr. Phys. 35 (1972) 717. 3) G. Clausnitzer and D. Fick, Nucl. Instr. and Meth. 47 (1967) 171. 4) W. Heberle, Helv. Phys. Acta Suppl. 6 (1960) 140. 5) Yu. A. Pliss and L. M. Soroko, Sov. Phys. Uspekhi 15 (1972) 318. 6) L. M. Soroko and N. A. Toropkov, Avtorskoye Svidetelstvo (Inventor's Certificate) No. 283423. 7) W. E. Lamb and R. C. Rutherford, Phys. Rev. 79 (1950) 549.
I35 (I976)
497-5OI;
© N O R T H - H O L L A N D P U B L I S H I N G CO.
LAMB SHIFT POLARIMETER OF THE SLOW P R O T O N BEAM Y U . A. PLISS and L. M. SOROKO
Laboratory o f Nuclear Problems, Joint Institute for Nuclear Research, Dubna, U.S.S.R. Received 15 March 1976 The Lamb shift polarimeter of the slow proton beam for the polarized ion source of the strong current phasotron of the Laboratory of Nuclear Problems, Joint Institute for Nuclear Research, is described. The proton polarization of the polarized ion source has been found to be P = -(0.594-0.04). The polarimeter enables one to investigate the depolarization of protons and deuterons during their injection and acceleration.
I. Introduction
The currently used technique for the polarization measurement of ions emerging from the polarized ion source (PIS) involves the acceleration of particles up to the energy above the threshold of the nuclear reaction and the measurement of the axial asymmetry of nuclear reaction products. For example, the tensor polarization of deuterons has been measured in the T(d,n)~ reaction at a deuteron energy of about 100keW). To measure the vector polarization of deuterons or protons the beam energy must be increased still further. The discussion of this technique has been given elsewhere2). The updated requirements to the accuracy of spin polarization measurements on the one hand, and the need for the precise investigation of the process of spin depolarization during the acceleration of particles on the other hand, can be satisfied simultaneously by measuring the spin polarization of ions directly at the H S exit. As the energy of ions extracted from PIS is equal to 1-10keV, that is, much lower than the threshold of any nuclear reaction, the unique way is to else atomic processes, sensitive to the spin polarization of the atomic nucleus. The metastable atoms formed in the charge exchange of polarized ions offer a promising way. These atoms can be effectively produced and easily transported from one part of PIS to the other. The idea of using the metastable atoms for measuring the spin polarization of the atomic nucleus is not r~Lew3-6). The background of this idea is the relation l:,etween the populations of the hyperfine substates of the 2 $1/2 metastable state in the external magnetic and electric fields, on the one hand, and the spin polarization of the nucleus, protons or deuterons, of these metastable atoms, on the other hand. However, up to new nobody has built a device based on this principle. The Lamb shift polarimeter (LSP) of the slow proton
beam has been designed and built at the Laboratory of Nuclear Problems, JINR. It consists of a cesium vapour target, Helmholtz magnetic coils, two Geiger counters sensitive to Lyman photons of 1216 ~, an electromagnet to analyse the proton beam of energy of about l keV, and the vacuum system. The typical counting rates amount to about 102 counts/s which correspond to a relative error of measurement of _+0.01 x/T, where T is the measurement time in hours. The LSP can also analyse the spin polarization of the slow deuteron beam. The LSP is intended to optimize the working conditions of various parts of the P|S of the Laboratory of Nuclear Problems, J I N R , such as a radio-frequency transition unit, a Penning ionizer and also to investigate the depolarizing processes during polarized ion injection and acceleration. 2. Theory There are two possible ways to convert the spin polarization of the atomic electron to the spin polarization of the atomic nucleus. The first one, quantummechanical, is accomplished via the adiabatic transition of the atoms from the region of the strong magnetic field into the region of the weak magnetic field, where the ionizer runs (Pmax<½-). The second technique includes the radio-frequency transitions and the ionization of atoms in the strong magnetic field (P ..... < 1). The last technique gives a possibility to change the proton spin polarization alternatively from P2 = P .... to P1 = 0, and, thus, to measure the spin polarization by means of atomic transitions between the substates of the metastable atoms. The structure of the hyperfine substates of hydrogen atoms in the 2 S u 2 and 2P1/2 states is given in fig. 1 in accordance with notations used by Lamb and Ruther-
498
YU.
A. P L I S S
AND
fordV). In the absence of any field, either electric or magnetic, the lifetimes of the metastable 2S~/z and 2P1/2 states are equal to r s = + s and rp = 1.6 x 10 - 9 S, respectively. The external electric field induces the Stark effect, and the lifetime of the metastable state diminishes greatly. At an electric field of E < 100 V/cm and a magnetic field B = 0, the lifetime of the 2S1/2 state is equal to "Cs =
--
× 10 - 6 s .
L. M. S O R O K O
The 1 and 2 levels and, respectively, the 3 and 4 levels cannot be separated by means of our photon counters and their populations must be grouped in pairs. The counting rates of two Geiger counters of the LSP, NI and N 2 can be considered as the indicators of the populations of a and fl levels which, in turn, have been produced in the cesium cell in a weak magnetic field. Namely,
(1) N1 = q l N ( f l ) = ½ q l [ l - ½ P ( l
The magnetic field induces a more severe effect on the lifetime of the metastable state than the electric one. Namely, the lifetimes of e and fl levels in the magnetic field B ~ 574 G, when these substates cross each other, are equal to r s = 1 . 1 3 x E - 2 [ ( 5 7 4 + B ) 2 + 7 1 6 ] x 10-9 s,
(2)
where E is the transversal electric field, V/cm, the plus sign refers to the a-level, and the minus sign refers to the fl-level. The lifetime of the c~-level is 1850 times longer than that of the fl-level. The relative populations of four levels: 1, 2, 3 and 4, depend on the spin polarization, P, of protons in the metastable atoms as follows: N(1) - I + P 4 '
N2=q2
N(3) 1-P 4 '
m I
40+x 2
,
(4)
N;-N~
P1 = 201 - - ,
N~-
(5)
P2 = 2 Cx - - , N2
(3)
m~
1000
0<
'
N2 -N;
where x = B/Bc, B is the magnetic field inside the cesium cell, and Bc = 63.4 G for hydrogen. '9 MH,e
2
where q l and q2 are the factors which depend on the temperature of the cesium cell, the beam intensity, the efficiency of the counters and other conditions. It has been supposed that the q-factors should vary in time only slowly. The two counting runs with radiofrequency transition unit " o n " , N +, N f , or "off", N t , N~, determine the two independent values of proton polarization
N(2)= ~1[ 1 - P ~/(l~x x 2i] ,
N(4) = ~ 1[ l + n x / ( 1 - +xx zi] ,
N ( ~ ) = ½q211 + ½ P ( l
40+
'~2
5O0
[]0 -500
~N -150(
Fig. 1. Hyperfine structure of the 2S1/z and 2P~/2 levels of hydrogen atoms in the magnetic field.
where
Cl = (t
____L__x )-1 4(1 + x 2)
In view of the time variations of the ql and q2 factors, these two values must be averaged in accordance with their statistical errors, APt and AP2: p = (AP1) z P2 + (AP2) 2 P, (AP1) 2 + (AP2) 2
(6)
The advantage of our counting technique consists in the fact that it is not necessary to measure the photon polarization similar to ref. 3. The polarized proton beam at the exit of the ionizer with a strong magnetic field has a longitudinal polarization. The proton velocity vector rotates over an angle of 90 ° in the analysing magnet. Due to the anomalous magnetic moment of protons the longitudinal component of the proton spin becomes ElL = P cos 90°(g/2--1) = P cos 161 ° = - P C 2 ,
499
LAMB S H I F T P O L A R I M E T E R
where C2 = 0.947. The transverse polarization does not play any role in the pick-up processes inside the cesium cell.
Due to crossing of the 2Sx/2 and 2P1/2 levels in the magnetic field B ~ 574 G as a result of the Stark effect in the electric field on the electrodes (15), the metastable atomic levels 2St/2 undergo the decay accompanied by the emission of Lyman photons 2 = 1216A. These photons are detected by the first photon counter (16). The second quenching electrodes (17) were placed downstream in the region of weak magnetic field. The second photon counter (18) views this quenching region. During the tuning runs the Faraday cup (19) can be interposed now and again in the beam to measure the total beam intensity. The operating part of the cesium cell has the shape of a cylinder 20 cm long and with an inner diameter of 2 cm. The restricting diaphragms at the entrance and at the exit of the cell having an inner diameter of 1.1 cm were designed to withstand the diffusion of cesium atoms of the operating part of the cesium cell. The oven (8) containing a 5 g cesium sample in the form of the break-seal vial (9) was connected with the operating volume through the heatable stainless steel valve (7). The oven together with the valve is heated separately from the operating volume. Two electric heaters were made of NiCr wire wounded on the tubes. The ends of the operating part of the cesium cell and the valve (10) are water-cooled. The temperature of the cesium cell is measured by a thermocouple on the digital voltmeter. To prevent cesium oxidation during the time between experiments argon was used as ballast gas.
3. The experimental systems A schematic view of the experimental system is shown in fig. 2. There are two principal parts: the polarized ion source (PIS) and the Lamb shift polarimeter (LSP), the PIS is shown only partly. The sextupole magnet (I) separates the ground states of the hydrogen atom according to their spin polarization. The hydrogen atomic beam H ° at room temperature velocities passes through the radiofrequency transition unit (2) which includes the permanent magnet with the transverse magnetic field B ~ 12 G growing downstream and the inductance coil excited by the hf-generator at a frequency v = 11.5 MHz and t]he amplitude of the rf magnetic field H ~ 1 G. The ionizer (3) using a high vacuum Penning discharge in the region of the strong longitudinal magnetic field, B ~ 1.5 kG, produces the proton beam of about 1 keV accompanied by various heavier ions. The ion beam emerging from the ionizer (3) crosses t]he analysing electric magnet (4). The protons of interest are deflected over an angle of 90 °, then focused by the einzel lens (5) and passed through the cesium cell (6). The rest of the charged components is deflected by the electrostatic field fed to the plates (12). At the entrance of the Helmholtz coils (13) the beam consisting of the fast atoms partly in the metastable 2Sx/2 state, partly in the ground state, are restricted by the diaphragm (14). 17 18
19
16
13
4. The experimental details In the course of the preliminary experiments we have found that the oven (8) and the valve (7) should be
4
9
3
Nt /n/.n.-~ 3-103
10
Ar~_...~ 11
~
-,-~ 2.103
Fig. 2. Schematic diagram of the experimental systems: (1) sextupole magnet, (2) rf transition unit, (3) ionizer, (4) analysing magnet, (5) einzel lens, (6) cesium vapour cell, (7) heatable valve, (8) cesium oven, (9) cesium vial, (10) unheatable valve, (11) argon vessel, (12) deflecting plates, (13) Helmholtz coils, (14) diaphragm, (15) 1st quenching plates, (16) 1st photon counter, (17) 2nd quenching plates, (18) 2nd photon counter, (19) Faraday cap.
103
o
1~o
. . . .
+~ t'~
Fig. 3. Counting rate of the first photon counter vs the cesium cell temperature.
500
YU.
A.
PLISS
AND
heated at different temperatures, namely, ~ 2 5 0 ° C at the valve (7), and ~ 120°C at the cesium cell. The temperature stable state is reached after an hour of heating. The counting rate of the first counter versus the temperature of the cesium cell is given in fig. 3. As can be seen, the small variations in temperature stongly affect the counting rate. There is a maximum in this curve, after which the counting rate diminishes considerably. To reduce the effect of random variations of temperature on the counting rates we made measurements simultaneously with two counters. The counting rates of the first and second photon counters versus the magnetic field in the centre of the Helmholtz coils are shown in fig. 4 for the case of the unpolarized beam. It is seen that the efficient quenching of the /?-levels of the 2S1/2 metastable state begins with a magnetic field of about 400 G. When the magnetic field is above ~540 G, the effective quenching volume moves upstream, the first counter stops to view this region and therefore its counting rate diminishes. The second photon counter responds to a-levels of the 2Stn state quenched by the strong electric field. The second curve in fig. 4 shows that with very high magnetic fields of about 500 G, the counting rate of the second photon counter for the unpolarized proton beam is equal to about ½ of the initial counting rate at B = 0 . To measure vacuum ultraviolet photons of 2 = 1216 A the window of the Geiger photon counter was made of a thin LiF crystal. The measured dead time of the Geiger counter is equal to about 1 ms. The intensity of the proton beam at the entrance of the cesium cell was typically about 10 9 protons/s. The intensity of metastable atoms amounts to 108 atoms/s and this induces the counting rate of 100-300 pulses/s.
L. M. S O R O K O
The following measurement procedure has been accepted: 1) The magnetic field in the centre of the Helmholtz coil was taken to be B = 500 G. The electric fields at the deflecting plates and at the electric field at the first quenching electrode were 20-30 V/cm and 10V/cm, respectively. The electric field at the second quenching electrode was 100 V/era. 2) The maximum proton intensity at the exit of the analysing electromagnet was set up. 3) The background counting rates, N b and N b, from two photon counters were measured. The "background" conditions correspond to the absence of the hydrogen atomic beam at the exit of the sextupole electromagnet, but to the presence of the Penning discharge in the ionizer. 4) The counting rates N~- and N 2, corresponding to the unpolarized proton beam, that is with open hydrogen gas supply and discharge in the dissociator and with the radiofrequency transition unit "off", were measured. 5) The counting rates N + and N~, corresponding to the polarized proton beam, that is with the radiofrequency transition unit " o n " , were measured. 5. Results Typical results of the measurements with a running time of 3 T = 300 s are the following: N + = 21020,
N~- = 23420,
N b = 13140,
N2 =
N2 =
Nb =
7930,
6980,
3570.
After the corrections for the dead time of the Geiger counter we have: N +(corr) = 25940, N ~ (corr) = 29560, Nb(corr) = 14990, N+(corr)= 8560, N~-(corr)= 7467, Nb(corr)= 3698. The polarizations P1 and P2 are equal to ( - P1)C1 C2 = 0.50_+0.04, (-P2)CIC2 0.58_+0.07. The average of these two mutually statistically consistent values is equal to ( - Po)CI C2 = 0.525 4- 0.035 and finally to
Nt
=
2.10z
2103
Po = - (0.59___0.04). 10:
1~
i
i
Fig. 4. C o u n t i n g H e l m h o l t z coils.
i
i
soo
i
8.; o ~o
~
i
8,o
r a t e v s t h e m a g n e t i c field in t h e c e n t r e o f t h e
As the running time is equal to 5 rain, the statistical error of the measurement is equal to 4-0.01 v/T, where T is the total measurement time in hours. The use of the photon counters with open photomultiplier would permit substantially to increase the counting rates and thus to diminish the necessary running time considerably.
LAMB SHIFT POLARIMETER
The authors wish to express their gratitude to V. G. Zinov for helpful discussions and to V. M. Grebeniuk for the assistance with the electronics. References 1) A. Galonsky, H.B. Willard and T.A. Welton, Phys. Rev. Lett. 2 (1959) 349.
501
2) G. G. Ohlsen, Rep. Progr. Phys. 35 (1972) 717. 3) G. Clausnitzer and D. Fick, Nucl. Instr. and Meth. 47 (1967) 171. 4) W. Heberle, Helv. Phys. Acta Suppl. 6 (1960) 140. 5) Yu. A. Pliss and L. M. Soroko, Sov. Phys. Uspekhi 15 (1972) 318. 6) L. M. Soroko and N. A. Toropkov, Avtorskoye Svidetelstvo (Inventor's Certificate) No. 283423. 7) W. E. Lamb and R. C. Rutherford, Phys. Rev. 79 (1950) 549.