Beam-loss monitoring system with free-air ionization chambers

Nuclear Instruments and Methods 174 (1980) 401-409 © North-Holland Publishing Company

BEAM-LOSS MONITORING SYSTEM WITH FREE-AIR IONIZATION CHAMBERS H. NAKAGAWA *, S. SHIBATA, S. HIRAMATSU, K. UCHINO and T. TAKASHIMA Natzonal Laboratory for High Energy Physics, Oho-machi, Tsukuba-gun, [baraM-ken, 305, Japan Received 21 January 1980

A monitoring system for proton beam losses was installed in the proton synchrotron at the National Laboratory for High Energy Physics in Japan (KEK). The system consists of 56 air iomzation chambers (AIC) for radiation detectors, 56 integrators, 56 variable gain amplifiers, two multiplexers, a computer interface circuit, a manual controller and a high tension power supply. The characteristics of the AIC, time resolution, radiation measurement upper hmit saturation, kinetic energy dependence of the sensitivity, chamber activation effect, the beam loss detection system and the results of observations with the monitoring system are described.

1. Introduction

However, the preamplifier was frequently damaged by radiation. From the experience with the beam-loss monitoring systems described above, we decided to introduce a new type of monitoring system using long ionization chambers [2]. The requirements suggest that: (i) the system should be able to resolve beam losses to within one half cell (longitudinal resolution). (ii) The integrated signals from individual detectors should have time resolution of less than about 5 ms which is one tenth of the 50 ms stacking cycle. (iii) The sensitivity should have a 4 to 5 decade dynamic range. In order to observe the beam losses with no blind areas, 56 such chambers are installed around the ring. There are no active electronic components in the accelerator tunnel. The damage to the monitoring system due to radiation is thus avoided. Because slow response type ionization chambers (1 m s ) h a v e enough capability to observe beam losses, a free-air ionization chamber is used for the detector. The fast structure of beam losses can be observed with a few photo-multipliers. Thus argon chambers are not needed in our system, although an argon chamber has advantages at larger radiation dose rates than expected in our accelerator.

The beam intensity of the KEK Main Ring and Booster synchrotron has been increased greatly by continuous efforts in tuning and other improvements. About 2 "~ 3 X 1012 protons per pulse have been accelerated in the Main Ring (12 GeV). A large amount of activation will be produced by beam losses at higher energies and higher intensities. In order to operate the accelerator without beam losses, it is necessary to know: (i) when and where does the beam loss occur in the accelerator, (ii) what is the number of lost particles? The beam-loss monitoring systems were used at KEK, scintillation counter type detectors and ionization chambers with an argon mixture [ 1 ]. The former has a fast response time o f less than a microsecond but it was difficult to equalize the sensitivity of all the detectors. The argon chamber has a medium response time (1/as), and is 40 cm long, installed close to the vacuum chamber of the accelerator. The system required preamplifiers and sample/hold circuits in the ring, in order to match impedance between the chamber and the signal transfer cable and to avoid noise pick-up. This system was able to observe the beam loss distribution around the ring at any time with a manual control multiplex scheme.

2. Outline of the air ionization chamber (AIC)

* Graduate Student, Department of Physics, Htroshima University, Htroshima, Japan.

The xonization chamber and electronics should be dc coupled to be able to integrate the charge during 401

H Nakagawa et a l / A beam-loss momtoring system

402 ACCELERATOR

IONIZATION "CHAMBER

ROOM

CONTROL ROOM

The chambers are mounted on the cable rack on the outer wall of the accelerator tunnel. The chambers are thus 2 m away from the beam duct, but the separation between the chamber and Q-magnets is kept constant.

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Fig. 1. Simplified electronic ctrcult o f the AIC.

In air, an ionized electron from an atom is easily captured by oxygen,

an acceleration cycle of about 2 s. The circuit is shown in fig. 1. The chamber is made of a hollow coaxial cable, with the inner conductor supported by a spiral polyethylene insulator. The inner diameter o f the outer conductor (b) is about 22 mm, the outer diameter of the inner conductor (a) is 9 mm, the length is 6 m, and the effective volume is estimated to be about 1700 cm 3. 500 V high tension (HT) is applied to the outer electrode as the bias voltage (V) for the chamber. The average electric field strength (E) between the inner and the outer conductor electrodes is approximated as follows: V

E=(b_a)/2

7 7 0 V c m -1 ,

E/P ~ 1 V cm -1 Torr -1 ,

(1) (2)

where P is atmospheric pressure. The end structure of the chamber is shown in fig. 2. It is composed o f only four parts - two insulators, an aluminium cover (shield), and a BNC connector. One end of the coaxial cable is used to pick up the signal, where the BNC center pin is connected to the detector inner conductor. The other end is for the HT terminal, where the BNC is connected to the outer conductor. The insulataon resistance between the inner and the outer conductors is greater than 1014 ~ at 500 V under normal atomospheric conditions.

2 02 ~ 02 + O; + e- ~ O; + O~* -~ O; + O~. The ratio of electrons (e) to created ion pairs is,

Ne/No = exp(-r/X),

(4)

where Ne is the electron density, No the density of created ion pairs, rl the electron attachment probability by oxygen per unit drift distance, and X the electron drift distance. The attachment coefficient rl/P is (This data is extrapolated from ref. [3])

rl/P ~ 0.1 cm -1 Torr -1 .

(5)

The calculated density of the free electrons in the chamber space is reduced to 1/1000 after a flight of 0.9 mm by electron attachment. The time-of-flight is less than 0.1 /as, therefore most of the signal ~s due to ion current. The ion drift velocity v_+is given by

v+_=/a+E/P,

(6)

where the mobdities /a_+ are /1+_= 1140 cm 2 Torr V -1 s -1 for positive ions and /a_ = 1520 cm 2 Torr V -1 s -1 for negative ions [4].

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(3)

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Fig. 3. (Rise time) -1 versus high tension.

H. Nakagawa et al. / A beam-loss monitoring system

Ion collection time t is given b y

dP 0.21 t =d/o_ - -- -tl..E V '

(7)

where d is the distance between electrodes, d = (b - a)/2 = 0.65 cm and this gives the signal rise-time of the AIC. The measured dependence of the rise time on the HT is shown in fig. 3. The results agree quite well with those obtained b y calculation.

403

point x, Q" the charge induced on the inner conductor b y the space charges, and ¢ the azimuth angle around the chamber axis. Calculation o f eq. (8) shows that the electric field at point x is reduced to near zero b y a space charge o f q+(b) = Q_(b) = 1.1 /~C, which is equivalent to a current o f about 2.4 m A or 9 × 1012 ion pairs cm -s s -I . (2) Density effect recombination. Recombination An(x) is An(x) = a m ( x ) n_(x),

4. Radiation measurement upper limit saturation As the beam loss increases, an ion chamber becomes unable to collect all the ions produced in the chamber, due to space charge effects and 1on recombination effects [5]. (1) Effect of space charge on the electric field. The electric field is reduced by the space charge, this reduces ion and electron current flow and recombination takes place. In this case of a cylindrical chamber, the electric field E ( x ) at a point x is estimated as follows, E(x) =

- Q c + Q+(x) - Q_(x) + Q'_ - Q" 2~rXeo

(s) (9)

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(11)

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(12)

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log( b / x )/log(x /a ) 1 + log(b/x)/log(x/a)

where a 1S called the recombination constant, and is 1.4 X 10 -6 cm 3 s -1 for air [6], n+(x), n_(x) are the ion densities in the air chamber. If the radiation is stationary, the ratio o f the recombination to the created ion pairs can be calculated and we can estimate the original radiation. Since the beam loss in the accelerator is not stationary, the recombination ratio changes with time, Thus it is preferable that the AIC be used in its linear range, where recombination is negligible. If the recombination ratios allowed to be about 10%, an approximate calculation o f the recombination ratio can be done simply, under the conditions that the created ion pairs no(cm -a s -1) are stationary with time and uniform in the chamber. The fractional loss f is, An f - rr(b 2 - a 2) no

2 X 104arlo /-t+p-V2 '

x d~ d x ,

(13)

where C is the capacitance o f the chamber, q÷(x)and q_(x) the positive and negative charge densities at

(15)

where An is the total recombination. Eq. (15) is calculated from a formula from ref. [5], where g = 1 and p = 1 are used. Let f = 0.1, then the created ion pairs are, no ~ 1.5 X 1012 pairs cm -3 s -1 at V = 500 V.

0

(14)

(16)

This is equivalent to a current of about 400/~A. It IS possible to find the upper limit b y simulation with lower radiation and reduced HT, because the recombination ratio is reversely proportional to the square o f the HT. The recombination hmxt' was measured during slow extraction, where the beam loss was stationary. In fig. 4, the ion current from the chamber is plotted as a function o f the bias voltage. F r o m these results, the upper limit due ot recombination at 500 V is about no = 8 X 1011 pairs cm -a s -1, which is equivalent to a chamber current of 210/~A. This IS o f the same order of magnitude as the calculated value. A check o f the lineadty near the maximum radla-

404

H. Nakagawa et al. / A beam-loss monitormg system

NORMALIZED CURRENT (A) RATIO I 2~lp10-9

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Ftg. 4. Saturation of AIC, points are experimental data, the line is eq. (15), which is normalized at 50 V. tion was investigated at a beam energy of 12 GeV. The beam was not extracted and was lost in somewhere in the machine room. The experimental results are shown in fig. 5. The results show the linear relation between the current and the numbers of lost partlcles. From this experiment, the peak current of one chamber is (charge/0.5 ms ---)200 #A, when 1012 protons were lost in the ring during a very short time (<0.5 ms). Thus the ion current is expected to be proportional to the number of lost particles up to 1012 protons/300 ms. These results show that the AIC has

5

Fig. 6. Signal versus kinetic energy. enough capability to measure the beam losses at the KEK Main Ring.

5. Dependence o f the sensitivity on the kinetic energy of the beam The dependence of the sensitivity on the kinetic energy of the beam was obtained experimentally using a fast beam scraper [7] which is able to scrape the beam in several milliseconds at an arbitrary timing. Fig. 6 shows the relation between the signal (normalized by the number of lost particles) and the kinetic energy. The ion currents from the 56 AIC were summed and normalized as follows,

S I GNAL

s = ~ (~)dz~v.

250 . (DIGITS)

200

150

IOO

50

(17)

(AS)i is the beam-loss signal from the ith chamber, the total number of the lost particles. The errors are due to the quantization error of the 8-bit analog to digital convertor and the result shows that the signal is proportional to the beam kinetic energy.

//

6. Activation measurement The low input leakage current of the integrator enabled the measurement of the ionization current produced by activation down to the order of 10 -a Grayh -1 (1 G r a y = 1 0 0 r a d = 2 . 3 × 10" ion pairs cm-3).

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LOST PARTICLES AT 12GeV (x IOIIpI~P)

Fig. 5. AIC lmearity at 12 GeV.

The residual radiation produced by activation after turning off the beam was measured after two weeks operation. The result shows that the beam-loss signal

H. Nakagawa et al. / A beam-loss monitoring system

is about 100 times larger than the signal produced by the residual radiation, where both of the signals are integrated during an acceleration cycle. The residual radiation distribution is proportional to the beam loss distribution in normal operation. The system can monitor large activated areas, but it is not completely the same as the activation distribution on the beam duct because of the kinematical effects of the lost particles or differences of the decay times of activation.

7. Dynamic range of the system The dynamic range of the monitoring system needs to be at least 5 decades. Since this system is used for the tuning of the Main Ring, the requirement on dynamic range comes from a consideration of the numbers of lost particles under various conditions. The range of the radiahon dose produced by typical beam losses is estimated to be composed of the following factors; (i) 100 ttmes, for display resolution, (ii) 24 times, kinematical energy dependence (0.5-12 GeV), (iii) 9 times, single or 9 bunch injection, (iv) 4 times, single point or symmetrically distributed about the points in 4 superperiods. The total dynamic range is 100 )<24 )<9 )<4 = 86400 times.

(18)

In this system, the photo-isolation buffer (PIB) for the signal distribution limits the dynamic range. The system diagram is shown in fig. 7. The noise level of the present PIB is about 20 mVp_p. The signal-tonoise ratio, which is 10 V/20 mV = 500, limits the dynamic range of the electromc circmts. To improve the signal-to-noise ratio, variable gain amplifiers are installed between the integrators and multiplexers. The gains of the amplifier are )<2 and )<54, thus the dynamic range is extended to 500 × (54/2) = 13 500. When taking data with the computer the 8-bit analog to digital converter (A/D) limits the dynamic range. The A/D converts its input voltage - 1 0 ~ +10 V to a digital number 0 to 256. A second variable gain buffer amplifier is installed between the ANALOG multiplexer and PIB, the gains are )
405

further, some residual beam losses are not symmetrical over the four quadrant superperiod. In consequence, a range of 4 decades is sufficient capacity to observe beam losses under various operational conditions.

8. Electronics A schematic diagram is shown in fig. 7. The output current from each chamber is sent directly to the integrator in the control room via the coaxaal cable (1.5D-2V with a length of 100 ~ 250 m). The integrated signal is displayed on an oscilloscope through a manually controlled multiplexer, or transfered to the computer network to display the beam losses around the ring for a specific period. The circuit diagram of the integrator is shown in fig. 8. As shown in section 4, the maximum ionization charge from one chamber is on the order of #C. The capacitance ( G ) of the integrator is deterrmned in order to make the output voltage be 10 V at maximum beam loss. The capacitance is CI = 200 nC/5 V = 47 nF. The capacitors of the integrators are reset at the beginning of every machine cycle. This integrator can measure a charge of 20 pC over a time interval of 2 s. The dual mode system is provided for convenience. The computer display mode is called ANALOG, and the real time oscilloscope observation mode is called VIDEO. The output of the ANALOG multiplexer is connected to one of the satellite computers of the MELCOM-70 computer network [8] through a PIB and an A/D in the I/O module of the computer. Direct memory access with external synchronization is used. It takes about 1 ms for the 56 data points which is the conversion speed limitation of the A/D. A conversion timing schedule for the interrupt generator is set by the computer with the following information. (i) total number of snap shots per machine 'cycle (maximum 16), (ii) reference times (injection ready, first injection kicker pulse, acceleration start, fiat top start), (iii) delay time for data taking from reference time. This circuit enables the reading of the beam loss distribution at arbitrary times and the minimum time between successive loss pattern measurements is 2 ms. The VIDEO multiplexer is switched either by corn-

H Nakagawa et al. / A beam-loss monitormg system

406

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Fig. 7. System diagram.

High tension for the bias of the 56 AICs is supplied from a single power supply. No noise pick up was observed with a static shield and noise filter for each chamber. The HT power supply and the shield of HT cable is grounded at the control room to the integrator's ground, as shown in fig. 1.

puter or manual controller. The computer can automatically show the time dependent structure of the beam losses at the position of maximum radiation. The mode select is provided for real time observation at a specific point, or a histogram display of radiation in the tunnel until a given time.

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. . 47NF POLYPROPYLENE FILM CAPACITOR CONTROL Fig. 8. Circuit of integrator. R1. input bias current compensation; R2: offset compensation.

H Nakagawa et al / A beam-loss monitormg system

407

9. Software The software consists of three tasks, (1) control, (2) data taking and processing, (3) display. The "Control" task will perform, (i) set the timing of data taking, (ii) the selection of chamber location for VIDEO signal observation, Off) gain control. These can be performed automatically or at the operator's i n s t r u c t i o n . The satellite computer is capable of taking a maximum of 16 loss pattern distributions per machine cycle, and the data are stored in the memory (2 kB). This task is initiated from the center computer (CC) and at the same time the number of data patterns to be taken is sent from the CC to the satellite. The task is started by a signal from the interrupt generator, which is synchronized to the specific timing of the acceleration. At the end of data acquimion, the satelhte hoists its completion flag and returns a ready status to the CC. The center computer reads the data from the satellite and displays them on a graptuc display. The data displayed on the screen are, (i) radiation distribution by beam losses in the tunnel, and the total radiation around the ring, (li) variation of the total radiation in each acceleration. The integrator is reset at "injection ready" time and integrates the current during each cycle. The amount of the beam losses in the time interval between T1 and T2 is calculated from the data at time T1 and T2 by subtraction. The average distribution of the radiation upto a specific time in each acceleration can be displayed.

10. Results Typical displays are shown in figs. 9 - 1 4 . The integrated output of the raw signal, VIDEO mode, is shown in fig. 9. The location IS where the beam losses occur at the time of lnjecUon and T-energy transition. In fig. 9(a), losses at injection seem to be simple step functions. The fine structure of the time dependent beam losses are observed with a photo-multlpher. The typical radiation distribution in the tunnel is shown in fig. 10. Let the number of lost particles be

INJECTION

PHASE TRANSITION Fig. 9. Integrated output of raw signal. (a) at injection (I-1D) GAIN X 54, 2 V/dw, 0.2 s/dlv, (b) at transmon (III-5F) GAIN X 2, 0.2 V/ally, 0 2 s/div.

investigated, the rejected particles for the experiments from the Booster to the Main Ring are 4 ~ 5 × 10 lI protons per pulse. Thus the total rejected particles are about 4 × 1012 protons with 9 bunch injection. About 2 × 1012 protons are accelerated to 12 GeV, so the number of lost particles at mjectioll is about 2 × 1012 protons. Considering the gain and the dependence of the radiation and the beam kinetic energy, the observed total beam loss is estimated to about 2 × 1012 protons. The relation between the ion current and particle number shown in sections 4 and 5 is confirmed under normal operation. That is, (CURRENT) cx (LOST PARTICLE) × (KINETIC E N E R G Y ) .

(19)

408

H. Nakagawa et al. / A beam-loss monitoring system

KEN MAIN RING LOSS MONITOR

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Fig. 12. Actwation distribution around Main Ring GAIN X 54X4. KEK MAIN RING LOSS MONITOR

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79/06/28 20 3 7 5 7 14596 -002

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The distribution o f lost particles at extraction is s h o w n in fig. 11 w h i c h shows the differential b e a m losses. From the data the extraction efficiency can also be estimated. Finally the activation around the AIC i m m e d i a t e l y after b e a m stop ts s h o w n in fig. 12. This shows a good proportionality to the total radiation b y beam losses as s h o w n in fig. 10(b). Activation, w h i c h is created during a machine cycle, around the AIC is shown as

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Ftg. 10. Radmtion distribution beam losses. (a) Up to acceleration start, GAIN X 54 X 1 ; (b) up to flat top end, GAIN X 2Xl.

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0.5 where T is the b e a m k i n e u c energy at w h i c h the beam losses occur, A N is the n u m b e r o f lost p a m c l e s . Figure 13 shows a comparison b e t w e e n the loss m o n i t o r signal and activation on the surface o f the

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H Nakagawa et al. / A beam-loss monitoring system

RADIATION 3 (ARBITRARY) Feb I0 1979 Q Mar I0 1979

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areas are even after the beam is turned off. Improvement an the S/N ratio o f the PIB and the analog to digital converter in order to obtain wider dynamic range is in progress. The software for data processing and display will be extended. For example, there will be a three dimensional display o f detector posiUon, time and number o f lost particles. An alununmm chamber will be investigated to measure the activation o f the magnets and beam duct.

I 60 (MIN)

TIME

Fig 14 Decay of actwation around the AIC.

beam duct reported b y the Radiation Safety Control Department. The two sets o f data almost agree with each other. The lifetime o f the activation was measured with AIC to identify the source o f the radiation. The posslbihty o f the presence o f three components with halflives o f 10 min, 20 ~ 30 min, 13 h IS shown in fig. 14. The possible candidate for the halflife o f 10 rain would be 62Cu, or 6°mCo, that o f 20 ~ 30 min would be 6°Cu, and for a halfhfe o f 13 h would be 64Cu. It seems that the activation measured b y the loss momtor is due to chamber materials and some cables near the outer wall.

11. Conclusions This system has sufficient capability to observe the ra&atlon produced b y beam losses at the KEK Main Ring. The merits o f this system are: (1) very useful for tuning the machine. (ii) Maintenance is very easy, because all electronic components are outside the radiation affected area. (in) The system can show where the high radiation

We would like to acknowledge the encouragement of Prof. T. Kamei. We wish to thank Dr. R.L. Wltkover for his helpful advice in the design o f this system. Thanks are due to Dr. H. lshimaru for his useful suggestion and to Dr. S. Mitsunobu for many helpful discussion. We are indebted to Dr. R.P. Bissonnette for his careful reading o f the manuscript.

References [1] S. Hiramatsu, H. Ishimaru, H. Nakagawa, H. Nll~amshi, Y. Kojlma, S. Shibata and A. Takagl, 2nd Symp. on Accelerator Science and Technology, Tokyo (1978) p. 127. [2] R L. Wltkover, IEEE Trans. Nucl. Sci. NS-26 (1979) 3313. [3] L.M. Chanin, A.V. Phelps and M S. Biondi, Phys. Rev. Lett. 2 (1959) 344. [4] S. hda, K Ono, H. Kanzakl, H. Kumagai and S. Sawada, m. Shinpan Butsun Teisuhyo (in Japanese) (Asakura Shoten, Tokyo, 1978) p. 115. [5] J Sharpe, In: Nuclear Radiation Detectors (Methuen, London, 1964) ch. 6. [6] A. yon Engel, in: Iomzed Gases (Clarendon, Oxford, 1965) ch. 6. [7] H Ishimaru, Z. Igarashi, H. Nishlmura and S. Shlbata, IEEE Trans. Nucl. Sci. NS-26 (1979) 3358. [8] T. Katoh, K. Uchino, T. Kamm, M. Tejlma, T. Takashima, K. Ishii, S. Ninomiya and E. Kadokura, IEEE Trans Nucl. Sci. NS-24 (1977) 1789.