Large scintillation cells for high sensitivity radon concentration measurements

Nuclear Instruments and Methods 212 (1983) 403-412 North-Holland Publishing Company

LARGE SCINTILLATION MEASUREMENTS

403

CELLS FOR HIGH SENSITIVITY

RADON CONCENTRATION

B.L. C O H E N , M. E L G A N A Y N I a n d E.S. C O H E N University of Pittsburgh, Pittsburgh, PA 15260, U.S.A. Received 11 February 1982 and in revised form 20 October 1982

Methods for improving the sensitivity of scintillation cells for radon concentration measurements were studied with emphasis on improving light collection efficiency, This allows the length and hence the volume of the cell to be increased. Variables studied were choice of scintillator material, its method of application and thickness, length of cell, cell material, type and configuration of reflectors, choice of photomultipliers, and factors affecting background. Response from various areas of the cell surface was studied with an alpha source and with radon filling. Coating the window with phosphor was found to be counter-productive. The optimum results obtained were with the inside of the cell (other than the window) covered with a thick layer of ZnS(Ag), or with a thick layer of reflective material coated with a thin layer of phosphor. With it, a 10 cm diameter plexiglass cell can be extended to at least 50 cm length without difficulty from insufficient pulse height.

1. Introduction Probably the most widely used instrument for measurements of radon concentration in air is the scintillation cell, often called the "Lucas cell" [3,9]. It consists of a chamber whose inside walls are lined with crystals of silver activated zinc sulfide - ZnS(Ag) - and which can be evacuated and filled with the air sample; a transparent end (window) of the chamber is placed on the face of a photomultiplier tube which then detects the scintillations of light produced by alpha particles from the decay of radon and its daughters impinging on the fluorescent ZnS(Ag). Its principal advantage is a remarkably high detection efficiency, producing a usable count for 60-80% of all alpha particles originating within its volume. This corresponds to 1.8-2.4 counts per 222Rn atom disintegration since each such disintegration is followed within about 1.5 h by two alpha decays of its daughters, 218po and 214po. Although scintillation cells for radon measurement have been widely used for a quarter century and are manufactured routinely by at least five commercial suppliers, there is relatively little scientific information available on them in the literature. This does not imply that such information has not been collected, but such work has generally been done by groups with practical development goals and little taste for publication in the scientific literature. As a result, the use of scintillation cells is surrounded by a lore propagated largely by private conversations and heavily influenced by isolated experiences and hearsay evidence. While this lore has been very valuable, it is also somewhat variable among 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland

different practitioners and is not always reliable. The purpose of this paper is to attempt to put some of this information on a more scientific basis and hopefully to introduce a trend toward using the scientific literature rather than this lore transmitted by word of mouth for propagating information on scintillation cells. In particular, we report on studies of 10 cm inside diameter cells mounted on 5 inch (12.7 cm) photomultiplier tubes, with the primary goal of maximizing sensitivity.

2. Elementary considerations The obvious way of increasing the sensitivity of a cell is to increase its volume by increasing its length. The obvious disadvantage in doing this is that the amount of scintillation light that reaches the photocathode of the photomultiplier is reduced, giving smaller output pulses. This process cannot be tolerated if the pulses become comparable in size to background pulses not arising from alpha particles. Most of the work here is therefore directed toward identifying factors that maximize the difference in pulse size between true and background pulses. Since scintillation spectrometry played a central role in experimental nuclear and elementary particle physics from about 1950-1965, there is an extensive scientific literature on that subject including several excellent books and review articles [2,4,6,8,10,11], so our discussion here leans heavily on it. It has been conventional to discuss the pulses derived from scintillation detectors in

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B.L. Cohen et al. / Large scintillation cells

terms of their number of photoelectrons, N, originating at the photocathode of the photomultiplier. For a given applied voltage, the pulse height derived from the system is proportional to N. A method for calibrating this relationship by determining the absolute value of N for some pulse height - voltage combination is given in the appendix. A reasonably accurate substitute is to assume that the largest pulses (i.e. those from the photoelectric interaction) from gamma rays incident on a wellpackaged NaI(T1) scintillation detector in contact with a photomultiplier face correspond to about one photoelectron for each 500 eV of energy; this is the value given as typical in the literature, and it corresponds closely to that found in our case by the method given in the appendix. For example, the photoelectric peaks in the gamma-ray spectra for 241Am(60 keV) and 137Cs(660 keV) correspond to N = 120 and 1320, respectively. A ZnS(Ag) scintillator produces one photon (average wavelength = 4500 A = 2.8 eV) for each 9 eV of radiation energy deposited [7], and a typical photocathode (Type S-II) produces one photoelectron for every 7 photons in the sensitive wave-length range that strike it [11], so we may expect one photoelectron for every (7 x 9 =) 63 eV of charged particle energy deposited. Since the energy of the alpha particles from radon and its daughters is about 6 MeV, we might ideally expect N - 1 0 5 . The obvious source of background is thermionic emission of electrons from the photocathode, known as "dark current". It is readily shown that from statistical considerations this is expected to give very few pulses with N = 10 or larger. This would seem to give a factor up to (105/10 = ) 104 in signal to background ratio available to trade off against various desirable features of a cell. Unfortunately, maximum pulse sizes are at least an order of magnitude smaller, and background pulses are an order of magnitude larger than indicated by the above crude considerations, and some desirable features of a cell such as large volume require further order of magnitude sacrifices in quantity of light collected and therefore in N. The margin between signal and background is therefore a matter of primary concern in the design of a scintillation cell.

3. Experimental Three types of ZnS(Ag) phosphor were used in these studies: (1) U.S.R. Optimix (Box 409, Hackettstown, NJ 07840), formerly U.S. Radium Corp., Type BL300A ($120/kg). This is the type used by EPA-Las Vegas in their widely circulated and periodically up-dated Laboratory Procedures, and it is used in commercial cells by Rocky Mountain Glass Blowing Co. Average

size of individual crystals is about 8/~m, In the discussion, we designate this USR. (2) Sylvania (Precision Materials Grp., Towanda, PA 18848) Type 1330 ($18.75/kg, minimum order $200, samples available from the authors). This is the type used in commercial cells by Eberline, and in scintillator plastics often used in lining cells by William B. Johnson Assoc. Average particle size is 7/~m. We designate it SYL. (3) Dupont (Photo Products Dept., Wilmington, DE) Type 1101 ($75/lb), the type used by Environmental Measurements Laboratory in its widely used Procedures Manual HASL-300. We designate it DUP. Average particle size is 23 t~m, giving it a ' 'sandy" texture as contrasted with a "powdery" texture for USR and SYL. In addition, some work was done with (4) Plastic with scintillator imbedded, purchased from William B. Johnson Assoc. (Research Park, Montville, NJ 07045) which we designate JHN. (5) Plastic with scintillator imbedded purchased from Eberline Instrument Corp (P.O. Box 2108, Santa Fe, NM 87501), which we designate EBP. (6) A commercial cell, Eberline Model S.C-6, which we designate EBC. (7) A home-made cell coated with USR phosphor by Rocky Mountain Scientific Glass Blowing Co. (2520 Galena St., Aurora, CO 80010), which we designate RMG. Three principal methods were used for applying the phosphor: (1) EML method: A bonding solution consisting of Dow-Corning Methyl Silicone fluid DC200 (200,000 centipoise viscosity) - 30 ml, benzene - 285 ml, and cyclohexane 285 ml, was used to wet the surfaces and then drained off. After about 2 min of drying, the ZnS(Ag) was added and shaken around to give it maximum opportunity to stick to the surfaces, and then the excess was removed. (2) EPA-Las Vegas method: 50 gm of ZnS(Ag) was suspended in a solution consisting of butyrate dope 10 ml, amyl acetate - 100 ml, and acetone - 200 ml by constant mixing with a magnetic stirrer, and sprayed on the surface with an air-brush; several coats with interspersed drying are needed. (3) About 100 g of ZnS(Ag) was mixed with about 100 ml of a solution made up of 1 part of butyrate dope to 3 parts of thinner (Sig thinner, Sig Mfg. Co., Montezuma, IA 50171), and stirred to form a slightly syrupy liquid. In applying this to inside tube walls, the tube was rotated in a lathe while the liquid was spread over the surface, and turning in the lathe was continued until the coating was dry and hard. In applying to the flat top plate, the liquid was s imply distributed over the surface held in a horizontal position, and slow flow due to gravity plus manual spreading gave a uniform coating. Our method (3) is

B.L. Cohen et al. / Large scintillation cells

somewhat similar to that used by EPA-Montgomery. Additional methods used were: (4) Coating surfaces with stop-cock grease (various thicknesses) and shaking the ZnS(Ag) over it to obtain maximum sticking before removing the excess. This method which has been used at Argonne and by hobbyists seeking simplicity is essentially equivalent performance-wise with the bonding solution method, but the amount of phosphor adhering is a little less (5-10 m g / c m 2 vs 10-15 m g / c m 2 for DUP), coatings are a bit less uniform and are more easily damaged, and the method is a little more time consuming (once a stock of bonding solution is made up) - 8 min vs 5 min to coat a chamber after surfaces are cleaned - so this method was abandoned as being the less advantageous of the two. It would be useful and acceptable, however, if bonding solution or its ingredients is not readily available. (5) Covering surfaces with the scintillator imbedded plastic, J H N . The spray-on method (2) is exceedingly difficult with the D U P scintillator, as the large crystal sizes make its suspension in the solution difficult and short-lived. Some surfaces were coated with this combination for experimental purposes, but it can definitely not be recommended. The finer crystal size of the U S R and SYL scintillator make the spray-on method much easier for them. The bonding solution method is more favorable for the D U P phosphor; coating thicknesses are only about half as thick (typically 6 m g / c m 2 vs 12 m g / c m 2) for the U S R and S U L phosphor. This is understandable in terms of the smaller crystal size. If the crystals are of half the diameter of the D U P and the surface is coated at all points with one crystal, as seems to be roughly the case under microscope examination, the coating is just half as thick. Method (3), coating by pouring a syrupy liquid over the surface, was used to obtain thick coatings, 50 m g / c m 2 and more. The other methods were not feasible for this purpose. It is believed that the choice of bonding agent makes very little difference, as long as it is transparent and colorless. A few cells were coated with polystyrene dope, but our choice of butyrate dope was based on its ready availability in hobby shops (it is widely used as a cement for model airplanes) in addition to favorable experience with it by EPA-Las Vegas. It is believed that the choice of a thinner is even less important, since it evaporates away. Several different thinners were used in our work. The photomultipliers used in these studies included four RCA 2065 and one R C A 4522, all reclaimed from extensive prior spectroscopic use, and a new EMI-9530B, r e c o m m e n d e d by E n v i r o n m e n t a l M e a s u r e m e n t s Laboratory. The EMI-tube runs at considerably lower voltages (it is 11 stage while the others are 10 stage), but

405

otherwise there was little difference among the tubes. The photomultiplier anode was connected to an emitter follower, the output of which was fed to a linear amplifier with a gain of about 50 and an 0.5 tts time constant, producing a double-differentiated bipolar pulse. Experiments on light attenuation were carried out with a light source consisting of a gamma ray emitter on a packaged NaI(TI) crystal with light output through a plexiglas window designed normally to be placed on a photomultiplier face for gamma ray spectroscopy. The pulse height of the peak from photoelectric interaction of the gamma ray with the NaI(T1) is proportional to the amount of light reaching the photomultiplier face. A typical application is to measure the light attenuation by a coated surface by suspending the light source in air above the photomultiplier face and inserting surfaces with and without coatings between. The ratio of the pulse heights is then taken to be the percentage of light transmission through the coating. Radon filling of cells was obtained by use of a bubbler in which helium gas is bubbled through a solution containing 50 nCi of radium, into a 10 I sampling bag (Calibrated Instruments, 731 Saw Mill River Rd., Ardsley, NY 10502). The latter is then pumped full of air with an aquarium pump; the ratio of helium to air in the bag is only a few percent, so for alpha particle stopping considerations, the bag is effectively filled with air. The scintillation cell is evacuated and then filled from this sampling bag; the flexibility of the bag assures that the pressure in the cell is accurately equal to the local atmospheric pressure. No effort was made to determine absolute counting efficiencies for radon. Scintillation cells were constructed of 4" inside diameter, 1 / 4 " wall thickness plexiglas or aluminum tubes, a plexiglas window at the bottom and a plexiglass top with valves for evacuation, filling, and attachment of a vacuum gauge. The three pieces were held together by O-rings in grooves machined in the window and the top piece, so they were easily taken apart and interchanged. Tubes of various lengths were readily substituted for one another to vary the length of the chamber. The windows were of ultra-violet transmitting plexiglass. Standard electronic instrumentation was used for the ~mplifiers, multichannel analyser, discriminators, calibration pulser, and scalars.

4. Light transmission and response to monoenergetic alpha particles A basic question in trying to understand the light output in a scintillation cell is the response of the scintillator-photomultiplier combination to monoenergetic alpha particles, with the scintillator on the photomultiplier face. This introduces the problem of how to

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get the alpha particles into the scintillator since even a very thin layer of scintillator as applied by the bonding solution method substantially attenuates light. For the D U P phosphor, a 12 m g / c m 2 coating typically transmits 60% of the light from the NaI(TI) light source, while for the USR and SYL phosphor, a 6 m g / c m 2 coating typically transmits only 40-45%. This difference can be explained in terms of the different crystal sizes. With thin films, alpha particle range becomes important. From the range-energy curves shown in fig. 1 we see that the range of 241Am alpha particles in ZnS is about 9.2 m g / c m 2, or 23 p,m, so it has a reasonable chance of depositing all of its 5,5 MeV energy in the thin layer of D U P scintillator, but only a fraction of its energy in the USR, or SYL scintillator: in 8 # m ( = 3.2 m g / c m 2) of ZnS(Ag), the average crystal size for these, according to the energy loss curve in fig. 1 it would deposit (3.2 × 0.43 = ) 1.4 MeV, or in 5 m g / c m 2 (13/,m), the average layer thickness, it would deposit (5 × 0.43 = ) 2.2 MeV. Several different geometries were used involving thin and thick layers of scintillator. Fig. 2 shows the results for one of them in which a thick layer of scintillator was placed on top of the photomultiplier with a 241Am alpha particle source, transparent to light, sandwiched between, and an aluminum reflector on top. Some of the conclusions from fig. 2 are: (1) The U S R and SYL scintillators give larger pulses than DUP. This implies that 8 # m crystal size is better than 23/*m. In various tests, no consistent difference was found between pulse heights produced by USR and SYL. (2) The commercially available scintillator loaded

Fig. 2. Response of various phosphors to alpha particles from a 241Am source (S) in the geometry shown. Curves are arbitrarily

shifted vertically.

plastics, J H N and EBP, give much smaller pulse heights than thick layers of phosphor. For the former, the reflector plays an important role as evidenced by the smaller pulses indicated in fig. 2 when the aluminum reflector is not used. This suggests that the thick layers of phosphor many times the thickness needed to stop the alpha particles, may play an important role as a reflector, (3) The N = 105 derived from theory in sec. 2 is not nearly achieved. Even under the most favorable circumstances, very few pulses have N = 10 4, and most are several times smaller. This loss of output light may be explained by quenching of excited states in the crystals, inhomogeneities in the material, and internal reflection from crystal surfaces.

5. Reflectors

It seems to be common practice not to use reflectors on the outside walls of plexiglass scintillation cells. They are not recommended or used by Environmental Measurements Laboratory, by Eberline in its Commercial cells, or by the Rocky Mountain Scientific Glass Blowing Co. in its glass wall cells, and we know of no Laboratory that uses them. However, they provide an obvious advantage in returning light that would otherwise be lost, and no one has offered a reason for not using them._ Their effect on a 10 cm long chamber is shown in fig. 3. The aluminum reflector is simply household aluminum foil wrapped around the cell and held in place by tape. The TiO 2 reflector was a commercial reflective coating, Nuclear Enterprises (931 Terminal Way, San Carlos, CA 94070), Type NE560 painted on

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the surface. According to i n f o r m a t i o n in the literature [10], TiO 2 is not an o p t i m u m reflector since its reflectivity falls from 88% for 4500 A, a n d 30% for 3700,~, whereas the M g O reflectivity is a b o u t 97% and a l u m i n u m has a reflectivity exceeding 85% over this wavelength range. [ZnS(Ag) scintillation light peaks at 4500 A, and the S-11 p h o t o c a t h o d e peaks at 4300 A, falling to 50% of peak value at 3600 A and 5600 A]. However, it is i m p o r t a n t that painted coatings adhere reliably a n d do not contain binders that turn yellow with age, and the commercial coating used here has been optimized on those bases. Moreover, according to the data in fig. 3, it performs better t h a n a l u m i n u m (of course the b o n d i n g to the surface is quite different). The convenience in using it to help make the cell light tight also argues in its favor. Where very thick p h o s p h o r coatings are used, the value of a reflector is, of course, m u c h diminished, since the p h o s p h o r itself acts as the reflector and little light can reach an external reflector.

6. Pulse height dependence on impact location In a scintillation cell, alpha particles impact on the ZnS(Ag) all over the inside surface, so in order to u n d e r s t a n d cell p e r f o r m a n c e it is useful to determine the dependence of pulse size on impact area. A n u m b e r of preliminary studies were carried out by placing our light source at various locations a n d orientations. The pulse height falls off with the distance of light source from the photomultiplier face, by a b o u t a factor of two for each 7 cm of distance. Directing the light from the

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source perpendicular to the direction to the photomultiplier, thus relying on reflection from the cell walls, decreased the pulse height by a factor of about three. A n u m b e r of studies was done with an alpha particle source in various geometries and limited areas of the cell covered by scintillator. As an example, fig. 4 shows the pulse height spectra with the 241Am source, transp a r e n t to light, placed on the cell surface at various points. F r o m these studies it was found that the pulse height depends sensitively on: a) The distance from the photomultiplier face; this is the most p r o m i n e n t feature of fig. 4. b) The orientation of the scintillator surface relative to the direction toward the photomultiplier; note the differences between positions 4 and 5, and between positions 3 and 5 in fig. 4. This indicates that the top of the cell is an i m p o r t a n t contributor; there has been some tendency to downgrade the importance of its contribution. c) The direction of the alpha particle, as indicated by studies other than the one shown in fig. 4. But the most direct way to determine the contribution to the pulse height spectrum from various impact areas is to prepare a cell with only one of these areas covered with p h o s p h o r at a time, a n d observe the resulting pulse height spectra when the cell is filled with radon. This was done with a 17 cm long cell using J H N plastic as the phosphor. The results are shown in fig. 5 where we see that they are generally consistent with expectations from fig. 4, although the differences between pulses from the various areas is not as large. This ,nay be partly due to the fact that much larger areas were covered to get reasonable c o u n t rates; in fig. 4,

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binder. However, 1 ml of butyrate dope was found to harden into only 110 mg of solid, so a mixture of 100 gm of ZnS(Ag) with 25 ml of dope results in only 3% of the mass being inert. For a surface covering of 100 m g / c m 2, the total inert matter is only 3 m g / c m z which is barely enough to cause a problem if it were all on the surface. Actually, it seemed to be well distributed through the phosphor which makes this problem negligible.

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positions 2 and 4 correspond to the extreme bottom and top respectively of the side walls, whereas in fig. 5, one third of the side wall height was covered in each test.

7. Radiation physics considerations If the alpha particle originates in the air filling the chamber, as is certainly the case for the 2 2 2 R n decay, and is perhaps sometimes the case for the 2JSpo and 214po daughters, there is appreciable energy loss before reaching the phosphor. We see from fig. 1 that the energy loss curve partially compensates for this - for example, if a 6 MeV alpha particle expends 75% of its range in air, it is still left with 33% of its energy to transfer to the phosphor - but this effect is probably over-compensated by the fact that scintillators respond non-linearly in giving less light per unit energy absorbed for low energy alpha particles. The curves in fig. 1 indicate that some of the thinner ZnS(Ag) coatings used in our studies do not stop full energy alpha particles. The extreme case is for a 2n4po alpha particle incident normally on a 5 m g / c m 2 surface, depositing only 1.9 MeV of energy. In general, however, this effect is relatively unimportant in view of the larger variations in pulse height due to other factors. In principle, there might be some concern that the surface of the phosphor is covered with an inert layer of

Traditionally, the inside of the cell window is not coated with scintillator so as to maximize the light transmission from other parts of the cell to the photomultiplier. However, some Laboratories do coat this window, and this is strongly recommended by Environmental Measurements Laboratory to increase detection efficiency. If this is done, the discussion in sec. 4 clearly indicates that the bonding solution method with D U P phosphor should be used since this gives the maximum light transmission, typically about 60%. The smaller pulse height yield from this coating is not a problem here because scintillations originating on the window give much larger pulses than those from other parts of the cell (cf. fig. 4). It was found that coating the cell window increases the count rate from a given radon filling by about 15% for a 17 cm long cell. This is in rough agreement with the fact that the window represents 11% of the inside surface area of the cell, especially considering the fact that all parts of the surface (e.g. corners), are not equally exposed to the alpha particles. In most situations, a 15% additional count rate (i.e. sensitivity) does not nearly compensate for a 40% loss in pulse height since lengthening the cell gives a larger increase in sensitivity with a lesser sacrifice in pulse height. Direct experimental data on this question will be presented in sec. 10.

9. Miscellaneous factors affecting light collection It seems to be conventional not to use an optical coupling fluid between the photomultiplier face and the window of the scintillation cell. This saves trouble if the cell is frequently removed to be used for sample collection. In other areas of scintillation spectrometry, an optical coupling is always used, as it improves energy resolution by improving light collection. A measurement with our light source hanging in a scintillation cell indicated that optical coupling with Dow-Corning silicone fluid DC-200, which is widely used for this purpose, increased the pulse height by 40%. There are at least two easy methods for sample

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collection without removing the cell from the photomultiplier face. One is to leave the two connected and carry the photomultiplier along with the cell (this requires careful handling), and the other is to collect samples in a sampling bag and use the latter to fill the cell. It should be noted that either of these procedures also avoids constantly opening and closing the light seal, which not only avoids that inconvenience and the attendant risk of destroying the photomultiplier by exposure to light under voltage, but also allows the photomultiplier to be kept in permanent darkness, which is advantageous since noise resulting from exposure to light can persist for several hours or even for a few days. The sampling bag method has been adopted here.The light loss due to transmission through, and reflections from the surfaces of the plexiglas cell window was measured by hanging the light source in the middle of the cell volume and measuring its pulse height with and without a cell window between it and the photomultiplier face. The window reduced the pulse height by 17%. Light absorption in plexiglas is reported to be only about 50% per meter, so the 17% loss here is essentially entirely due to reflections from surfaces.

I0. Cell performance The performance of a cell may be judged by the size of the pulses it produces, which we express as the F

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distribution of N values. This is determined by filling the cell with air containing an appreciable quantity of radon, and measuring the pulse height distribution it produces after waiting at least 1.5 h for equilibrium with the radon daughters. Typical count rates were 2000 cpm. Early measurements were mostly made with D U P phosphor applied by the bonding solution method, with the cell window coated - this is the Environmental Measurements Laboratory (EML) procedure. Some of the results are shown in fig. 6. Also included there are two cells with the cell window clear (i.e. not coated), shown by dashed lines. In general, cells were plexiglass covered with a reflector on the outside, but fig. 6 also shows data for a cell made from aluminum tubing (polished by lathe on the inside with emery cloth) and for a plexiglass cell not covered with a reflector. One striking observation from fig. 6 is the rapid deterioration of pulse height with increasing length of the cell for this coating technique. The pulse height of the peak of the distribution falls by a factor of 2 as the length is increased from 10 cm to 17 cm, and by at least another factor of 2 as it is increased from 17 cm to 28 cm. Since the curves in fig. 9 have been arbitrarily shifted vertically, there is no significance in the areas under them. If we interpolate between the curves, it appears from fig. 6 that leaving the window uncoated on a 17 cm long cell gives a pulse height distribution equivalent to that of a 12.5 cm long cell. Thus, if our starting point were a 12.5 cm long cell with a clear window and we wanted to i

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N Fig. 8. Pulse height distributions from various length cells coated with thick ( - 100 mg/cm 2 ) USR phosphor. Curves are arbitrarily shifted vertically.

increase its sensitivity by sacrificing about 40% in pulse height, we could either (a) coat the window with phosphor, which increases its sensitivity by 15%, (b) extend its length to 17 cm, which increases its sensitivity by 30%. The obvious choice is (b); by extending this argument it is always more favorable to to increase sensitivity by extending the length rather than by coating the window with phosphor. This reinforces our conclusion from sec. 8. A n o t h e r striking observation from fig. 6 is the poor p e r f o r m a n c e of a solid a l u m i n u m tube when coated by the b o n d i n g solution method. With the thin coating, the surface appears gray rather than white; evidently the a l u m i n u m does not serve very well as a reflector. However, the poorest performance in fig. 6 is for a plexiglas cell with no reflector on the outside. The peak of the distribution is at a b o u t 40% of the pulse height of the same cell with a reflector, and is lower than that for a 28 cm cell with a reflector. By extrapolation we estimate that approximately the same pulse height distribution is obtained from a 30 cm long cell with a reflector as from a 17 cm long cell without a reflector. Since the former has nearly twice the sensitivity of the latter and application of an external reflector is so simple, it is difficult to justify not using it. The dependence on p h o s p h o r type of the perform a n c e of a cell coated by the b o n d i n g solution method

was studied. Little difference a m o n g the three types was found. By far the most i m p o r t a n t factor affecting cell performance was found to be the thickness of the p h o s p h o r coating. Data on this are shown in fig. 7 where coatings are with U S R p h o s p h o r (except for the J o h n s o n Lab plastic ( J H N ) and the Eberline Commercial cell (EBC) which use SYL), a n d all cells are 17 cm long plexiglass with an outside reflector. Recall again that curves have been arbitrarily shifted vertically, so no information on c o u n t rates or efficiency should be inferred from fig. 7. The thick ( - 100 m g / c m 2) coating, applied by m e t h o d (3), gives by far the largest pulse height. The next largest pulses were obtained from the cell coated by Rocky M o u n t a i n Scientific Glass Blowing C o ( R M G B ) which are estimated to be about 40 m g / c m 2 thick. Somewhat smaller pulses were obtained from our spray coating [method (2)] which is estimated to be a b o u t 20 m g / c m 2 thick. Our spray coating was much smoother a n d more uniform the R M G B coating is quite rough and n o n - u n i f o r m but apparently the thickness is of over-riding importance. The thin (6 m g / c m 2) coating obtained by the b o n d i n g solution m e t h o d gives very much smaller pulses, only a b o u t one-third the size of those from the thick coating. Since the thicknesses used here greatly exceed the range of the alpha particles, these results can only be explained as due to high efficiency of the p h o s p h o r for reflecting light. The p h o s p h o r does not become completely opaque until the thickness exceeds 100 rag/cruZ; for example, the R M G B coating transmits 12% of the light from our light source. The dependence of pulse height on cell length for thick coatings is shown in fig. 8. We see that it is very much less than the dependence for thin coatings shown in fig. 6, and this allows the use of very much longer cells. A 38 cm long cell with a thick coating gives larger pulses and a more favorable distribution of pulse sizes than a 10 cm long cell with a thin coating. In fact, it appears from fig. 8 as though cells can be made well over 50 cm long without pulse height limitations causing difficulty (in our situation, the length was limited by the equipment for applying the coating). This is three times longer than cells in current use, and therefore their sensitivity would be three times higher. (Note that the curves in fig. 8 have been arbitrarily shifted vertically so there is no significance to the areas under them.) We have pointed out that the reason for the large pulse heights from the thick p h o s p h o r coatings is that the thick p h o s p h o r acts as a highly efficient reflector. This suggests that a thick coating of an alternative reflecting material would serve equally well. In order to test this, cells of various types were coated with thick layers of M g O or of alpha alumina, b o t h of which are well recognized to be highly efficient reflectors, and ZnS(Ag) was sprayed on top. They gave pulse sizes

B.L Cohen et aL / Large scintillation cells

equivalent to those from thick ZnS(Ag) layers. The former method is more difficult and there was some loss in detection efficiency because of imperfect covering with the sprayed scintillator, so it was abandoned. Cells were also constructed with a MgO reflector on the outside of a plexiglas cylinder whose inside was coated with a thin layer of scintillator, but its performance was much poorer than with the thick scintillator on the inside. An attempt was made to use a thick efficient reflective layer (MgO) on the inside of the cylinder with J H N plastic covering it, but this performed no better. As a result of these tests it was decided to adopt as standard a 38 cm long cell with its inside walls coated with a thick ( - 100 m g / c m 2) layer of ZnS(Ag).

11. Background The background for a scintillation cell is the count rate due to anything other than radon or its short half-life daughters. It may be conveniently measured by evacuating the cell, or filling it with an aged gas such as nitrogen from a tank. However, understanding its sources can be somewhat complex. Sources originating in the photomultiplier tube can be separated out by taking data with the scintillation cell removed. The results for our six tubes are shown in fig. 9. Four of them gave essentially identical results while the other two gave somewhat higher background. Our conclusion is that, with a threshold set at N = 150 or larger, background from the photomultiplier is not usually an important problem. If lower photomultiplier backgrounds are advantageous, they are easily obtained by relatively small increases in discrimination level. It is interesting to discuss the source of this back-

I

I

I

3-RCA 2 0 6 5 I - RCA 45P 2 A

4

Z

2

~-RC~2065\ \ ~ \ PlexicjIoss

~ 0.4

g

~ 0.2

0. I

~

E 510

M

I I IO0

-

"I 150

I 200

N Fig. 9. Count rates for pulses with N larger than the abscissa value for various photornultipliers without scintillation cells. Dashed curve is the addded effect of a long plexiglass cell not coated with phosphor.

411

ground. The most obvious source is thermionic emission from the photocathode, but this would very seldom give pulses with N > 10. Multi-electron noise pulses have been recognized in the literature for a long time, and some of the sources that have been discussed [!] are field emission, scintillations in the glass, electroluminescence in the glass, light from the anode current, and positive ions in the residual gas which would be accelerated into the photocathode. Another source of background is the plexiglass of which the cells are constructed which behaves as a scintillator or Cherenkov radiation detector for cosmic rays. To test its contribution, a newly constructed large chamber to which ZnS(Ag) had not yet been applied was mounted, and the pulse height distribution is shown by the dashed line in fig. 9. In practical 38 cm long scintillation cells coated with thick layers of phosphor, elimination of background when counting environmental air samples with typical count rates of 5 cpm requires a discrimination level of about N = 450. This gives about 0.5 cpm of background. The peak in the radon spectrum is typically at about N = 1500. Signal becomes equal to background for a radon concentration of 0.05 pCi/1 which means that concentrations down to 0.01 pCi/1 can be measured without difficulty in a few hours. One source of background that is often discussed is 2t°po, a residue from decay of radon daughters which itself decays by alpha particle emission with the 22 year half-life of its 21°pb parent. We can expect one eventual count from this for every three counts of regular operation, with the former initially at a rate based on its being spread out over (22/ln2 = ) 32 y. If the air in the chamber gives an average of 6 counts per minute, after one year this background rate is ( 1 / 3 x 6 x M / 3 2 M = ) 0.06 cpm where M = 5 x 105 is the number of minutes in one year. This background increases linearly with time, so it should not become appreciable for several years with ordinary environmental monitoring, even disregarding the loss of this material on the frequent occasions when the cell is evacuated. If the cell is filled with a high concentration of radon, say 1000 cpm for two hours to measure a pulse height spectrum a n d / o r obtain a count rate-voltage relationship, the added background is ( 1 / 3 x 120 × 1000/32 x 5 x 105 =)0.002 cpm. There is therefore no apparent reason not to fill the cell with a high concentration of radon periodically to make checks of this type. We have found it necessary to repeatedly pump and flush the cells with air for many hours after doing this; presumably radon gas adsorbs onto surfaces and requires this type of treatment for removal. Of course, particles of radium or other long half life alpha emitters must not be allowed to enter the cell. To avoid this, scintillation cells should be filled through filters capable of removing radioactive dust. We have

412

B.L. Cohen et al. / Large scintillation cells

been using 0.6 micron pore size nuclepore filters for this purpose.

the anode is the peak voltage, V, of the pulse produced, easily obtained with a calibrated oscilloscope, which is related to Q by

The authors are indebted to D. Kulwicki and K. Warner for help in various aspects of this work.

V = Q/C,

Appendix Determination of absolute value of N

2.36/~N,

(1)

where I / ~ / N is the fractional standard deviation of N due to statistical fluctuations, and the numerical factor converts from standard deviation to full width at half maximum in which terms A E is conventionally expressed. Actually eq. (1) gives an under-estimate of A E since further statistical spreading occurs in the electron multiplication process. Taking this into account and solving eq. (1) for N gives [10] N-

5.56 S ( A E / E ) 2 S - 1'

(2)

where S is the average electron multiplication per stage in the tube. For a given gamma ray line, A E / E is easily measured from the observed pulse height spectrum so eq. (2) gives a relationship between N and S. Another relationship between N and S is obtained from the electron multiplication process leading to collection of electric charge Q at the photomultiplier anode, Q = UeSq,

where C is the capacitance to ground from the anode. A simple way to determine C is to add a known capacitance, C' (e.q. a short length of cable of known capacitance per foot) and observe the new voltage, V'. Then Q = C V = ( C + C') v',

When a scintillator detects a radiation that is sharply defined in energy (E), such as a gamma ray from a single nuclear transition interacting with the crystal through the photoelectric effect, the spread in observed pulse heights is due to statistical fluctuations in the process. If all of these statistical fluctuations, including those in the conversion of radiation energy into photons, the collection of these photons, their conversion into photoelectrons, and the multiplication of electrons in the photomultiplier, are effective in causing a statistical spread in values of N, the number of photoelectrons emitted from the photomultiplier cathode in response to each event, the spread in observed energy, ,~ E, for the peak would be AE/E=

(4)

(3)

where q is the number of multiplication stages in the tube, known from its design, and e is the well known charge of the electron. What is ordinarily measured at

which may be solved to give C as C=

C'V'/V 1 -

(5)

v'/v

Using eq. (5) in eq. (4) to determine Q and inserting its value in eq. (3) then converts that equation into a second relationship between N and S which can be solved simultaneously with eq. (2) to determine N (and S). The simultaneous solution is simple if various values of S are assumed and used to calculate values of N from eqs. (2) and (3) until these two are equal. Actually eq. (2) gives an under-estimate of N since other statistically variable factors contribute to A E. The relative importance of these can be minimized by making the contribution of variations in N [i.e. a E / E in eq. (2)] as large as possible, as by using a low energy gamma ray (e.g. 241Am) on a NaI(TI) crystal some distance away from the face of the photomultiplier.

References [1] J.A. Baicker, IRE Trans. Nucl. Sci. NS-7 (1960) 74. [2] J.B. Birks, The theory and practice of scintillation counting (MacMillan, New York, 1964). [3] R.J. Budnitz, Health Phys. 26 (1974) 145. [4] C.E. Crouthamel, Applied gamma ray spectrometry (Pergamon, New York, 1960). [5] S.C. Curran, Luminescence and the scintillation counter (Academic Press, New York, 1953). [6] W.H. Jordan, Annual Rev. Nuc. Sci. (1952) 207. [7] H. Kallmann, Phys. Rev. 75 (1949) 623. [8] G.F. Knoll, Radiation detection and measurement (Wiley, New York, 1979). [9] H.F. Lucas, Rev. Sci. Instr. 28 (1957) 680. [10] W.E. Mott, and R.B. Sutton, 1958, in Handbuch der Physik, vol. 45 (Springer, Berlin, 1958) p. 86ff. [11] J.H. Neiler, and P.R. Bell, in Alpha, beta, and gamma ray spectroscopy, ed., K. Siegbahn (North-Holland, Amsterdana, 1966) p. 245ff.