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Activation of air near a target bombarded by 3 GeV protons
NUCLEAR
INSTRUMENTS
AND
METHODS
75
(I969)
93-IO2;
©
NORTH-HOLLAND
PUBLISHING
CO.
A C T I V A T I O N OF AIR NEAR A T A R G E T B O M B A R D E D BY 3 GeV P R O T O N S M. A W S C H A L O M et al.
National Accelerator Laboratory, Batavia, Illinois, U.S.A. F. L. L A R S E N a n d W. S C H I M M E R L I N G
Princeton-Pennsyh, ania Accelerator, Princeton University, Princeton, New Jersey, U.S.A. Received 23 J u n e 1969 M e a s u r e m e n t s o f 140, 150, 13N, l l C created in air a r o u n d internal a n d external accelerator targets are reported for an incident p r o t o n energy o f 3 GeV and Fe, Pt a n d Pb targets. For incident p r o t o n currents o f 1011 p/see, average radionuclide
concentrations o f 5.4 x 10 9/~Ci/cm z o f l Z N a n d 8.0 x 10-9/~Ci/cm 3 are f o u n d in a typical experimental cave. A l s o , a precision m e a s u r e m e n t o f the 11C-half life is reported as T÷(11C) = 20.404-0.04 min.
1. Introduction
beam. Irradiation times were of the order of 20 min at beam intensities of 2 x 1011 protons/see. The purpose of these measurements was to obtain adequate normalization of activity to beam intensity, and to estimate the parameters of the angular distribution; as well as to measure the concentration of radioactive nuclei produced by activation of the components of air. In the external beam caves, these measurements were limited by the sample retrieval time of approximately five minutes to radioactive nuclides of half-lives greater than a few minutes. The measurements around the internal target were oriented toward ascertaining the presence of shorter lived radionuclides, as well as to measure the airborne radioactivity. A quick sample retrieval system ( " r a b b i t " ) was constructed for the retrieval of samples exposed around the internal beam target. In order to have the sample as close to the target as possible in a known reproducible geometry and to minimize the time necessary for bringing the samples to the counting equipment, the rabbit operated between the vicinity of the internal target and the counting room. The interval between the end of the irradiation and the start of counting was one minute on the average. The irradiation times were of the order of 40 min of continuous machine operation in order to approach saturation (about two half-lives of 11C). Counting intervals in any one run varied from 30 sec to 5 min, depending on the half-life of the dominating activity. Sample materials to represent pure oxygen and nitrogen were distilled water and analytical grade a m m o n i u m nitrate (NH4NO3). All the nuclides of interest are positron emitters with the exception of 7Be. In order to take advantage of this fact, this experiment was designed to relate the initial activaties of each nuclide to the decay rate of
A measurement has been made of the concentrations of certain airborne radionuclides produced by activation of air in the vicinity of targets bombarded by 3 G e V protons. The permissible occupational exposures to concentrations of radioactive nuclides in air have been found not to be exceeded at the Princeton-Pennsylvania Accelerator. There are two main target areas where secondary particles for experimenters are produced: an internal target in the synchrotron room itself, and external targets in the external beam caves. Access to these areas is permitted only for maintenance and other operational purposes when the machine is turned off. Therefore, it becomes a matter of importance to measure the concentration in air of various radionuclides as a function of time from machine turn off. Of the nuclides produced by the activation of air, ~ 5 0 , 1 3 N , and 1~C had been previously identifiedl'2"3). However, the large uncertainties in the early measurements of their concentrations made it necessary to measure them again. The radioactive products from Argon, have been reported elsewhere4). These measurements took place during a two year period and they are described in a logical rather than chronological sequence. 140,
2. Experimental Two series of measurements have been performed to obtain different complementary information. One of these was a series of measurements of the angular distribution of the integrated particle fluxes around external beam iron and lead targets. These were in the forms of cubes 3.18 cm on a side. Activation detectors were placed at a distance of 30 cm and at angles of 25, 35, 85, 95, 145, 155 degrees with respect to the incoming 93
94
M. AWSCHALOM et al. =10 3
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Fig. 2. Location o f single channel discriminator window locations with respect to the 0.511 MeV photopeak. The peak above 6.1 V is due to amplifier saturation. 20 I0 42
"Lower
Edge"
"Upper
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Fig. 1. Block diagram o f the counting system used to f o l l o w the decay in time o f the 0.511 M e V g a m m a rays.
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the positron annihilation gammas. The samples for both series of measurements were counted in a 3 x 3inch NaI(TI) crystal with a i} inch diameter, 1½ inch deep well. Relatively long counting times and large changes in counting rates were to be expected. Hence, a possibility to monitor and correct for effects of drift in the counting system was provided. This counting system was also used for both series of measurements. Fig. 1 shows a block diagram of the set up. Fig. 2 shows the relative location of the discriminator levels with respect to the 511 keV photopeak. Channelno. 2 is a narrow window on the lower edge of the annihilation peak. Channel no. 3 is a narrow window on the upper edge. Channel no. 4 is a wide window accepting the whole 511 keV peak. Channel no. 1 is an integral discriminator used for testing purposes only. Provided the thresholds in channels 2 and 3 remain constant with respect to the photopeak, then the ratio between the countingrates in the two channels is a very sensitive measure of any drift. A drift of less than 0.1% in the relative location of the photopeak was easily detected and corrected for with the precision attenuator. These
;3.4 ~= {3-
Operating ~ Point
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Attenuator Setting
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Fig. 3. The ratio o f the number o f counts of channel no. 3 divided by channel no. 2 ( H I G H / L O W ratio) is shown vs precision attenuator setting. For reference, the photopeak position is also shown. Note, the H I G H / L O W scale is logarithmic, the photopeak position is linear.
effective gain changes were of the order of a few tenths of one per cent. This ratio was monitored throughout the experiment. Changes were necessary only at the beginning of each run when the counting rates were highest. Note that the counting rate in channel no. 4 will be only slightly affected by shifts in the window thresholds. Fig. 3 shows the change in counting rate as a function of location of the thresholds. Before and during the runs, the precision pulser and the second amplifying channel were used to check the relative position of the discriminators' thresholds. Fig. 4
ACTIVATION
OF A I R N E A R A T A R G E T B O M B A R D E D
shows the counting rate in channel no. 4 as a function of window width. Of the several devices monitoring the internal beam at P.P.A., a triple-scintillation telescope viewing the target at 90 ° was used as a reference since its output is directly proportional to the flux of secondary particles produced by the protons hitting the target. The external proton beam current was monitored using a tuned cavity and activation of aluminum foils. The total number of protons was calculated from the 27Al(p,5p5n)18F: activity1°). Results from the synchrotron room measurements were normalized to 10 3 counts of beam on target scintillation telescope ( " B A T " ) . Results from the external beam measurements were normalized to 1 x l011 3 GeV incident protons/sec. Results for the internal target have been normalized to the same intensity using the considerably more uncertain conversion factor of 1 B a T = 3 x 10 9 protons/sec. The air flow inside the synchrotron r o o m was measured with a thermo-anemometer. The accuracy of this instrument was -t-2 ft/min or ___3%, whichever was larger, as quoted by the manufacturer. A regular pattern was found to exist (fig. 5). Air flows upward alongside the magnets, at an angle of approximately
BY
3 GeV
50 ° with the median plane of the synchrotron and at an average velocity of 50 ft/min (25 cm/sec). A circular overhead flow pattern is maintained by a fan and an exhaust system. Air flowing upward is captured into this pattern 4 to 7 ft above beam height. Circulation is confined to the synchrotron room. R o o m air pressure is kept slightly above the pressure in the experimental areas as a hydrogen safety precaution.
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Fig. 5. Air flow patterns in the s y n c h r o t r o n room. The air flowing alongside the magnet is inclined 50 ° into the plane of the figure (measured from a normal to the paper). Air speeds are about 40-60 It/rain.
501~
3.
Analysis
of
the
measurements
Let a(R,
d
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•
(R)
tet tj
2.0
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4.0
N* 6.0
8.0
Window Width (V) U s
Fig. 4. Counting rate in channel no. 4, as a function of the window width. The threshold was held fixed at 0.75 V.
As
specific activity in microcuries per gram induced in a sample at position R, after activation time t,; the vector from the particle source to the sample, in cm; the flux of secondary particles, in = c m - 2 . min-1 • the cross section for the production of a certain isotope ct in samples, in barns; length of irradiation, in min; = time delay between end of irradiation and beginning of counting interval A tj, in min; = calculated number of counts for Atj after t~; = measured value of N*; the decay probability of isotope ct, rain-1; the atomic weight of the sample s, in grams;
ta) =
M. AWSCHALOM et al.
96 No
= 6.025 x 102a atoms/mol (Avogadro's number); n = number of counting intervals in any one run; A(R, t) = activity in sample at location R, after irradiation time t in pCi/g.
in the jth interval. The four normal equations will be:
#=x
j=l -- ~
Wjb,jNj,
a = 1,...4,
(6)
j=l
Then, for negligible absorption in the sample material we have, at the end o f the irradiation time t,:
A ( R , t,) =
Wj is a weight based on statistical error. In matrix form, if
~(R_______~) N O Y asa [1 - e x p ( - ata)] 3.7x 104 A~ 7' [pCi/g],
£ ~ [Wj b,j b#j] =-- Ha,, (1)
and
where the summation is over all radionuclides produced. For compactness in notation let us rewrite:
A ( R , t,) = ~, as(R, ta).
(2)
If the j t h counting interval of length Atj is started at time tj after the irradiation has ended, then the number of disintegrating nuclides of the a-th species in this interval will be equal to:
A,(R, t j, At j) = a,(R, t,). b,j,
(3)
where
b,j = [ 1 - e x p ( - 2 , d t j ) ]
exp(-2,tj).
(4)
If the efficiency of the counter for detecting isotope is e,, then the number of counts measured as a function of time should be:
N* = Y
(5)
Ideally, one would try to fit this equation to the measured decay curve, with the half-lives, initial concentrations, decay rates and the number of nuclides present as parameters. However, there are considerable difficulties involved in fitting an experimental multidecay curve of this type by a least squares methodS). The matrices involved are generally ill-conditioned due to both inherent and computational errors. Consequently, a less sophisticated approach was chosen. We assume that not more than four nuclides of known half-life, 140, 150, aaN and 11C, were present in measurable quantities. Also, 5, = e, for a = l, ... 4, since they are all pure positron emitters. The least squares fit would then minimize the expression
Z J
(7)
j=l
N j) ,
where Nj is the actually recorded number of counts
[WgbpjNj] = Yp,
(8)
j=l
then,
/-/p, a,(R, ta) --- Y#,
(9)
and the formal solution of the problem will be
a,(R, ta) -- (H- x)p, yp .
(10)
Accordingly, we have written a F O R T R A N IV program that: 1) corrects N~ for background and dead time, 2) computes unknown partial activities for a given number of isotopes and half-lives, normalized to a number of protons on target and one grammolecule of sample, 3) computes errors in these results due to counting statistics and 4) averages over several measurements. Decay curves reconstructed from computed parameters, and original decay curves usually agreed within 1%. The errors in the half-life values used in the least squares fitting procedure will have a large influence in the analysis of the measurements. The values given by Nuclear Data Sheets 6) showed discrepancies of the order of 5% in the half-life measurements of the nuclides assumed present. A remeasurement of all these half-lives was considered outside of the scope of this experiment. However, the half-life of 11C, which had some of the larger variations in the published data, was accurately measured (see appendix). The angular dependence of radionuclide formation from a given sample at a known distance has been measured and reported elsewhere7). This dependence has the functional form:
A(Ro, O) = A(Ro, O ) e x p ( - b O ) + B ,
(11)
where A (Ro, 0) = the activity per unit volume at a distance
ACTIVATION OF AIR NEAR A TARGET BOMBARDED BY 3 GeV PROTONS Ro from the target and an angle 0 rad from the beam direction; A (Ro, 0) = the activity per unit volume of the exponential term, extrapolated to zero angle; b = the slope dependent on the energy of the activating secondaries and the atomic number A of the target. If the activation of threshold detectors is measured, the dependence on the threshold energy Eth has been found to beT): b ( r a d - i) = KE,~A).
(12)
For an iron target ~(Fe) = 0.64 and for a lead target ct(Pb) - 0.34. Then, the average activity in a volume V is
97
Then, ( A ) = 31-C.
(17)
This relationship is independent of the exact shape of
f(O) and lends itself in many cases to an easy estimate of the average air-activity around a target without knowing the angular distribution of the activity. f(O) is a decreasing function of 0, so the maximum length of the radius vector describing an isoactivity surface is found at forward angles. If the activity at minimum forward angle and radius Re is CI, then we know that the average activity within the sphere of radius R~ and center at the target is less than ~ C I, and that the activity anywhere outside of that sphere is less than C !. 4. Results
(13)
( A ) = (1/V) ~v A(R,0)dV.
One way to evaluate the hazards due to air activation around a target is to find a volume whose average concentration is equal to an established value (outside this volume the concentrations will be lower). Then one would follow the movements of this volume. It is obvious that such a volume can be found easily if we let the boundary of the volume be an isoactivity surface. The equation for the isoactive surface of concentration C is
C = A(Ro, O) (Ro/R) 2 = A (R o, 0) (Ro]R) 2 exp ( - bO) +
(14
+ B(Ro/R) 2 = f(O)/R 2 ,
f(O) = [A(Ro,0) exp (-bO)+B]R2o.
(15)
The average concentration in a volume limited by an isoactive surface is then
(A) =
fo
dO
fRo.t°r (o)
dR (f(O)/R 2} R 2 sin 0,
# R,,,.,,.
~0~ fRouter(0) dR R 2 sin 0 2 7~ dO a]Rlnner
Activity of 29 g of air
I
['Activity of one mole'] _ = 0.78 [_ of N H g N O 3 .J ['Activity of one-] (18) - 1.92 L mole of H 2 0 .J"
where
2 rc
Table 1 shows the concentrations of the radionuclides produced in air at six different angles and at a distance of 30 cm from the external beam targets iron and lead, normalized to an incident beam intensity of 1 x 1011 protons/sec. The errors shown are the statistical errors propagated by the computer program. These concentrations were obtained from the counting rate in ammonium nitrate and distilled water, respectively. We assume one gram-molecule, or 29 g of air is made up of 78 percent N 2 and 21 percent 02 (by volume). Then:
, (16)
where Router(O) = x/{f(O)/C}. Little error is made by setting R i.... = O, since the target volume usually is much smaller than the volume of integration and the integrands behave properly f o r R ~, O.
Further we assume 29 g of air at STP occupy a volume of 22.4x10 a cm 3, and thus arrive at the concentrations given in table 1. Due to the limiting sample retrieval time, two calculations were performed for the decay of the external target samples. One included only the 10.1 min 13N and the 20.40 11C activity, whereas the other also assumed the 2.1 min 150 isotope was present. As would be expected, no significant difference within the experimental reproducibility can be seen for the 13N and HC results of both calculations. The oxygen results are discussed below. Table 2 shows the activity obtained in the samples irradiated near the internal targets, as well as the calculated activity due to ~3N and 1~C in air, using formula (19). The oxygen activity is shown for illustrative purposes only, and errors are deliberately not
M. A W S C H A L O M et al.
98
TABLE 1
A n g u l a r distribution o f radionuclide concentrations produced in air due to secondaries from the external beam target, normalized to 1 × 10 n incident 3 GeV protons/sec. Distance from target = 30 cm, target is cube 3.18 cm on the side. The concentrations are given in units of 10 - s (/~Ci/cm3). Target
Angle (degrees)
Concentrations assuming that two radionuclides are produced 11C
13N
Concentrations assuming that three radionuclides are produced
150
13N
11C
Fe
25 35 85 95 145 155
604-9 174-2.2 5.04-2.2 114-7 -2.14-1.4
334-9 244-1 4.84-1.0 1.94-3.0 5.14-1.4 4.14-0.7
444-51 584-169 -174-6
22.64- 3.7 14.44- 3.5 2.64-3.6 7.44-14.7 -1.14- 0.9
42.04-1.3 24.44-1.1 5.44-1.2 2.74-4.5 6.24-1.6 4.34-1.0
Pb
25 35 85 95 145 155
324-5 -13.04-1.6 19.34-1.3 8.14-2.8 7.94-1.3
59.44-2.4 13.94-8.2 10.14-0.7 14.84-0.5 4.84-1.3 5.44-0.8
624-63 ----
29.44- 9.4 84-38 11.64- 1.2 20.54-2.8 10.04- 2.1
59.94-3.3 104-14 10.54-0.4 14.64-0.9 4.64-1.1 4.94-0.8
TABLE 2 Radioactivation of air a r o u n d internal target.
Sample nuclide
HsO
0) NH4NO3
Air 3) (calc.)
Activity 4- E r r o r 2) (dps/mol x BOT × 10 -3)
O a4 015 N TM C 11
175 1573 153-k 1964-
014
0 (degrees)
Ro (cm)
Activity 4- Error 2) (dps/mol × BOT × 10 -3)
5 2
108 1396 134± 208±
4 2
N a3 C 11
222 2290 8264- 14 9104- 6
123 2190 8604728-k
6 2
N 13 C 11
3534- 35 3404- 13
015
~,
71 °
~ ~ II ~ ~-~ ~ ~.
4164- 30 170± 9
Target
oo
104 °
305 2313 2834-137 3544- 63
(1) NHaNO8
014 O15 N 13 C 11
374 2635 9 3 3 ± 54 11124- 22
29 1245 4604- 50 4 7 6 ± 22
Air 3) (calc.)
N TM C 11
187+630 190-[-278
684-333 1054-157
(1) Normalized to flux on water sample, (d-QNm/d~C2n2o) = 0.35. (2) Statistical error propagation only. ~3) a (Ro, air) = 0.782 a (Ro, NH4NO3) - 1.92 a (Ro, HzO), (18).
Ro (cm)
Pt
014 015 N 13 C 11
HzO
0 (degrees)
tl
151 978 152± 81 139± 35
Be
A C T I V A T I O N OF AIR NEAR A T A R G E T B O M B A R D E D BY 3 G e V
PROTONS
99
TABLE 3 Concentration of radionuclides produced in air due to secondaries from the internal platinum target, normalized to 1 × 1011 protons/see (using conversion 10a BOT = 3 × l012 protons/see). The concentrations are given in units of l0 8 (/~Ci/cm3). Target
Angle
Distance
Concentrations assuming that four radionuclides are produced
(degrees)
(cm)
1aN
11C
13N
11C
Pt
71 ° 104 ~
71.3 69.6
1.41 ~0.14 1.67-k0.12
1.37-E0.05 0.68-4-0.04
8.02±0.79 8.98 5=0.65
7.7 ±0.3 3.66~z0.22
Be
71" 104 °
71.3 69.6
0.8 ~2.5 0.3 ±1.3
0.8 ± 1 . 1 0.4 ±0.6
4 ~14 1.4 ± 7
4.3 ~ 6 . 3 2.3 ±3.4
quoted. The existence of four isotopes was assumed by the computer program. T h e o x y g e n a c t i v i t y i n a i r as c a l c u l a t e d b y (18) gives u n p h y s i c a l r e s u l t s in a v a r i e t y o f cases. A d i s c u s s i o n o f t h e p o s s i b l e r e a s o n s f o r t h i s is o u t s i d e t h e s c o p e o f t h i s r e p o r t . It m a y b e c o n j e c t u r e d , h o w e v e r , t h a t a) t h e a m m o n i u m n i t r a t e is p r o b a b l y h y d r a t e d , b) h y d r o x y l r a d i c a l s a r e f o r m e d in t h e w a t e r o f h y d r a t i o n , a n d c) t h e h y d r o x y l r a d i c a l s a c t as a r e d u c i n g species i n t r a n s f o r m i n g t h e n i t r a t e i n t o n i t r i t e , w i t h eventual formation of molecular oxygen. In this way, a sizable fraction of the irradiated oxygen may be i
10-6f '
i
i
i
lost, r e s u l t i n g in t h e a p p a r e n t l y v e r y l o w o x y g e n activity of the ammonium nitrate. Finally, table 3 shows the normalized concentration o f l a N a n d 1~C in air, c a l c u l a t e d o n t h e b a s i s o f t a b l e 2, f o r c o m p a r i s o n w i t h t h e r e s u l t s o f t a b l e 1. T h e c o n t e n t s
f~6 , g
i
External
'13 N
Fitted Using
Fe - T a r g e t P b - Targef Internal Beam P t - Target •
O= 2: , 0 = 3 Nuclides O= 2 . O = 3 N u e l i d e s
Beam
Fe-Target Pb- Target Internal B e a m Pt- Target •
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External Beam
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F i t t e d Using 0:2,0=3 Nuclides [3=2,11=3 N u c l i d e s
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,~
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45
65
85
105
125
145
Degrees
,sal z,5
I 45
I 65
85
I 105
I 125
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Degrees
Fig. 6. a3N activity in air around external beam targets of iron and lead at a distance of 30 cm. Targets are cubes 3.18 cm on the side. Two points ( + ) are also given for an internal platinum target 3.18 cm long. The activities are given in/zCi/cm a for 1011 incident protons/sea Some typical error bars are shown.
Fig. 7. 1 1 C activity in air around external beam targets of iron and lead at a distance of 30 cm. Targets are cubes 3.18 cm on the side. Two points ( + ) are also given for an internal platinum target 3.18 cm long. The activities are given in pCi/cm a for 1011 incident protons/see. Some typical error bars are shown. A "hand fitted" function to the observed points is shown, [6.8 exp ( - 2.9 if) +0.48] x l0 7/~Ci/cm a per l011 incident p/sec.
M. AWSCHALOM et al.
100
of table 1, as well as the results for an internal platinum target, are plotted in figs. 6 and 7. It may be seen that the results appear consistent. The M a x i m u m Permissible Concentrations in air, for 40 h per week occupancy, (MPC),, for 1sO and lSN have been found by A. C. George et al. s) to be 2 x 10 -6 #Ci/cm 3. It seems reasonable to assume a similar value for 11C. To establish a comparison with our results, we calculate the average concentration in the accelerator r o o m taking into consideration an approximation to the air flow. I f the air flow in fig. 5 is taken as 1500 cm/min, then the air in a hemisphere 250 cm in radius surrounding the target will remain there less than a time At = 10 see. As shown in ref. 2, the average concentration in the outer ring, Aav , will be given by:
Let us take the concentration at 0 = 0, R = 30 cm, and find the concentration at 0 = 0, R = 250 cm. Then, using eq. (18), and multiplying by a factor of 3.81cm 21.5g/cm 3 (207g/mol~ ~ •
-.
~-2.3,
3.18cm l l . 3 g / c m 3 ~,l--~g/--~ol/ to account for the different 3.81 cm Pt-target, assuming geometric cross-sections, one has the following results for the average concentration in the synchrotron room:
(Ip
= 1 × 1011
protons/see)
Aav(13N) = 2.1 x 10-1°/aCi/cm 3 , Aav(11C) = 2.9 x 10-1° #Ci/cm3 .
where T = 5 min is the time for the mass of air to move around the synchrotron. We may use eqs. (15), (16) and (18) to find .4, the average concentration in the volume of reference. To do this, one can obtain the functional dependence f(O) by hand-fitting the points of figs. 7 and 8. This has been done, and the corresponding expressions for a 3.18 cm Pb target and 1011 protons/sec incident, are
It may be seen that the concentrations of airborne radioactivity due to 13N and 11C is entirely insignificant in the synchrotron room, and is likely to remain so even if the machine intensity increases by an order of magnitude. The average concentration of the activities in the external beam caves, without taking dilution by ventilation into account, can be approximated by the average concentration in a sphere 250 cm in radius to be :
f(O, 1aN)= {4 e x p ( - 4 . 0 0 ) + 0 . 7 } x 10-7 #Ci/em 3 ,
(I v = I x 1011 protons/see)
A~ "~ A [ { 1 - e x p ( - 2 A t ) J / { 1 - e x p ( - 2 T ) } ] ,
Aav(13N) = 5.4 x 10 -9/aCi/cm 3 ,
f(0,11C) = {7 e x p ( - 2 . 9 0)+0.5} x 10 -7 #Ci/cm 3,
Aav(11C) = 8.0x 10-9#Ci/cm 3 .
and for Fe,
f(O, ~3N) = {18 exp(5.70)+0.4} x 10-7 #Ci/cm 3 . The H C productions for lead and iron targets were essentially undistinguishable. 107
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i
Time of Day
Fig. 8. Typical raw data for one 11C decay vs time. The background has not yet been subtracted and no corrections have been made for instrumental dead time. As it stands, this curve implies T~.(11C) ~ 20.37 min.
It may be seen that here too, the exposure to airborne radioactivity from 13N and 11C does not present a problem. It must furthermore be pointed out that this refers to initial activity and does not take into account the short half-lives involved. Finally, the average concentrations of radioactive daughters produced in argon, the next most abundant component in air (0.9 pereen0, have been calculated using the concentration at 20 degrees f r o m ref. 2 ) a t the isoactivity surface one meter from the target. The results of this calculation are shown in table 4. Here the results from ref. 2) have been multiplied by (1 m/2.5 m) 2 to obtain the activity at 250em, by (1 x 1011/1.5 x 101°) to normalize to the beam intensity in this report, by (9 x 10 -3) to convert from cm a of pure argon to cm a of air, and by (3) to obtain the average activity within the isoactive surface according to eq. (18). The (MPC)a are also shown, as given by ref. 9). Again, the argon daughter activity concentrations are well below established limits.
ACTIVATION
OF A I R N E A R A T A R G E T
TABLE 4 Average airborne radioactivity due to argon in air for l x 10 11, 3 GeV protons/see incident on a 3.18 cm thick lead target. Nuclide
A.. (/tCi/cm z)
41Ar 84mC1-84C1 a7S 24Na
3.2 3.8 8.3 8.3
x x × ×
10-7 10-8 10-9 10 -9
(MPC)*a
2× 1x 1x I×
10 -6 10-6 10-6 10 -7
* (#Ci/cm 8) for a 40 h week.
5. Conclusions The existence of 140, xSO, 13N and llc produced by secondaries from internal and external targets bombarded by 3 GeV protons was ascertained by unfolding the multidecay curves of distilled water and a m m o n i u m nitrate samples activated at a known position with respect to various targets. The calculated concentrations of these radionuclides in air have been ascertained to be negligible quantitatively for 13N and 11C as compared with available (MPC)a values. The presence of the oxygen isotopes has been shown qualitatively to be of the same order of magnitude, but effects discussed above prevent us from obtaining a quantitative result. A formula for calculating the average activity in a given volume approximately is given and also applied to argon activation measurements reported elsewhere to show that concentrations of airborne radioactivity due to the argon content of air are not a hazard at the Princeton-Pennsylvania Accelerator. N o effort to determine the effects of other constituents of air, or of various chemical species has been made, as this was considered to be outside the scope of this report.
Appendix PRECISION MEASUREMENT OF THE 161C HALF-LIFE
Graphite samples about l cm a were irradiated near a Pt-target bombarded by 3 GeV protons. The irradiations lasted 23 h. The sample was situated at an angle of about 30 ° from the incident beam, one meter away from the target. The sample was counted in a 3 x 3 inch NaI(TI) crystal with a ~ x ½ inch hole. The counting electronics were the same as described above. As soon as the samples were retrieved, they were
BOMBARDED
BY
3 GeV
PROTONS
101
put into the counter and data taking began. In addition to the t i C the only other possible activity was 7Be. In each run the activity was followed for six to seven half-lives. The counts versus time curve plotted on semi-log paper did not seem to deviate from a straight line except at the very beginning. A typical decay curve is shown in fig. 8. The analysis of the data was made as follows: A F O R T R A N IV program was written which subtracts the r o o m background and ]Be gammas from the measured counts. Then a least square fit is made to the corrected radioactivity decay curve, with N O and 2(C) as unknown parameters. That is to say, we made a least square fit to
N(t) - [B + B(Be) e x p { - 2 (Be) t}] = N Oexp { - 2 (C)t}, (A1) where
N(t)
= measured counts per 5 min; = counts per 5 rain at the beginning of the counting period; B = constant background per 5 min; B(Be) = counts from Be-activity, per 5 min; 2(Be) = decay constant for 7Be, T½ = 53.4 daysS); = 0.90 x 10 -5 m i n - l ; 2(C) = decay constant for 1~C; t = elapsed time from beginning of counting period.
No
B and B(Be) were determined as follows: The l~C activity was allowed to decay so that it would contribute only 1 counts/5 min (waiting about 20 half-lives). The remanent counting rate was then assumed to be a mixture of 7Be and r o o m gammas. A 20 min run was then made, which gave B + B ( B e ) . This was followed by another 20 min run in which the exposed graphite sample was replaced by a "clean" graphite sample of same mass and shipment, which gave B. The initial counting rates were comparable to the reciprocal of the nominal dead time of the system. Hence, a careful study of dead time corrections on the data was made. The procedure was: a series of nominal dead times were assumed, then a half-life was calculated and the weighted mean square deviation (A 2) evaluated. Finally, the assumed dead time which led to a least A 2 was adopted for that run. A 2 = ~ [ N o e -a~otj_ N(tj)]2 ( n - 1) s '
(A2)
with n = number of data points considered. Having thus found an appropriate dead time for the electronics, we investigated the changes in A2 as we excluded the first j data points (where the dead time
102
M. AWSCHALOM et al.
corrections were largest). The final choice of 2(C) was that for which A 2 was again a m i n i m u m .
TABLE
Run
A2
T~
5
[T~ ( a v ) - T~] ~
ai
Table 5 shows the calculated T÷ = in 2/2(C) for each of the five runs in which A 2 was minimized. The a ' s in table 5 are the statistical uncertainties associated with the measurements in each o f the runs. These uncertainties were used in calculating the weighted mean of the average T+'s. T~ (~ ~C) = 20.40 + 0.04 min.
no.
1
0.99
20.37
0.0009
0.010
2
3.86
20.44
0.0016
0.007
3
0.70
20.41
0.0001
0.022
4
1.67
20.35
0.0025
0.013
5
0.85
20.36
0.0016
0.010
Where _+0.04 is the root mean square deviation o f the half-life. We want to thank Messrs J. Ives, J. Fennimore and O. Griesbach, who helped with the electronics and couting as well as Messrs S. H o c h m a n and L. Pizzarello who helped with the design and construction o f the " r a b b i t ' . References
Y.~ = In 2/2(11C).
T~(av) = ~ ( T ;i,i a ,2 ~ ) /.~ ( l /.a ~,2) + i
i 2' ,2 L~(a~/a, )/~tl/ai
i
,2
)] •
i
o'~ = relative deviation, however, very small errors are # made setting a~ = a~. T½(av) = __10250"8___ 502.53 1_502.53.]
= 2 0 . 4 0 _ 0.005.
The spread in the measurements is [~(T~(av)-
T~)21'(n-- 1)3 ~- =
[0.0017] ~ = 0.041 .
i
Hence T½ (av) = 20.40 :t: 00.04.
t) M. Awschalom, F. L. Larsen and R. B. Sass, Proc. USAEC Symp. Accel. tad. dosimetry and experience, B.N.L. Report CONF-651109 (Nov. 1965) p. 57. 2) M. Awschalom, F. L. Larsen and W. Schimmerling, 1EEE Trans. Nucl. Sci. NS-14, no. 3 (1967) 980. a) A. Rindi and S. Charalambus, Nucl. Instr. and Meth. 47 (1967) 227. 4) M. Awschalom, F. L. Larsen and W. Schimmerling, Health Phys. 14 (1968) 345. 5) Proc. Syrup. Applications o f computers to nuclear and radiochemistry', Report NAS-NS3107 (Gatlinburg, Oct. 1962). ~) Nuclear Data Sheets, compiled by K. Way et al. (Printing and Publishing Office, National Academy of Sciences, National Research Council, Washington 25, D.C.). 7) M. Awschalom and W. Schimmerling, Angular distribution of integrated hadron fluxes due to 3 GeV protons, to be published. s) A. C. George, A. J. Breslin, J. W. Haskins, Jr. and R. M. Ryan, Proc. USAEC Symp. Accel. rad. dosimetry and experience, B.N.L. Report CONF-651109 (Nov. 1965) p. 513. 9) Code o f federal regulations, title 10, part 20, appendix B. lo) J. B. Cumming, Ann. Rev. Nucl. Sci. 13 (1963) 261.
INSTRUMENTS
AND
METHODS
75
(I969)
93-IO2;
©
NORTH-HOLLAND
PUBLISHING
CO.
A C T I V A T I O N OF AIR NEAR A T A R G E T B O M B A R D E D BY 3 GeV P R O T O N S M. A W S C H A L O M et al.
National Accelerator Laboratory, Batavia, Illinois, U.S.A. F. L. L A R S E N a n d W. S C H I M M E R L I N G
Princeton-Pennsyh, ania Accelerator, Princeton University, Princeton, New Jersey, U.S.A. Received 23 J u n e 1969 M e a s u r e m e n t s o f 140, 150, 13N, l l C created in air a r o u n d internal a n d external accelerator targets are reported for an incident p r o t o n energy o f 3 GeV and Fe, Pt a n d Pb targets. For incident p r o t o n currents o f 1011 p/see, average radionuclide
concentrations o f 5.4 x 10 9/~Ci/cm z o f l Z N a n d 8.0 x 10-9/~Ci/cm 3 are f o u n d in a typical experimental cave. A l s o , a precision m e a s u r e m e n t o f the 11C-half life is reported as T÷(11C) = 20.404-0.04 min.
1. Introduction
beam. Irradiation times were of the order of 20 min at beam intensities of 2 x 1011 protons/see. The purpose of these measurements was to obtain adequate normalization of activity to beam intensity, and to estimate the parameters of the angular distribution; as well as to measure the concentration of radioactive nuclei produced by activation of the components of air. In the external beam caves, these measurements were limited by the sample retrieval time of approximately five minutes to radioactive nuclides of half-lives greater than a few minutes. The measurements around the internal target were oriented toward ascertaining the presence of shorter lived radionuclides, as well as to measure the airborne radioactivity. A quick sample retrieval system ( " r a b b i t " ) was constructed for the retrieval of samples exposed around the internal beam target. In order to have the sample as close to the target as possible in a known reproducible geometry and to minimize the time necessary for bringing the samples to the counting equipment, the rabbit operated between the vicinity of the internal target and the counting room. The interval between the end of the irradiation and the start of counting was one minute on the average. The irradiation times were of the order of 40 min of continuous machine operation in order to approach saturation (about two half-lives of 11C). Counting intervals in any one run varied from 30 sec to 5 min, depending on the half-life of the dominating activity. Sample materials to represent pure oxygen and nitrogen were distilled water and analytical grade a m m o n i u m nitrate (NH4NO3). All the nuclides of interest are positron emitters with the exception of 7Be. In order to take advantage of this fact, this experiment was designed to relate the initial activaties of each nuclide to the decay rate of
A measurement has been made of the concentrations of certain airborne radionuclides produced by activation of air in the vicinity of targets bombarded by 3 G e V protons. The permissible occupational exposures to concentrations of radioactive nuclides in air have been found not to be exceeded at the Princeton-Pennsylvania Accelerator. There are two main target areas where secondary particles for experimenters are produced: an internal target in the synchrotron room itself, and external targets in the external beam caves. Access to these areas is permitted only for maintenance and other operational purposes when the machine is turned off. Therefore, it becomes a matter of importance to measure the concentration in air of various radionuclides as a function of time from machine turn off. Of the nuclides produced by the activation of air, ~ 5 0 , 1 3 N , and 1~C had been previously identifiedl'2"3). However, the large uncertainties in the early measurements of their concentrations made it necessary to measure them again. The radioactive products from Argon, have been reported elsewhere4). These measurements took place during a two year period and they are described in a logical rather than chronological sequence. 140,
2. Experimental Two series of measurements have been performed to obtain different complementary information. One of these was a series of measurements of the angular distribution of the integrated particle fluxes around external beam iron and lead targets. These were in the forms of cubes 3.18 cm on a side. Activation detectors were placed at a distance of 30 cm and at angles of 25, 35, 85, 95, 145, 155 degrees with respect to the incoming 93
94
M. AWSCHALOM et al. =10 3
I
I
I
I
I @
161<
@2 #3
O
,
,
,
--+~ . . . . . . .
2
It~l
Bose Line
I
°"' II 41'2
Discrim.
Di$crim.
I 14k3Discrim'
I
Fig. 2. Location o f single channel discriminator window locations with respect to the 0.511 MeV photopeak. The peak above 6.1 V is due to amplifier saturation. 20 I0 42
"Lower
Edge"
"Upper
Edge"
"Photo
4.0 +~
Peok m
Fig. 1. Block diagram o f the counting system used to f o l l o w the decay in time o f the 0.511 M e V g a m m a rays.
i
3= o
3.8
.c:
3.6
Q_ CL O
-1-
the positron annihilation gammas. The samples for both series of measurements were counted in a 3 x 3inch NaI(TI) crystal with a i} inch diameter, 1½ inch deep well. Relatively long counting times and large changes in counting rates were to be expected. Hence, a possibility to monitor and correct for effects of drift in the counting system was provided. This counting system was also used for both series of measurements. Fig. 1 shows a block diagram of the set up. Fig. 2 shows the relative location of the discriminator levels with respect to the 511 keV photopeak. Channelno. 2 is a narrow window on the lower edge of the annihilation peak. Channel no. 3 is a narrow window on the upper edge. Channel no. 4 is a wide window accepting the whole 511 keV peak. Channel no. 1 is an integral discriminator used for testing purposes only. Provided the thresholds in channels 2 and 3 remain constant with respect to the photopeak, then the ratio between the countingrates in the two channels is a very sensitive measure of any drift. A drift of less than 0.1% in the relative location of the photopeak was easily detected and corrected for with the precision attenuator. These
;3.4 ~= {3-
Operating ~ Point
5.2
~
30
l
o
',
~
~
Attenuator Setting
~
Fig. 3. The ratio o f the number o f counts of channel no. 3 divided by channel no. 2 ( H I G H / L O W ratio) is shown vs precision attenuator setting. For reference, the photopeak position is also shown. Note, the H I G H / L O W scale is logarithmic, the photopeak position is linear.
effective gain changes were of the order of a few tenths of one per cent. This ratio was monitored throughout the experiment. Changes were necessary only at the beginning of each run when the counting rates were highest. Note that the counting rate in channel no. 4 will be only slightly affected by shifts in the window thresholds. Fig. 3 shows the change in counting rate as a function of location of the thresholds. Before and during the runs, the precision pulser and the second amplifying channel were used to check the relative position of the discriminators' thresholds. Fig. 4
ACTIVATION
OF A I R N E A R A T A R G E T B O M B A R D E D
shows the counting rate in channel no. 4 as a function of window width. Of the several devices monitoring the internal beam at P.P.A., a triple-scintillation telescope viewing the target at 90 ° was used as a reference since its output is directly proportional to the flux of secondary particles produced by the protons hitting the target. The external proton beam current was monitored using a tuned cavity and activation of aluminum foils. The total number of protons was calculated from the 27Al(p,5p5n)18F: activity1°). Results from the synchrotron room measurements were normalized to 10 3 counts of beam on target scintillation telescope ( " B A T " ) . Results from the external beam measurements were normalized to 1 x l011 3 GeV incident protons/sec. Results for the internal target have been normalized to the same intensity using the considerably more uncertain conversion factor of 1 B a T = 3 x 10 9 protons/sec. The air flow inside the synchrotron r o o m was measured with a thermo-anemometer. The accuracy of this instrument was -t-2 ft/min or ___3%, whichever was larger, as quoted by the manufacturer. A regular pattern was found to exist (fig. 5). Air flows upward alongside the magnets, at an angle of approximately
BY
3 GeV
50 ° with the median plane of the synchrotron and at an average velocity of 50 ft/min (25 cm/sec). A circular overhead flow pattern is maintained by a fan and an exhaust system. Air flowing upward is captured into this pattern 4 to 7 ft above beam height. Circulation is confined to the synchrotron room. R o o m air pressure is kept slightly above the pressure in the experimental areas as a hydrogen safety precaution.
:.'.'; Y:.~.':']%/j'.:I~:'(:'"C6 h'c r e I e
I
I
I
I
I
!
R o bf.:i.~.£: .L:.i.~:l-./?(".: .% L_
.7~y~..~t:...:::...;.~...%~.:,v.::...:.:.::....,-.::....:.-.'i~: ~~:: .~:..:-:~-:~.: ~:~ :
:...:.....:~ .... ~ " "
•
!~!~.-:.% :.,:...:~..~.:::::
:': o:.."....~.y: .,~.: ~:.:.:e;(: .... ' ....... ' ' " '"'
I~:':..~':,i~] T
""
"
......
'"
. . . . ~. : .".. i!l ...... 'i'~,.
(Into Figure Plane]
"]-['~:
?i
Prot o ri ' ~ ~ i n [Out ot Figure Plane) I
95
PROTONS
I
t
e
g
r
a
t
ion
Foundation
Fig. 5. Air flow patterns in the s y n c h r o t r o n room. The air flowing alongside the magnet is inclined 50 ° into the plane of the figure (measured from a normal to the paper). Air speeds are about 40-60 It/rain.
501~
3.
Analysis
of
the
measurements
Let a(R,
d
R o
•
(R)
tet tj
2.0
I
I
I
4.0
N* 6.0
8.0
Window Width (V) U s
Fig. 4. Counting rate in channel no. 4, as a function of the window width. The threshold was held fixed at 0.75 V.
As
specific activity in microcuries per gram induced in a sample at position R, after activation time t,; the vector from the particle source to the sample, in cm; the flux of secondary particles, in = c m - 2 . min-1 • the cross section for the production of a certain isotope ct in samples, in barns; length of irradiation, in min; = time delay between end of irradiation and beginning of counting interval A tj, in min; = calculated number of counts for Atj after t~; = measured value of N*; the decay probability of isotope ct, rain-1; the atomic weight of the sample s, in grams;
ta) =
M. AWSCHALOM et al.
96 No
= 6.025 x 102a atoms/mol (Avogadro's number); n = number of counting intervals in any one run; A(R, t) = activity in sample at location R, after irradiation time t in pCi/g.
in the jth interval. The four normal equations will be:
#=x
j=l -- ~
Wjb,jNj,
a = 1,...4,
(6)
j=l
Then, for negligible absorption in the sample material we have, at the end o f the irradiation time t,:
A ( R , t,) =
Wj is a weight based on statistical error. In matrix form, if
~(R_______~) N O Y asa [1 - e x p ( - ata)] 3.7x 104 A~ 7' [pCi/g],
£ ~ [Wj b,j b#j] =-- Ha,, (1)
and
where the summation is over all radionuclides produced. For compactness in notation let us rewrite:
A ( R , t,) = ~, as(R, ta).
(2)
If the j t h counting interval of length Atj is started at time tj after the irradiation has ended, then the number of disintegrating nuclides of the a-th species in this interval will be equal to:
A,(R, t j, At j) = a,(R, t,). b,j,
(3)
where
b,j = [ 1 - e x p ( - 2 , d t j ) ]
exp(-2,tj).
(4)
If the efficiency of the counter for detecting isotope is e,, then the number of counts measured as a function of time should be:
N* = Y
(5)
Ideally, one would try to fit this equation to the measured decay curve, with the half-lives, initial concentrations, decay rates and the number of nuclides present as parameters. However, there are considerable difficulties involved in fitting an experimental multidecay curve of this type by a least squares methodS). The matrices involved are generally ill-conditioned due to both inherent and computational errors. Consequently, a less sophisticated approach was chosen. We assume that not more than four nuclides of known half-life, 140, 150, aaN and 11C, were present in measurable quantities. Also, 5, = e, for a = l, ... 4, since they are all pure positron emitters. The least squares fit would then minimize the expression
Z J
(7)
j=l
N j) ,
where Nj is the actually recorded number of counts
[WgbpjNj] = Yp,
(8)
j=l
then,
/-/p, a,(R, ta) --- Y#,
(9)
and the formal solution of the problem will be
a,(R, ta) -- (H- x)p, yp .
(10)
Accordingly, we have written a F O R T R A N IV program that: 1) corrects N~ for background and dead time, 2) computes unknown partial activities for a given number of isotopes and half-lives, normalized to a number of protons on target and one grammolecule of sample, 3) computes errors in these results due to counting statistics and 4) averages over several measurements. Decay curves reconstructed from computed parameters, and original decay curves usually agreed within 1%. The errors in the half-life values used in the least squares fitting procedure will have a large influence in the analysis of the measurements. The values given by Nuclear Data Sheets 6) showed discrepancies of the order of 5% in the half-life measurements of the nuclides assumed present. A remeasurement of all these half-lives was considered outside of the scope of this experiment. However, the half-life of 11C, which had some of the larger variations in the published data, was accurately measured (see appendix). The angular dependence of radionuclide formation from a given sample at a known distance has been measured and reported elsewhere7). This dependence has the functional form:
A(Ro, O) = A(Ro, O ) e x p ( - b O ) + B ,
(11)
where A (Ro, 0) = the activity per unit volume at a distance
ACTIVATION OF AIR NEAR A TARGET BOMBARDED BY 3 GeV PROTONS Ro from the target and an angle 0 rad from the beam direction; A (Ro, 0) = the activity per unit volume of the exponential term, extrapolated to zero angle; b = the slope dependent on the energy of the activating secondaries and the atomic number A of the target. If the activation of threshold detectors is measured, the dependence on the threshold energy Eth has been found to beT): b ( r a d - i) = KE,~A).
(12)
For an iron target ~(Fe) = 0.64 and for a lead target ct(Pb) - 0.34. Then, the average activity in a volume V is
97
Then, ( A ) = 31-C.
(17)
This relationship is independent of the exact shape of
f(O) and lends itself in many cases to an easy estimate of the average air-activity around a target without knowing the angular distribution of the activity. f(O) is a decreasing function of 0, so the maximum length of the radius vector describing an isoactivity surface is found at forward angles. If the activity at minimum forward angle and radius Re is CI, then we know that the average activity within the sphere of radius R~ and center at the target is less than ~ C I, and that the activity anywhere outside of that sphere is less than C !. 4. Results
(13)
( A ) = (1/V) ~v A(R,0)dV.
One way to evaluate the hazards due to air activation around a target is to find a volume whose average concentration is equal to an established value (outside this volume the concentrations will be lower). Then one would follow the movements of this volume. It is obvious that such a volume can be found easily if we let the boundary of the volume be an isoactivity surface. The equation for the isoactive surface of concentration C is
C = A(Ro, O) (Ro/R) 2 = A (R o, 0) (Ro]R) 2 exp ( - bO) +
(14
+ B(Ro/R) 2 = f(O)/R 2 ,
f(O) = [A(Ro,0) exp (-bO)+B]R2o.
(15)
The average concentration in a volume limited by an isoactive surface is then
(A) =
fo
dO
fRo.t°r (o)
dR (f(O)/R 2} R 2 sin 0,
# R,,,.,,.
~0~ fRouter(0) dR R 2 sin 0 2 7~ dO a]Rlnner
Activity of 29 g of air
I
['Activity of one mole'] _ = 0.78 [_ of N H g N O 3 .J ['Activity of one-] (18) - 1.92 L mole of H 2 0 .J"
where
2 rc
Table 1 shows the concentrations of the radionuclides produced in air at six different angles and at a distance of 30 cm from the external beam targets iron and lead, normalized to an incident beam intensity of 1 x 1011 protons/sec. The errors shown are the statistical errors propagated by the computer program. These concentrations were obtained from the counting rate in ammonium nitrate and distilled water, respectively. We assume one gram-molecule, or 29 g of air is made up of 78 percent N 2 and 21 percent 02 (by volume). Then:
, (16)
where Router(O) = x/{f(O)/C}. Little error is made by setting R i.... = O, since the target volume usually is much smaller than the volume of integration and the integrands behave properly f o r R ~, O.
Further we assume 29 g of air at STP occupy a volume of 22.4x10 a cm 3, and thus arrive at the concentrations given in table 1. Due to the limiting sample retrieval time, two calculations were performed for the decay of the external target samples. One included only the 10.1 min 13N and the 20.40 11C activity, whereas the other also assumed the 2.1 min 150 isotope was present. As would be expected, no significant difference within the experimental reproducibility can be seen for the 13N and HC results of both calculations. The oxygen results are discussed below. Table 2 shows the activity obtained in the samples irradiated near the internal targets, as well as the calculated activity due to ~3N and 1~C in air, using formula (19). The oxygen activity is shown for illustrative purposes only, and errors are deliberately not
M. A W S C H A L O M et al.
98
TABLE 1
A n g u l a r distribution o f radionuclide concentrations produced in air due to secondaries from the external beam target, normalized to 1 × 10 n incident 3 GeV protons/sec. Distance from target = 30 cm, target is cube 3.18 cm on the side. The concentrations are given in units of 10 - s (/~Ci/cm3). Target
Angle (degrees)
Concentrations assuming that two radionuclides are produced 11C
13N
Concentrations assuming that three radionuclides are produced
150
13N
11C
Fe
25 35 85 95 145 155
604-9 174-2.2 5.04-2.2 114-7 -2.14-1.4
334-9 244-1 4.84-1.0 1.94-3.0 5.14-1.4 4.14-0.7
444-51 584-169 -174-6
22.64- 3.7 14.44- 3.5 2.64-3.6 7.44-14.7 -1.14- 0.9
42.04-1.3 24.44-1.1 5.44-1.2 2.74-4.5 6.24-1.6 4.34-1.0
Pb
25 35 85 95 145 155
324-5 -13.04-1.6 19.34-1.3 8.14-2.8 7.94-1.3
59.44-2.4 13.94-8.2 10.14-0.7 14.84-0.5 4.84-1.3 5.44-0.8
624-63 ----
29.44- 9.4 84-38 11.64- 1.2 20.54-2.8 10.04- 2.1
59.94-3.3 104-14 10.54-0.4 14.64-0.9 4.64-1.1 4.94-0.8
TABLE 2 Radioactivation of air a r o u n d internal target.
Sample nuclide
HsO
0) NH4NO3
Air 3) (calc.)
Activity 4- E r r o r 2) (dps/mol x BOT × 10 -3)
O a4 015 N TM C 11
175 1573 153-k 1964-
014
0 (degrees)
Ro (cm)
Activity 4- Error 2) (dps/mol × BOT × 10 -3)
5 2
108 1396 134± 208±
4 2
N a3 C 11
222 2290 8264- 14 9104- 6
123 2190 8604728-k
6 2
N 13 C 11
3534- 35 3404- 13
015
~,
71 °
~ ~ II ~ ~-~ ~ ~.
4164- 30 170± 9
Target
oo
104 °
305 2313 2834-137 3544- 63
(1) NHaNO8
014 O15 N 13 C 11
374 2635 9 3 3 ± 54 11124- 22
29 1245 4604- 50 4 7 6 ± 22
Air 3) (calc.)
N TM C 11
187+630 190-[-278
684-333 1054-157
(1) Normalized to flux on water sample, (d-QNm/d~C2n2o) = 0.35. (2) Statistical error propagation only. ~3) a (Ro, air) = 0.782 a (Ro, NH4NO3) - 1.92 a (Ro, HzO), (18).
Ro (cm)
Pt
014 015 N 13 C 11
HzO
0 (degrees)
tl
151 978 152± 81 139± 35
Be
A C T I V A T I O N OF AIR NEAR A T A R G E T B O M B A R D E D BY 3 G e V
PROTONS
99
TABLE 3 Concentration of radionuclides produced in air due to secondaries from the internal platinum target, normalized to 1 × 1011 protons/see (using conversion 10a BOT = 3 × l012 protons/see). The concentrations are given in units of l0 8 (/~Ci/cm3). Target
Angle
Distance
Concentrations assuming that four radionuclides are produced
(degrees)
(cm)
1aN
11C
13N
11C
Pt
71 ° 104 ~
71.3 69.6
1.41 ~0.14 1.67-k0.12
1.37-E0.05 0.68-4-0.04
8.02±0.79 8.98 5=0.65
7.7 ±0.3 3.66~z0.22
Be
71" 104 °
71.3 69.6
0.8 ~2.5 0.3 ±1.3
0.8 ± 1 . 1 0.4 ±0.6
4 ~14 1.4 ± 7
4.3 ~ 6 . 3 2.3 ±3.4
quoted. The existence of four isotopes was assumed by the computer program. T h e o x y g e n a c t i v i t y i n a i r as c a l c u l a t e d b y (18) gives u n p h y s i c a l r e s u l t s in a v a r i e t y o f cases. A d i s c u s s i o n o f t h e p o s s i b l e r e a s o n s f o r t h i s is o u t s i d e t h e s c o p e o f t h i s r e p o r t . It m a y b e c o n j e c t u r e d , h o w e v e r , t h a t a) t h e a m m o n i u m n i t r a t e is p r o b a b l y h y d r a t e d , b) h y d r o x y l r a d i c a l s a r e f o r m e d in t h e w a t e r o f h y d r a t i o n , a n d c) t h e h y d r o x y l r a d i c a l s a c t as a r e d u c i n g species i n t r a n s f o r m i n g t h e n i t r a t e i n t o n i t r i t e , w i t h eventual formation of molecular oxygen. In this way, a sizable fraction of the irradiated oxygen may be i
10-6f '
i
i
i
lost, r e s u l t i n g in t h e a p p a r e n t l y v e r y l o w o x y g e n activity of the ammonium nitrate. Finally, table 3 shows the normalized concentration o f l a N a n d 1~C in air, c a l c u l a t e d o n t h e b a s i s o f t a b l e 2, f o r c o m p a r i s o n w i t h t h e r e s u l t s o f t a b l e 1. T h e c o n t e n t s
f~6 , g
i
External
'13 N
Fitted Using
Fe - T a r g e t P b - Targef Internal Beam P t - Target •
O= 2: , 0 = 3 Nuclides O= 2 . O = 3 N u e l i d e s
Beam
Fe-Target Pb- Target Internal B e a m Pt- Target •
i
External Beam
i
r "\
g
i
I
HC
i
F i t t e d Using 0:2,0=3 Nuclides [3=2,11=3 N u c l i d e s
x\~ ' ~ ,vZL.._.__
o ¢= 0) ¢3 o > ,<
,~
-9 I0
I 25
I
I
I
I
I
I
45
65
85
105
125
145
Degrees
,sal z,5
I 45
I 65
85
I 105
I 125
I 145
Degrees
Fig. 6. a3N activity in air around external beam targets of iron and lead at a distance of 30 cm. Targets are cubes 3.18 cm on the side. Two points ( + ) are also given for an internal platinum target 3.18 cm long. The activities are given in/zCi/cm a for 1011 incident protons/sea Some typical error bars are shown.
Fig. 7. 1 1 C activity in air around external beam targets of iron and lead at a distance of 30 cm. Targets are cubes 3.18 cm on the side. Two points ( + ) are also given for an internal platinum target 3.18 cm long. The activities are given in pCi/cm a for 1011 incident protons/see. Some typical error bars are shown. A "hand fitted" function to the observed points is shown, [6.8 exp ( - 2.9 if) +0.48] x l0 7/~Ci/cm a per l011 incident p/sec.
M. AWSCHALOM et al.
100
of table 1, as well as the results for an internal platinum target, are plotted in figs. 6 and 7. It may be seen that the results appear consistent. The M a x i m u m Permissible Concentrations in air, for 40 h per week occupancy, (MPC),, for 1sO and lSN have been found by A. C. George et al. s) to be 2 x 10 -6 #Ci/cm 3. It seems reasonable to assume a similar value for 11C. To establish a comparison with our results, we calculate the average concentration in the accelerator r o o m taking into consideration an approximation to the air flow. I f the air flow in fig. 5 is taken as 1500 cm/min, then the air in a hemisphere 250 cm in radius surrounding the target will remain there less than a time At = 10 see. As shown in ref. 2, the average concentration in the outer ring, Aav , will be given by:
Let us take the concentration at 0 = 0, R = 30 cm, and find the concentration at 0 = 0, R = 250 cm. Then, using eq. (18), and multiplying by a factor of 3.81cm 21.5g/cm 3 (207g/mol~ ~ •
-.
~-2.3,
3.18cm l l . 3 g / c m 3 ~,l--~g/--~ol/ to account for the different 3.81 cm Pt-target, assuming geometric cross-sections, one has the following results for the average concentration in the synchrotron room:
(Ip
= 1 × 1011
protons/see)
Aav(13N) = 2.1 x 10-1°/aCi/cm 3 , Aav(11C) = 2.9 x 10-1° #Ci/cm3 .
where T = 5 min is the time for the mass of air to move around the synchrotron. We may use eqs. (15), (16) and (18) to find .4, the average concentration in the volume of reference. To do this, one can obtain the functional dependence f(O) by hand-fitting the points of figs. 7 and 8. This has been done, and the corresponding expressions for a 3.18 cm Pb target and 1011 protons/sec incident, are
It may be seen that the concentrations of airborne radioactivity due to 13N and 11C is entirely insignificant in the synchrotron room, and is likely to remain so even if the machine intensity increases by an order of magnitude. The average concentration of the activities in the external beam caves, without taking dilution by ventilation into account, can be approximated by the average concentration in a sphere 250 cm in radius to be :
f(O, 1aN)= {4 e x p ( - 4 . 0 0 ) + 0 . 7 } x 10-7 #Ci/em 3 ,
(I v = I x 1011 protons/see)
A~ "~ A [ { 1 - e x p ( - 2 A t ) J / { 1 - e x p ( - 2 T ) } ] ,
Aav(13N) = 5.4 x 10 -9/aCi/cm 3 ,
f(0,11C) = {7 e x p ( - 2 . 9 0)+0.5} x 10 -7 #Ci/cm 3,
Aav(11C) = 8.0x 10-9#Ci/cm 3 .
and for Fe,
f(O, ~3N) = {18 exp(5.70)+0.4} x 10-7 #Ci/cm 3 . The H C productions for lead and iron targets were essentially undistinguishable. 107
I
'
"~
'
I
,
"-~...
4 '
~
I 8:00
I
I 8:30
I
t
t 9:00
9:50
n
I I0:00
n
I
I 10:50
i
Time of Day
Fig. 8. Typical raw data for one 11C decay vs time. The background has not yet been subtracted and no corrections have been made for instrumental dead time. As it stands, this curve implies T~.(11C) ~ 20.37 min.
It may be seen that here too, the exposure to airborne radioactivity from 13N and 11C does not present a problem. It must furthermore be pointed out that this refers to initial activity and does not take into account the short half-lives involved. Finally, the average concentrations of radioactive daughters produced in argon, the next most abundant component in air (0.9 pereen0, have been calculated using the concentration at 20 degrees f r o m ref. 2 ) a t the isoactivity surface one meter from the target. The results of this calculation are shown in table 4. Here the results from ref. 2) have been multiplied by (1 m/2.5 m) 2 to obtain the activity at 250em, by (1 x 1011/1.5 x 101°) to normalize to the beam intensity in this report, by (9 x 10 -3) to convert from cm a of pure argon to cm a of air, and by (3) to obtain the average activity within the isoactive surface according to eq. (18). The (MPC)a are also shown, as given by ref. 9). Again, the argon daughter activity concentrations are well below established limits.
ACTIVATION
OF A I R N E A R A T A R G E T
TABLE 4 Average airborne radioactivity due to argon in air for l x 10 11, 3 GeV protons/see incident on a 3.18 cm thick lead target. Nuclide
A.. (/tCi/cm z)
41Ar 84mC1-84C1 a7S 24Na
3.2 3.8 8.3 8.3
x x × ×
10-7 10-8 10-9 10 -9
(MPC)*a
2× 1x 1x I×
10 -6 10-6 10-6 10 -7
* (#Ci/cm 8) for a 40 h week.
5. Conclusions The existence of 140, xSO, 13N and llc produced by secondaries from internal and external targets bombarded by 3 GeV protons was ascertained by unfolding the multidecay curves of distilled water and a m m o n i u m nitrate samples activated at a known position with respect to various targets. The calculated concentrations of these radionuclides in air have been ascertained to be negligible quantitatively for 13N and 11C as compared with available (MPC)a values. The presence of the oxygen isotopes has been shown qualitatively to be of the same order of magnitude, but effects discussed above prevent us from obtaining a quantitative result. A formula for calculating the average activity in a given volume approximately is given and also applied to argon activation measurements reported elsewhere to show that concentrations of airborne radioactivity due to the argon content of air are not a hazard at the Princeton-Pennsylvania Accelerator. N o effort to determine the effects of other constituents of air, or of various chemical species has been made, as this was considered to be outside the scope of this report.
Appendix PRECISION MEASUREMENT OF THE 161C HALF-LIFE
Graphite samples about l cm a were irradiated near a Pt-target bombarded by 3 GeV protons. The irradiations lasted 23 h. The sample was situated at an angle of about 30 ° from the incident beam, one meter away from the target. The sample was counted in a 3 x 3 inch NaI(TI) crystal with a ~ x ½ inch hole. The counting electronics were the same as described above. As soon as the samples were retrieved, they were
BOMBARDED
BY
3 GeV
PROTONS
101
put into the counter and data taking began. In addition to the t i C the only other possible activity was 7Be. In each run the activity was followed for six to seven half-lives. The counts versus time curve plotted on semi-log paper did not seem to deviate from a straight line except at the very beginning. A typical decay curve is shown in fig. 8. The analysis of the data was made as follows: A F O R T R A N IV program was written which subtracts the r o o m background and ]Be gammas from the measured counts. Then a least square fit is made to the corrected radioactivity decay curve, with N O and 2(C) as unknown parameters. That is to say, we made a least square fit to
N(t) - [B + B(Be) e x p { - 2 (Be) t}] = N Oexp { - 2 (C)t}, (A1) where
N(t)
= measured counts per 5 min; = counts per 5 rain at the beginning of the counting period; B = constant background per 5 min; B(Be) = counts from Be-activity, per 5 min; 2(Be) = decay constant for 7Be, T½ = 53.4 daysS); = 0.90 x 10 -5 m i n - l ; 2(C) = decay constant for 1~C; t = elapsed time from beginning of counting period.
No
B and B(Be) were determined as follows: The l~C activity was allowed to decay so that it would contribute only 1 counts/5 min (waiting about 20 half-lives). The remanent counting rate was then assumed to be a mixture of 7Be and r o o m gammas. A 20 min run was then made, which gave B + B ( B e ) . This was followed by another 20 min run in which the exposed graphite sample was replaced by a "clean" graphite sample of same mass and shipment, which gave B. The initial counting rates were comparable to the reciprocal of the nominal dead time of the system. Hence, a careful study of dead time corrections on the data was made. The procedure was: a series of nominal dead times were assumed, then a half-life was calculated and the weighted mean square deviation (A 2) evaluated. Finally, the assumed dead time which led to a least A 2 was adopted for that run. A 2 = ~ [ N o e -a~otj_ N(tj)]2 ( n - 1) s '
(A2)
with n = number of data points considered. Having thus found an appropriate dead time for the electronics, we investigated the changes in A2 as we excluded the first j data points (where the dead time
102
M. AWSCHALOM et al.
corrections were largest). The final choice of 2(C) was that for which A 2 was again a m i n i m u m .
TABLE
Run
A2
T~
5
[T~ ( a v ) - T~] ~
ai
Table 5 shows the calculated T÷ = in 2/2(C) for each of the five runs in which A 2 was minimized. The a ' s in table 5 are the statistical uncertainties associated with the measurements in each o f the runs. These uncertainties were used in calculating the weighted mean of the average T+'s. T~ (~ ~C) = 20.40 + 0.04 min.
no.
1
0.99
20.37
0.0009
0.010
2
3.86
20.44
0.0016
0.007
3
0.70
20.41
0.0001
0.022
4
1.67
20.35
0.0025
0.013
5
0.85
20.36
0.0016
0.010
Where _+0.04 is the root mean square deviation o f the half-life. We want to thank Messrs J. Ives, J. Fennimore and O. Griesbach, who helped with the electronics and couting as well as Messrs S. H o c h m a n and L. Pizzarello who helped with the design and construction o f the " r a b b i t ' . References
Y.~ = In 2/2(11C).
T~(av) = ~ ( T ;i,i a ,2 ~ ) /.~ ( l /.a ~,2) + i
i 2' ,2 L~(a~/a, )/~tl/ai
i
,2
)] •
i
o'~ = relative deviation, however, very small errors are # made setting a~ = a~. T½(av) = __10250"8___ 502.53 1_502.53.]
= 2 0 . 4 0 _ 0.005.
The spread in the measurements is [~(T~(av)-
T~)21'(n-- 1)3 ~- =
[0.0017] ~ = 0.041 .
i
Hence T½ (av) = 20.40 :t: 00.04.
t) M. Awschalom, F. L. Larsen and R. B. Sass, Proc. USAEC Symp. Accel. tad. dosimetry and experience, B.N.L. Report CONF-651109 (Nov. 1965) p. 57. 2) M. Awschalom, F. L. Larsen and W. Schimmerling, 1EEE Trans. Nucl. Sci. NS-14, no. 3 (1967) 980. a) A. Rindi and S. Charalambus, Nucl. Instr. and Meth. 47 (1967) 227. 4) M. Awschalom, F. L. Larsen and W. Schimmerling, Health Phys. 14 (1968) 345. 5) Proc. Syrup. Applications o f computers to nuclear and radiochemistry', Report NAS-NS3107 (Gatlinburg, Oct. 1962). ~) Nuclear Data Sheets, compiled by K. Way et al. (Printing and Publishing Office, National Academy of Sciences, National Research Council, Washington 25, D.C.). 7) M. Awschalom and W. Schimmerling, Angular distribution of integrated hadron fluxes due to 3 GeV protons, to be published. s) A. C. George, A. J. Breslin, J. W. Haskins, Jr. and R. M. Ryan, Proc. USAEC Symp. Accel. rad. dosimetry and experience, B.N.L. Report CONF-651109 (Nov. 1965) p. 513. 9) Code o f federal regulations, title 10, part 20, appendix B. lo) J. B. Cumming, Ann. Rev. Nucl. Sci. 13 (1963) 261.