# Presentation: 784167

## Radioisotope Production with a Cyclotron

- David J. Schlyer
- Brookhaven National Laboratory

- Cyclotron

- Target

## Circular Accelerators

## Rolf Wideroe  1929

- R. Wideroe proposed an accelerator by using an alternating voltage across many alternating “gaps.”
- 	It was not without a myriad of problems
- 	- Focusing of beam
- 	- Vacuum leaks
- 	- Oscillating high voltages
- 	- Again, imagination
- His professor refused any further work because it was “sure to fail.”
- 	- Wideroe still published his idea in *Archiv fur Electrotechnic*

## Accelerator basics

**proton**

**+**

**-/+**

**+**

**-/+**

- 2. Acceleration of Charged Particle

- push

- pull

- push

**Accelerator basics**

## Ernest Orlando Lawrence

- 	In April 1929, UC Berkley’s youngest Physics professor happened across *Archiv fur Electrotechnic*.
- 	Not able to read German he just looked at the diagrams and pictures of the journal.
- 	Immediately after seeing Wideroes schematic, Ernest fully comprehended it’s implications.  He was excited !

## “R cancels R”

- Ernest quickly jotted down the following:
-   Fr = m*v*2/r   and
-   FB = q*v*B
- 		thus:
-  r = m*v*/qB
* **ω= 2πf *= *v*/r
*	**	*substitute:
-  *f* = qB/*2π*m
*	**	** **R cancels R !*

## What does this mean ?

- Ernest Lawrence recognized that the ion’s angular velocity does not depend on the radius.
- Mother nature was kind to cyclotroneers, for as the particle’s energy (speed) increased, so did it’s orbital path length.  For a fixed particles q/m and magnetic field the angular frequency is constant.

## Making the Cyclotron a Reality

- It was quickly realized that two hollow, semi-circular electrodes (named DEE’s for their shape) in a strong magnetic field would best serve as the accelerating gap and ion storage.

## A Little Bit About the RF Oscillator

- We know that:
- 			***f ***** = qB/2********m**
- The DEE has capacitance **C** so **L** is chosen:
- 			 ***f*****r=1/2********(LC)**
- For 1p1 & 1 Tesla B-field the ***f*****r ~ 15 MHz  ****(RF)**

## The First Cyclotron

- 	The first 4-inch cyclotron was crude, but successfully demonstrated the magnetic resonance principle.

## The 11 inch 1.1 MeVJanuary 1932

- Telegram to Lawrence:
- 	“Dr. Livingston has asked me to advise you that he has obtained 1,100,000 volt protons.  He also suggested that I add ‘Whoopee’!”

## Pace of Development was UnprecedentedFeburary 1932				September 1932  

- Even before the 11-inch was completed, the 27 inch was being designed.
- Left Photo is of Ernest Lawrence and M.S. Livingston (L to R)

## A Lesson to be Learned (1934)

- Joliot announced induced radioactivity using a small alpha source and Al targets.
- Although swamped in radioactivity for months, the Berkley Cyclotron “Rad-lab” missed the discovery:
- “...the Laboratory missed the discovery because the same switch operated the cyclotron and the Geiger counter.” – “We felt like kicking our butts.”
- 						[Thornton]

## IBA machine  Cyclone18/9

## GE machine  PET Trace

## BNL’s JSW BC1710 and Ebco TR19 Cyclotron

- We have 2 operating cyclotrons
- The Japan Steel Works BC1710
- The Ebco TR19 shown to the right

## The Cyclotron as seen by.....

**The Cyclotron as seen by.....**

## The Cyclotron as seen by.....

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## The Cyclotron as seen by.....

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## The Cyclotron as seen by.....

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## The Cyclotron as seen by.....

**The Cyclotron as seen by.....**

## The Cyclotron as seen by.....

**The Cyclotron as seen by.....**

## The Cyclotron as seen by.....

**The Cyclotron as seen by.....**

## Problems  in Cyclotron Targetry 

- Need to produce radioisotopes reliably to meet our research needs
- Need high specific activity for binding studies
- Need to produce large quantities to meet current and future demands

## Considerations in Targets

- Power deposited > 1 kwatt

## How are the Radioisotopes made

- An accelerator shoots
- a particle at high energy

- The particle reacts with a nucleus to form a new radioisotope

## Nuclear Reaction Cross Sections

**A a**

- a

**A**

**A**

**B**

**D**

- a

- b

- c

- b

- a

- INELASTIC
- SCATTERING

- NUCLEAR
- REACTION 2

- ELASTIC
- SCATTERING

- NUCLEAR
- REACTION 1

- +

- +

- +

- +

- +

## Reaction Yields

- Where:
- dn = number of reactions occurring in one second
- I0 = number of particles incident on the target in one second
- NA = number of target nuclei per gram
- ds = thickness of the material in grams per cm2
- σab = cross-section expressed in units of cm2

## Heat Deposition as particle slows down

- Heat Deposition

- Bragg Peak

## PET Radioisotope Production

- Production of F-18
- O-18 water target for fluoride
- Production of Carbon-11
- Nitrogen gas target

## PET Radioisotope Production

- Production of F-18
- O-18 water target for fluoride
- Production of Carbon-11
- Nitrogen gas target

## Considerations in F-18 Targets

## Fluorine-18 Fluoride Production

- Target is usually a depression cut into a metal block which contains the oxygen-18 enriched water

## Fluorine-18 Fluoride Production

- 18O(p,n)18F Nuclear Reaction in 95% enriched water
- Volume of the target from 0.3 mL up to 3.0 mL
- Cost of water is $35/g

## Boiling in the target

- There is substantial density reduction in the water during irradiation and loss of yield.

- [18O]
- H2O

- Cooling Water

- Beam

## Water Target During Irradiation

- Heselius SJ Schlyer DJ, Wolf AP, 1989

## Liquid Targets and  Density Reduction

**Problem**
- Density Reduction
- The yield is reduced in the target
- Higher pressures are created
- Mechanical action may affect surfaces

- Figure a –
- 5 μA beam current

- Figure b –
- 10 μA beam current

## Fluorine-18 Fluoride Production

- Heselius SJ Schlyer DJ, Wolf AP, 1989

## Fluorine-18 Fluoride Production

**Solution**
- The boiling in the water can be reduced by raising the pressure on the water, effectively increasing the density

- Heselius SJ Schlyer DJ, Wolf AP, 1989

## Fluorine-18 Fluoride Production

**Problem**
- If the beam is too tightly focused, the beam will cause the water to boil
**Solution**
- Use a diffuse beam for water targets

- Courtesy of Wake Forest

## Fluorine-18 Fluoride Production

**Problem**
- The amount of activity is limited by the beam current and power deposited
**Solution**
- Use an inclined plane water target

- Ebco Technologies, Inc

## Fluorine-18 Fluoride Production

**Problem**
- The surface chemistry occurring in the silver water target causes silver colloid to be created
**Solution**
- The water must be filtered

## F-18 Fluoride production

**Solutions**
- Use a foil support to keep the foil from bowing
- Use targets constructed from a more inert material (titanium or niobium)
- Supply more efficient cooling to the back of the target
- Completely fill the volume with water

- Cooling Water

- Target Body

- Foil Support

## PET Radioisotope Production

- Production of F-18
- O-18 water target for fluoride
- Production of Carbon-11
- Nitrogen gas target

## Considerations in Targets

## Carbon-11 Production

- Nitrogen Gas Target for 14N(p,α)11C Reaction

- Gas target body is made from polished aluminum

## Heat Transfer in Gas Targets

- Density Reduction is a result of target heating
- The heating is not uniform in the beam strike area.

- Beam

- Heselius, S.J., Lindblom, P., and Solin, O., 1982

## Density Reduction in Gas Targets

- Temperature distribution can be mapped out and can have an average temperature of several hundred degrees C

## Contributions to the profile

- Multiple scattering mainly in the foil, but also along the path of the beam
- Mainly an x and y spread but with a z component
- Energy straggling through the gas and the window
- Mainly a z spread
- Density reduction from thermal effects along the path of the beam
- Mainly a z spread

## Multiple Scattering in Gas Targets

- Multiple scattering in the front foil causes the beam shape to enlarge
- The Multiple Scattering in the target can be approximated by a simple model

## Target Beam spot

- This beam profile shows some hot spots
- The beam profile could be improved by using a thicker stripper foil for the beam extraction

- 1 cm

## Heat Transfer in Gas Targets

- There are convection flow patterns set up inside the target
- These currents  help in the convective heat transfer

- Protons at 14.7 MeV , 10.7 MeV on target, current of 15 μA and pressure of 675 kPa

- Nominal Range

## Heat Transfer in Gas Targets

- The fraction of heat transferred by convection is high except very close to the wall
- The fraction is relatively independent of gas temperature

- Hugel, E.  PhD Thesis 1988

## Heat Transfer in Gas Targets

- The flame in gravity has convection currents.
- The flame in microgravity has no convection currents

## Heat Transfer in Gas Targets

- The heat transfer is best determined when the amount of heat deposited in the target is a constant which occurs when the target is thick to the beam
- The length of the target and the exact shape do affect the transfer

## Spreading the beam

**Spreading the beam**

- As the beam spreads out, the yield and specific activity goes down
- As the beam spreads out, the convective heat transfer becomes more efficient

## Specific Activity

- Specific Activity = Amount of radioactivity (Ci) divided by the total mass (micromoles)
- Two ways to improve the number
- Make more activity
- Reduce the mass

## Specific Activity of Radioisotopes

- Ultimate Specific activities

| Nuclide | Half-life | Spec Act
Ci/μmole | nanomoles 1 millicurie |
| --- | --- | --- | --- |
| C-11 | 20.4 min | 9218 | 0.0001 |
| N-13 | 9.98 min | 18900 | 0.00005 |
| O-15 | 2.03 min | 91730 | 0.00001 |
| F-18 | 109.8 min | 1710 | 0.0006 |

## Carbon-11 Specific Activity

- If the carbon-12 concentrations are about 9000 carbon-12 atoms for every carbon-11 atom (1Ci/μmole) then
- In a high affinity receptor study this can mean that 20% of the receptor sites are occupied with non-radioactive compound

## Carbon-11 Specific Activity (20 Ci/μmole)

## How Much Added Mass

- Specific Activity – Sources of carbon-12 need to be thoroughly explored
- C-12 from Mass from Added Reactants
- C-12 from Mass from Gas Impurities
- C-12 from Foil Impurities
- C-12 from Target Wall Impurities
- C-12 from Transfer of Fluids In and Out of Targets

## Final Summary

- Simple targets can be designed, but the material and method of construction is critical
- The physical form of the target material often determine the recycling
- Non-radioactive isotopes have many sources
- High power targets are being developed

- Simple targets which do not degrade in performance
- Recycle enriched isotopes
- Minimize the amount of non-radioactive isotope in the final product
- Targets which will withstand high power deposition and use favorable nuclear reactions

## Shielding of the Targets

- The shield around the targets

## Slide 66

## Acknowledgements

- Joanna’s backyard 2001

- Department of Energy (Office of Biological and Environmental Research)
- The Group

## Slide 68

## Beam Profile Alteration

- in the cyclotron.  The original foils were 180 ug/cm2 polycrystaline graphite.  An assortment of foils from 400 to 1200  ug/cm2 were  purchased
- Beam spot shape was measured
- by irradiating a copper foil and
- imaging it with a phosphor plate
- imaging system.

- Calculated beam profile

- Measured beam profile

**Beam Profile Alteration**

## Home-made Ebco C-11 gas target

- Nominal fill pressure is 400 psi
- “Beam on” pressure is typically 700-800 psi (50 atm.)
- Have hit 1100 psi on occasion (75 atm.)

- Cooling Jacket
- Target BodyCut out section

## Original Ebco C-11 gas target

- Nominal pressure was 200 psi
- At 200 psi, the target was very thin due to density reduction

## New Ebco C-11 gas target

- Slightly longer so slightly better production
- The target was washed with organic solvents during manufacture and gave very bad specific activity
- The target was still thin at the recommended pressure

## The Solution:

- The decision was made to both make a longer target and also to purchase thicker graphite stripper foils to widen the beam.
- Calculations showed that the stripper foil would widen the beam by roughly 0.25 mm per 100 ug/cm2 of foil thickness.
- This target uses the 14N(p,α)11C reaction and the target is nitrogen gas with <600 ppm oxygen.  The target was machined with Electrical Discharge Machining.

## Specific Activity Ceiling

- 10-12 Ci/mole seemed to be a limit
- The amount of carbon did not depend on beam time or beam current
- The specific activity was dependent on the age of the Iodine tube in the MeI box

## Raclopride  Mass

- Baseline GE   Wet Method 	GE 4.5 H2 on GE CO2 adsorber added   MS 4A-3X in Ni trap

- Leak in cave
- xfer lines fixed

## Baseline GE	         Wet Method 	GE    4.5 H2 on GE      CO2 adsorber added   MS 4A => 13X in Ni trap

- Baseline GE	         Wet Method 	GE    4.5 H2 on GE      CO2 adsorber added   MS 4A => 13X in Ni trap

## I fully expect the petal array to out perform the standard hex array for a front grid.  With thicker grid spines, it still has more area

- exposed to the beam.  The spines as drawn are 0.5 mm across, or 20 mil.  I'm planning to use a 10-20 mil thick Al foil as the
- pressure foil.  I do believe the cooling spines of the grid should be at least as thick as the foil, purely for superstitious reasons.
- The radial spines as shown taper from 0.5 mm (20 mil) to 1.0 mm (40 mil) as you get closer to the edge of the beam.
- I expect a 15 mil front foil (6061) to hold about 750 psi at 50 uA beam current.  Mind you, 5056 is much stronger.  A 10 mil front foil
- will yield about 8% more thick target yield than a 15 mil foil, and 20% more than a 20 mil (18 MeV on front surface of foil),
- so it's something to consider.

## New Target Design

## Schematic of Wideroe’s Linac